Bomb,
That was not my intended meaning. Simply put, we're human.
Obviously you have your opinion and I'm not trying to argue, passive aggresively or otherwise. I may have reacted a bit sarcastic when I thought you overstated.
Perhaps you can explain something to me. In GMA's specs, they state phase shift acoustically over given frequencies. Does that mean impedance phase (reactance) or total phase? Either way, impressive. |
I believe one of the members had posted that the tweeter is 180 degrees out-of-phase in a 2nd order system. I believe that this is wrong info. If you read Roy Johnson's 1997 article in Audio Ideas Guide he says otherwise & I quote Contrary to popular belief, the 180-degree shift doesn't mean that the tweeter is out of phase with the woofer. It merely leads the woofer output by 180-degrees, or half of the wavelength of the crossover frequency....
Figure 5b also shows that the amplitude response (derived from steady-state tones) will have an infinite 'suck-out' Tw + W. At first glance, this cancellation does, indeed, seem exactly like that caused by a tweeter with reversed polarity. What is really true is that the time delay is causing cancellation on steady tones, because the time delay at the crossover frequency is half of a wave cycle, 1/(2f) seconds.
To 'fix' the problems of the system, a designer will often reverse the tweeter's wires to invert its polarity. Although the steady-state results shown in Figure 6a look much smoother, in reality this is because the graph for the tweeter's phase has had 180 degrees subtracted.
This seemingly minor point forms the basis for the many claims of 'phase-coherent' performance, which, at best, is a half-truth. The graph now shows 'a smooth rate of phase-angle change at the crossover point.' But it isn't smooth from the perspective of time passing by. In fact, regardless of how smooth this curve appears, the woofer and tweeter still have the same sequence of arrival as before.
Reversing the tweeter polarity only means that the tweeter is moving inward on the initial pulse while the woofer moves outward. This is evident in the square wave response in Figure 6b: The tweeter is pulling in while the woofer is pushing out. At the highest frequencies, the tweeter still arrives as before (in Figure 5), and now its absolute polarity will still be backwards.
I think that Al already provided us a link to this 1997 article so I will not repeat it here. I want to flip the switch and listen to music. Not interested in using an oscilliscope to figure out how to enjoy music. precisely! Even more reason to own a time-coherent speaker. Time-coherent speakers, when compared to non-time-coherent speakers, when properly designed, will be more dynamic, more true to the recorded music & simply more realistic. That's the kind of speaker to have in one's room if you want to simply "flip the switch and listen to music". With time-coherent speakers you will leave the oscilloscope on the test rack (where it belongs) & you listen to music. Problem is that we don't hear distortion as distortion, not unless you have the trained hearing of a professional under controlled conditions. It can be measured by instruments but human brains interpret those as frequency differences and there could be any number of explanations including comb filtering, diffraction, or the driver. Almost as easily, they can be masked.
So, forget the test, it's useless. I think that this is a bad attitude to have. Basically this says, "you are not trained to listen & you are not getting it, so drop this issue & don't make the effort to learn. keep the status quo". I say that if one wants to become a better audiophile, a better listener, a better consumer, one should challenge oneself to reach higher & try to understand things that are at this time outside one's grasp. One should challenge one's norm or the norm & you might find out that there is indeed a better way to do things & this enlightenment might bring more satisfaction & joy in listening to music. if you surf the Green Mtn Audio website, there are article on how to listen. http://greenmountainaudio.com/how-to-listen-to-music/http://greenmountainaudio.com/how-to-choose-speakers/One needs to understand that if a speaker is not time-coherent, no other parameter will make up for this. When it comes to time-coherence, you cannot juggle it/trade it off w/ other design parameters - you either have time-coherence or you don't. Time coherence is not a 'fascinating' concept or idea. It's real & properly implemented makes the difference between enjoying music & listening to top quality sound..... |
Ngjockey and Bombaywalla ... I find the issue of phase coherence to be intellectually interesting. I also find Roy's article and the article Ng... provided to be equally interesting. If I had the time, if there were more B&M stores around with a variety of speaker types, and if the salepeople were game to burn time to experiment, I'd love to build up a personal anecdotal portfolio of experiences.
To date I have only one data point, which is almost useless. I spent 90 mins about 2 years ago with Vandy Treos. They just didn't do it for me. Imagaging, soundstage and musicality were nonextant.
And if the pushback is that they weren't set up correctly, or wrong cabling, wrong positoning, not leveled, and so forth, my response is .... still not impressed. Even if the Treos would have taken me to the 8th dimension if all the foregoing variables were controlled ... I am still not convinced. I want to flip the switch and listen to music. Not interested in using an oscilliscope to figure out how to enjoy music.
This is a fascinating thread. I hope it will continue to attract more comments from folks who have relevant experiences.
BIF |
Bif,
Don't get me wrong. Phase is taken into consideration on any competently designed speakers. My first didn't but I wasn't particularly competent then. Not yet either. Still, with four 6-1/2's per side, they sounded more dynamic than any "coherent" speakers I've heard short of the Dunlavy SCIVa. Never considered the two were related.
Time coherence implies that the first cycle of a tone is more important than the second or third. That, I'm unsure of. Really, I don't know. Low order crossovers have advantages, including transient behaviour. I like them because they're simpler and cheaper but not for all drivers and circumstances.
Here's the test... Listen to any 2.5 way speaker. By design, the woofers roll off at different frequencies and must have increasing phase separation as frequencies increase. 45 degrees by 1000 Hz is fairly typical. According to the article, where our ears are most likely to detect. The .5 woofer will be lower SPL but still audible. Can you hear any phase distortion?
Problem is that we don't hear distortion as distortion, not unless you have the trained hearing of a professional under controlled conditions. It can be measured by instruments but human brains interpret those as frequency differences and there could be any number of explanations including comb filtering, diffraction, or the driver. Almost as easily, they can be masked.
So, forget the test, it's useless. More likely that someone will misinterpret that as all 2.5's are .... |
And here's my personal bottom line. Phase coherence is just one of many variables that is taken into account when designing a speaker system ... and there are many. As many folks have said, trust your ears and audition as many speakers as you can. But ... if I was asked to buy speakers based on their phase coherence characteristics as a stand-alone factor, I personally would not. Bifwynne, I disagree here. Of course, you are entitled to think about this the way you choose, just as I am. There's too much research - of the wrong type - that convinces people that time-coherence is one of the many issues/parameters to be resolved in speaker design. After hearing time-coherent speakers vs. others, the speaker should be designed around time-coherence & issues re. that speaker's design should be solved in the context of time-coherence. When that speaker is correctly built as a time-coherent speaker it will simply be far more realistic, dynamic & accurate to the recorded music than any other speaker in its price range. From my experience, I'm convinced - there ain't no other way to go..... Others who have also had such an experience feel the same way not surprisingly. sorry to see that (once again) this crowd has missed the point re. time-coherence. :( When you have a chance, listen to a time-coherent speaker (doesn't have to be Green Mtn Audio) & compare it a non-coherent speaker..... |
Ng... thanks for the phase coherence article. The article seems to be based on scientific controlled studies and tests. Rather than paraphrase the conclusions, I copied them here:
"So what conclusions regarding the audibility of phase distortion can we draw from the all of the above?
'Given the data provided by the above cited references we can conclude that phase distortion is indeed audible, though generally speaking, only very subtly so and only under certain specific test conditions and perception circumstances.
'The degree of subtly depends upon the nature of the test signal, the dB SPL level at which the signal is perceived, the acoustic environment in which the signal was recorded and/or played back as well as the Q & fo of any filter networks in the signal stream. Certain combinations of conditions can render it utterly inaudible.
'Room acoustics further masks whatever cues that the hearing process may depend upon to detect the presence of phase distortion."
And here's my personal bottom line. Phase coherence is just one of many variables that is taken into account when designing a speaker system ... and there are many. As many folks have said, trust your ears and audition as many speakers as you can. But ... if I was asked to buy speakers based on their phase coherence characteristics as a stand-alone factor, I personally would not.
Thanks again Ng.... Good article. It puts the issue into context.
Bruce |
I think that Kiddman makes good points here. There are so many interactions and variables in the design of loudspeakers that it's got to be very hard to isolate which factor results in a speaker that you ultimately love.
To wit: you couldn't have two more different small speakers than, say, GMA Chromas and Harbeth P3ESR's. "Leaky" plywood cabinet vs. synthetic stone cabinet. Metal dome tweet vs. cloth tweet. "Complex" crossover vs. "simple" 1st-order crossover. Inverted driver polarity (!) vs. not. "Radial" cone material vs. paper-composite. Flat face baffle vs. slanted baffle. And the list goes on and on.
Now, then, various audiophiles swear by one or the other of these speakers. But who can really say whether it's the cabinet material, driver material, crossover topology, phase/time coherence or (most likely) some combination of all of the above that gives the results that you get? And THEN, factor in the room and the associated equipment and the tastes and hearing of the listener and you have more variables than anyone can deal with.
So, yeah: I can see that you might become fixated on the theoretical merits of time coherence and then fall in love with a non-time-coherent speaker, anyway.
Reminds me of the joke about the unmarried scientist who feeds his theoretical preferences in women to a computer and then cross-references it to a database of women in his city. One day he approaches his lab assistant and says, "Well, it seems I have actually found the woman of my dreams. She meets every one of my requirements."
"Then why the glum face?" asks the assistant.
"I just don't like her," he sighs. |
Remember, all these are opinions of folks based on their perceptions, with most of them marketing something (though Geddes does not sell subs). You were not there for the experiments. It is all self-reporting. So time aligned, phase aligned, time and phase coherent, one sub, two subs, many subs....take it all for what it is: "Self-reported" opinions based on experiments where the experimenter was the only one observing.
You have to try the products yourself, or hear them in a good setup, to form your own conclusions. And still you won't know if it is the "main feature" the manufacturer is touting that is dominating the sound, or other details of the product.
Remember, we listen to music, not features.
Sometimes I feel that in audio, buyers and enthusiasts would rather talk features than simply sit and listen to see if it sounds like music. |
http://www.audioholics.com/room-acoustics/human-hearing-phase-distortion-audibility-part-2
That's bound to be misunderstood. Related articles in links.
The 30 milliseconds I mentioned previously came from a U-tubed Earl Geddes seminar on multiple subwoofers in a small room. |
Reversing the polarity of a mid in a 3-way or a tweeter in a 2-way is just a result of correcting for the phase shift of the crossover. The exception being 4th order. It's about getting the drivers to sum correctly. If they were 180 degrees out of phase, they cancel around the crossover point. In fact, one of the best ways to confirm the drivers are in phase is measuring the reverse null.
Now, if you want to talk about absolute phase and the half a millisecond @ 2000 Hz between cycles and whether that's audible, that's a different story. I don't know the audibility threshold up there but down around 100 Hz, it's around 30 milliseconds and several cycles for our ears to respond. Less for the trailing edge. |
I believe it. Hales uses fourth order crossover. It is of course not time coincident as one can see from the step response. You could speculate that part of the intent is to visually differentiate the transcendence from the revelation line, which it does. |
It's not crystal clear to me that Hales really said that, from how it is written. It does not appear to be a quote.
But it would be so very refreshing to hear that honesty from a designer. |
The best answer I've heard for using a sloped baffle came from Paul Hales,... "It looks cool"...
http://www.stereophile.com/floorloudspeakers/699hales/ |
Pay attention to those manufacturers that have intentionally run the mid driver in reverse phase to the tweeter and woofer. This indicates a bandaid approach that will ALWAYS compromise timbre, the accuracy of which is dependent on time and phase coherence. |
06-24-14: Kiddman
I would have to see the detailed measurements to accept the, I agree with this as well. It's just too bad that there aren't many to view/study.
IMO, unlikely results claimed by this manufacturer.
I disagree with this statement (given my personal experience) but I would have written "unverified claims by this manuf". Owning & listening to those speakers leads me to believe that such type of specs are achieveable by this manuf. Investing hours in driving to dealers, or flying, asking manufacturers to let you hear them, flying to audio shows, those are all much better ways to make an educated guess about how you will react to the sound in your home. indeed, I have done most of the items in this list - I have not asked manuf to loan me speakers to listen in my home - but I've done all the other items. I was at RMAF2013 & listened to 95% of the rooms in the Marriott Tech & walked over & listened to all the rooms in the Hyatt. I heard a lot of speakers - most speakers were in "bad" rooms (typical show environment) except for those speakers put into huge ballrooms & other conf rooms. Most of the speakers sounded very blah except a very few. And, one of them was the Green Mtn Audio room. I was there all 3 days & I listened to the Green Mtn Audio Eos speakers each day. After hearing so many blah sounding speakers, it was a relief to listen to the Green Mtn Audio Eos - they sounded like music, great dynamics & Roy was playing all kinds of music (not audiophile CDs - just regular redbook music). Those speakers sounded good on every genre. This excellent sonics caught the attention of Stereophile: http://www.stereophile.com/content/marigo-whirls-green-mountainFWIW. IMHO. YMMV. |
06-24-14: Bifwynne
I'm hoping Al and Ralph weigh in here to help us better understand the relative significance of phase coherence as compared to other factors. Hi Bruce, As I indicated earlier, speaker technology is not one of my areas of expertise, and about all I can offer is my suggestion that some insight into that degree of significance can potentially be gained by attempting to correlate one's subjective reaction to a wide range of quality speakers with JA's step response plots for those speakers. I'm hoping Al (Almarg) and Ralph (Atmasphere) get around to reading Roy's articles and sharing their comments.
I've read through the "Loudspeaker Phase Accuracy and Musical Timing" article. IMO it is a brilliant and informative article, which certainly inspires very high confidence in the designer. But my feeling, consistent with Kiddman's comments, is that in audio there are always multiple ways to achieve success. Best regards, -- Al |
TAD and Tannoy are not time coherent. The little KEF 50 with the coincident driver is not.
What does this mean to the listener? Without a listen, it means little. The TAD Reference represent some of the best speakers at any price. That little KEF is great. The Tannoy are some of the most loved speakers, and have had a very long life as a studio speaker, with uncountable numbers of musicians hearing themselves on them after playing. Would anyone postulate that these musicians don't know how they themselves sound?
Again, this would be a nice thing to have, as perfect radiation patterns, perfect frequency response, perfectly inert cabinets, zero harmonic distortion, zero intermodulation distortion, and zero driver resonances would be nice things to have. No speaker has it all, so the sonic result of the compromises chosen is the real key.
Again, listen. Do not fixate on any one or two features, that will lead you nowhere in your quest for a speaker that sounds most like real music to you. |
Maybe this is a subject for another discussion, but I was wondering if coaxial drivers (TAD, KEF, Tannoy) are more likely to be time and phase coherent? |
Bandying about technical claims by manufacturers and trying to determine how it affects the final product, whether it be a car, motorcycle, amplifier, airplane, or missile is almost totally useless. Investing hours in driving to dealers, or flying, asking manufacturers to let you hear them, flying to audio shows, those are all much better ways to make an educated guess about how you will react to the sound in your home. |
Excellent comments gents. I'm hoping Al (Almarg) and Ralph (Atmasphere) get around to reading Roy's articles and sharing their comments.
Although it has been said many times, designers of audio gear take into account many features and do their best to balance, maybe better said ... optimize, what comes out of the metaphorical oven. I'm sure phase coherence is just one of many important factors. That's why I'm hoping Al and Ralph weigh in here to help us better understand the relative significance of phase coherence as compared to other factors.
Bombaywalla, I agree that one shouldn't base expensive purchase decisions solely on reviewer comments. But there's a problem; one that has been touched on by many A'goners including me over the years. There are very few hi-end B&M stores around anymore. And to the extent such stores are around, setting up meaningful and fair auditions is very difficult. Oh ... and as far as Green Mountain is concerned, no dealers near me.
Not sure what else to say. Another potentially huge sleeper problem that is finally getting the press it deserves. |
I would have to see the detailed measurements to accept the, IMO, unlikely results claimed by this manufacturer. |
it is true that the measurements by reviewers are few for Green Mtn Audio speakers. I don't know why?
I believe that Green Mtn Audio speakers are different from conventional high-order x-over speakers that are prevalent in the market. If one is used to measuring the conventional high-order x-over speakers that are prevalent in the market that same technique is unlikely to work for Green Mtn Audio speakers because of the requirements of the distance & listening axis required to ensure that the drivers integrate. I believe that JA was not knowledgeable about this when he did the meausurements for the Diamante speakers. I also understand from the manuf that JA was advised by the manuf many times how to setup the speaker to measure it correctly but JA completely ignored the manuf's recommendations & did his own thing. The result is poor measurements & also measurements done incorrectly.
Yes, the slopes of 1st order x-over are shallow compared to 2nd or 3rd or 4th order x-over so signal will bleed thru at high levels. While 4th order x-over speakers can be driven to 120dB SPL, 1st order x-over speakers can be driven to something like 105dB SPL. If this is a no-deal for you, so be it. But 105dB SPL for in-room/in-house listening is plenty loud by any stretch of the imagination. Yes, it's not outdoor rock concert SPL.
Have you owned any Green Mtn Audio speakers by which you claim " All and all, a concept that does not really work, due to all the other problems it create."?? Or, are you one of those people who reads reviews & makes decisions based on the printed word? Or, are you a speaker manuf with an axe to grind?
Because I speak from personal experience & from the experience of my friends & relatives who own & have owned Green Mtn Audio speakers. Everyone that I know of has had & is having an excellent experience with these speakers. Yeah they have limitations (which speaker does not?) but their limitations are of ommission rather than of commission.
I do not intend to convert you to 1st-order speakers - never intended to for the get-go - please buy what you like & enjoy it. But I would not make such sweeping remarks without personal experiences.
Anyway, this thread is about "sloped baffle speakers", "time & phase coherence". There are very few speakers that meet the time & phase coherence criteria & I believe that Green Mtn Audio is one such brand hence it gets brought up time & again in such discussions as an available commercial product. I'd prefer that this thread not take a downward spiral focusing on Green Mtn Audio speakers or Green Mtn Audio bashing. Let's keep the discussion on "sloped baffle speakers", "time & phase coherence". Thanks. |
Hi guys, I'm back to the real world... first, Al, thank you for your gracious words...
If Green Mountain audio meet specks, what Bombaywalla posted are truly amazing, it takes a lot of crossover work to bring phasing in line to plus or minus 3 degrees. I've never seen a graph of a speaker with specks that tight. 6db pads done correctly will keep you within 15 degrees or so. I'm not a pro and have nothing to lose by showing my stuff. I'm using HiVi D6.8 woofers (same as Totem Forest) and a low to mid Scan Speak Tweeter in an MTM. I've never used photo bucket, so hopefully this works, but I just uploaded my graphs & crossovers to photo bucket... Hopefully, you can view this, if not, I'll try to figure it out and post again.
http://s28.photobucket.com/user/timlub01/media/XO_HiVi_D68_ScanSpeakD2608_MTMrev4_zps302ac070.gif.html |
>>>In both the Green Mtn Audio speaker specs, now, we are talking flat impedance & phase response. :-) Nothing like that in the Magico S5.
Interesting. Any independent measurements you can point us at to verify your claims? The ones we see in SP are not that impressive. In fact, they are quit awful (and are not even time coherent as claimed). Even the so polite JA had to say that: “..the listening axes on which the speaker is (almost) time-coherent and offers the most neutral response don't coincide”. Which is just one typical issue this concept have. Not to mention that distortion level with first order XO will be significantly high: “…occasionally thought I caught a hint of lower-midrange congestion”. All and all, a concept that does not really work, due to all the other problems it creates.
|
Hi Roy,
good to see that you discovered this thread. :-)
Duke, you are always gracious.... :-)
hope that Bifwynne takes advantage of your resp. presence here & gets some deep technical dialogue going..... :-) |
Just took a look at this thread; the title didn't obviously say "read me" so it took me a while to check it out.
Lowpass drivers lag behind highpass drivers in both time and phase, and a sloped baffle can introduce a limited amount of compensation for that, along with a tilting of the driver axes (which may be desirable in some cases). In and of itself, it's not enough compensation to make much difference, but could be part of a full-scale time-coherent effort such as Roy Johnson's magnificent speakers.
"Phase coherence" and other phase-based terms have been used loosely and don't necessarily mean what we think they mean. For instance, the woofer can lag the tweeter by 360 degrees at the crossover point, yet the system could be called "phase coherent".
Roy's designs are truly "time coherent", and that puts them in a different league; they're the real thing. His designs do everything we might have optimistically hoped for, such as waveform preservation, when we first came across the term "phase coherent".
As for how small manufacturers can do decent design work without access to an anechoic chamber, one answer is time-gating. That's a measurement technique wherein the microphone is turned on briefly to catch the direct sound from the speaker, and then turned off before the first reflection arrives. Time-gating has limitations relative to what can be done with a true anechoic chamber, but it sure costs a lot less.
Roy, I'd be very interested in reading any clarifications you care to post, and have zero problems with your making specific references to your products. The inherent rapid rolloff at the bottom end of the horns I use prevents me from ever doing what you do without totally changing directions (and going back to college), but that doesn't lessen my admiration for your work.
Duke
dealer/manufacturer |
Gentlemen,
I am glad to know my previous writings were appreciated, and it's become easier to explain since then.
I can clarify some basic technical concepts above, without reference to our products, if you like.
Please let me know.
Thank you,
Roy Johnson |
Bombaywalla, did you or Tim mention that one mark of a phase coherent speaker is one which has a flat impedance and phase plot. Take a look at the Magico S5's specs. Is there anything else apparent from the NRC tests that permit inferences about phase coherence? Bifwynne, don't know whether I or Timlub wrote this - I don't recall writing this in this thread. Maybe in some other thread you & I contributed to? Anyway, looking at the Magico S5 measurements, even tho' the imp & ph plots are less bumpy than many others in the market, there is still quite a bit of phase shift. Plus, at 2KHz the speaker goes from positive phase angle (inductive) to negative phase angle (capacitative). This can be quite burdensome for the amp - the phase relationship between the current & voltage changes very suddenly & the amp has to react to that instantaneously. This is difficult to do. Looking at these impedance & phase plots I cannot confirm whether or not this speaker model is time-coherent. Probably not as I've not seen Magico design any speaker thus far that has been time-coherent. Maybe the S5 is a departure from that norm - don't know - but if I'd have to guess, I'd say 'no'. OTOH, for, say, Green Mtn Audio speaker I'm used to reading specs that look like this. For the C3 speaker model, for example: Response +/- 0.75dB from 40Hz to 21kHz, -3dB at 35Hz and
23kHz, from 2.5 to 6m, on Soundfield Converged axis, on first arrival
tone bursts, across approx. 80dB dynamic window.
Phase shift +/- 3 degrees acoustically, from 160Hz to 8kHz.
Does not vary with loudness.
Impedance 6.5 Ohms, +/- 0.75 Ohms 150Hz to 20kHz. Does
not vary with loudness. My brother owns the Callisto bookshelf models & their specs are: Response Freefield at 2m: +/- 0.75dB from 55Hz
to 20kHz, -3dB at 47Hz and 21kHz. Typically -3dB
at 47Hz on 24” speaker stands at 8’ distant.
Phase shift +/- 2 degrees acoustically, from 200Hz to 8kHz
on listening axis. Does not vary with loudness.
Impedance 4.8 Ohms, +/- 0.75 Ohms 100Hz to
20kHz. Does not vary with loudness. In both the Green Mtn Audio speaker specs, now, we are talking flat impedance & phase response. :-) Nothing like that in the Magico S5. (Once again, I've experience with Green Mtn Audio speakers hence I bring them up time & again. If other members have similar specs for other time-coherent or phase-coherent speakers, please share those specs. Thanks.) |
There are time and phase coherent speakers that are
wonderful and there are those that are lackluster or worse.
There are speakers that are not both time and phase coherent
but are wonderful, and those that are lackluster or worse.
Time and phase coherency are two out of many attributes that
would be goals in designing a loudspeaker. But not having
it does not mean the speaker is not great, and having it
does not mean the speaker is great.
You have to listen.
Of my favorite speakers, one is and some others are not. It
does not bother me that some are not. I cannot detect the
effects of the lack of time and phase coherency of those
that are not. None of this means that it is not a valid
design goal, but it is only one of a very large number of
valid design goals. |
In speaker designs that I've only simulated for now, sloping the baffle 10 degrees allowed a slightly higher crossover with the same drivers, which is not so arbitrary. In a 3-way, it also allowed symmetric slope crossovers. In a 2-way, adjusted for BSC, it ended up asymmetric similar to the way I'ld do a flat baffle. Since a normal TM tends to lobe downwards, the sloped baffle tends to correct that and even allow a shorter height. The downside was that it was sensitive to listener height and distance. |
Bombaywalla, I'm somewhat familiar with NRC test facility. But I am not sure which of the NRC tests speaks to time and phase coherence.
Btw, I surmise that the reason many of the top speakers, like the Magico Q5, that have flat frequency response plots and probably use higher order cross overs is because the driver timing is tweaked at the cross over points to optimize wave cancellation and augmentation. But ... the drivers are not otherwise in phase outside of the cross over overlap region.
So ... even if the speaker specs well on the bench, it may very well be distorting complex sound patterns as Roy Johnson explains in his articles.
Bombaywalla, did you or Tim mention that one mark of a phase coherent speaker is one which has a flat impedance and phase plot. Take a look at the Magico S5's specs. Is there anything else apparent from the NRC tests that permit inferences about phase coherence?
http://www.soundstage.com/index.php?option=com_content&view=article&id=1043:nrc-measurements-magico-s5-loudspeakers&catid=77:loudspeaker-measurements&Itemid=153
Al, here's another Roy Johnson article I clipped. Perhaps if you get a chance to read his articles, please share comments and thoughts:
http://greenmountainaudio.com/speaker-time-phase-coherence/
Btw Al, you may find the square wave pics in the article to be of interest. It seems that using higher order cross overs creates an almost unsolvable electrical problem.
BIF |
Just a word of caution regarding some speaker measurements one might come across. In an effort to avoid room interactions skewing measurements, very often speaker measurements are taken closer to the speaker than where one would typically sit. Most speakers designed for time and phase integrity need to be taken further away to allow for driver integration (approximately 8'-12'), where listeners typically sit. Unless these further measurements are taken in an anechoic chamber, the room will now contribute more measurable distortions than if the same measurements were taken closer to the speaker. Most people don't listen to speakers at the distances many speaker measurements are taken. If speakers with time and phase integrity aspirations are measured at distance ranges other than where they were intended to be heard from; their square wave and step response, though probably still better than competing speakers without such design considerations, will have their ultimate measurable potential be compromised. |
06-22-14: Bombaywalla
Why the differences? Because the Vandy 7 is time-coherent & the Revel2 is not!
Bifwynne, if you click on the link provided by Tls49, you will see exactly why as you read the text there. Scroll down where Roy J talks about time-domain response & he shows how a step response looks when the speaker is time-coherent & when it is not.
The Revel2 are clearly not time-coherent. I agree. Bruce, taking the Magico Q5 as an example, whose measurements you also linked to, note that the frequencies of each of the four up and down oscillations that are shown in the step response figure become progressively lower (i.e., their periods/durations become progressively longer). That is because the first peak primarily represents sound arrival at the measurement mic from the highest frequency driver; the second peak primarily represents sound arrival at the measurement mic from the next highest frequency driver, and so on. So it can be inferred that the amount of delay between the start of the arrival of the sounds from each of those drivers and the occurrence of the step in the signal that is sent into the speaker are significantly different, and become progressively longer for progressively lower frequency drivers. While with the Vandersteen 7 they are not significantly different, resulting in the outputs of each of its drivers summing (at the position of the measurement microphone, at least) to a much closer approximation of an ideal step response. Best, -- Al |
Bifwynne, Ok ... if Tim or Bombaywalla catch this, here the ultimate Q. How can one tell whether a speaker is time and phase coherent? Critical listening is one of the methods - when a speaker is time-coherent then it will get the timing right for ALL genres of music. Recordings otherwise painful to listen to will be less painful (because the speaker is not distorting the music signal coming to it). In time-coherent speakers you forget the audiophile attributes (pin-point imaging, soundstage width/depth/height, etc, etc) & you focus on enjoying the music. The other method, as stated by Unsound & 2nded by Al, is the time-domain step response. Usually Stereophile (as indicated by Al already) & Soundatage reviews http://www.soundstage.com/index.php?option=com_content&view=article&id=16&Itemid=140 do a pretty good job of showing all the measurements they have done on various loudspeakers they have resp. reviewed. Thanks Tls49 - you have pointed Bifwynne to a really good website for reading on time & phase coherence. I always look to Roy Johnson when I have questions on this subject. To me he is skilled in this field & has dedicated his life to making time-coherent speakers since he began his biz in the 1980s. Green Mtn Audio speakers are the only speakers that I know of that are time-coherent across the entire audio band from my personal experience. I know that several people believe that Vandersteens & Thiel speakers are also time-coherent - I personally do not have any experience w/ either of those brands so I will not comment. First, the Vandy 7's step response plot shoots up like a rocket and then quickly rolls off and stays down. By contrast, the Studio 2 shoots up, rolls off and then rolls back up again. What does this mean? Why the differences? Why the differences? Because the Vandy 7 is time-coherent & the Revel2 is not! Bifwynne, if you click on the link provided by Tls49, you will see exactly why as you read the text there. Scroll down where Roy J talks about time-domain response & he shows how a step response looks when the speaker is time-coherent & when it is not. The Revel2 are clearly not time-coherent. I'll ask again, how important is time and phase coherence? To me it is everything! I used to own B&W speakers & I have heard speakers from many, many different brands now both auditioning at dealer locations & at multiple RMAF shows, at various friends' homes, etc. The sonics from a time-coherent speaker are in a totally different league. For me time-coherent speakers "get it" while all others simply do not. Once I heard Roy Johnson's Green Mtn Audio speakers & owned one for many years, I realized what the heck time-coherence was all about. Now, I cannot listen to any speaker if it's not time-coherent - very quickly I will realize the lack of time-coherence & I will immediately perceive that the music has no life & no PRaT & to me it will just good/superior sonics but not music. I want music from my speaker & not good/superior sound - I didn't pay top $ for good/superior sound; I paid top $ for music thru the speaker. These days I stick to time-coherent speakers - my selections are very few but it's worth every penny to me. This should be the case for everyone if you are interested in listening to music. Many of friends are into superior sound (& not music) so they are on a perpetual merry-go-round for speakers - speakers come & go thru their music rooms. They are always wow'd by a new speaker & later after a few months or a year, that speaker is sold & another one takes its place & the cycle repeats. In my mind I'm thinking that I've made my case for time-coherent speakers - you buy it once & then you sit back & listen to music because a time-coherent speaker will play every genre without tripping up because its physics is right. Of course, IMHO. YMMV. |
Just a supplemental fyi, here JA's bench test measurements for the Revel Salon 2:
http://www.stereophile.com/content/revel-ultima-salon2-loudspeaker-measurements
And one more, the might Magico Q7:
http://www.stereophile.com/content/magico-q5-loudspeaker-measurements
If nothing else, perhaps someone can explain how to properly interpret what these measurements mean. They do not look like the Vandy 7. |
Al and others, take a look at John Atkinson's step measurements of the Revel Studio 2:
http://www.stereophile.com/content/revel-ultima-studio2-loudspeaker-measurements
JA notes that "the speaker is time-coherent rather than time-coincident: each drive-unit's step smoothly hands over to the next lower in frequency. This correlates with the superb frequency-domain integration between their outputs ...."
Two observations on my part.
First, the Vandy 7's step response plot shoots up like a rocket and then quickly rolls off and stays down. By contrast, the Studio 2 shoots up, rolls off and then rolls back up again. What does this mean? Why the differences?
The other observation is that the Studio 2 has a ruler flat frequency response. I believe the Studio 2 uses high order cross-overs, like my Paradigm S8s.
Interpretive comments are welcome. |
Sounds_real_audio -- your comment re placement if the drivers is interesting. There are some brands that stuff drivers every-which-way into the front baffle. One brand that comes to mind is Nola. I wonder how the Nolas manage the time and phase coherence attribute. |
There is a really good book that explains a lot of this and is written for the person without a PHD. It's entitled "The Loudspeaker Design Cookbook".
I pulled my copy out and reread some of what was discussed above. I highly recommend it if anyone is interested in loudspeakers and how/why they work. |
Bifwynne, if you don't mind a little more reading, I suggest Time and Phase Coherence by Roy Johnson from his website. This is very informative with illustrations, and you don't have to sort through confusing comments as in the threads from the links that were provided previously. Hope this helps. |
Bruce (Bifwynne), thanks for the summons. I haven't yet taken the time to read the links Bombaywalla was good enough to provide, but I am pretty much in agreement with all that has been said by the various posters above. Including the comment by Kiddman just above, which I believe to be a correct statement of the bottom line. And also including the comments by others to the effect that speaker technology is not one of my areas of expertise :-) And in that connection I'll say also that over the years I have found Tim's (Timlub's) posts concerning speakers to be highly knowledgeable, credible, and informative. Another member whose inputs I suspect would be particularly valuable on these issues would be Duke (Audiokinesis); perhaps he'll spot this thread. Regarding your basic questions about about how to gauge time and phase coherence, and about how important it may be among the innumerable tradeoffs that are involved in speaker design, I think that Unsound summed up the answers very well when he said: Good step and square wave response are indications of wave form fidelity (time and phase). Again, as to it's importance, well that's up to debate, with opposing opinions prevalent on both sides. Realize that a theoretically ideal square wave (which does not exist in the real world, of course), having infinitely fast transition times between its two states (i.e., risetimes and falltimes of zero), perfectly flat tops and bottoms, and no overshoot or ringing, consists of the summation of an infinite number of sine waves, one being at its "fundamental frequency" (the frequency with which its pulses repeat), plus others at every odd multiple of that frequency (i.e., the 3rd, 5th, 7th, etc. harmonics). The amplitude of each harmonic decreasing as its order (i.e., its frequency) increases. In order for a real-world approximation of that square wave to be reproduced accurately by a speaker, at a given listening position, all of those harmonics (which collectively are generated by a combination of all of the drivers in a multi-way speaker) have to arrive at that listening position properly timed and phased relative to one another. No speaker will do that perfectly, of course, but results will vary widely among different designs. In the measurements section of most of the speaker reviews which appear in Stereophile, John Atkinson presents and discusses the speaker's step response (which is pretty much as meaningful as the square wave response, a square wave consisting of a continuous series of positive-going and negative-going steps). For example, Figure 9 here is the step response for the (very expensive) Vandersteen 7, measured at a certain height and distance. The step response shown is probably about as good as it gets. (Keep in mind, btw, that the step response of any speaker will not **look** like a step because, among other reasons, for it to do so would require that the speaker's frequency response extend down to close to zero Hz). So perhaps some degree of insight into the importance of time and phase coherence could be gained by comparing that measurement for speakers whose sonics one is familiar with and that have been reviewed by Stereophile, and trying to judge the degree of correlation that exists between the quality of that measurement and one's subjective preferences among those speakers. Although to be meaningful, that would have to be done by a given listener for a considerable number of speakers. I've never done that, btw, and unfortunately I don't think that such measurements are available for most of the speakers I've owned or extensively heard over the years. Best regards, -- Al |
Do not pick a speaker based on crossover slope, sloped baffle or not, type of driver, or any other feature or set of features. There are plusses and minuses of every design under the sun. Don't even be influenced by things such as this. Pick a speaker based on your ears. |
Hi Bif, I've found that time alignment is certainly worthwhile, but let me just say that all you may need is a way to visualize how and why it would be important. I'm not technically as well versed on the subject as some, but I do have some direct user experience with it. I'm in the process of triamping my rig. Each amp I have (and will be adding) will have a suite of (digital) tools which include a pair of xovers, EQ and time delay. Each pair of tweeters, mids and woofers will have their own amp...and each of those frequency bands can be delayed the appropriate amount - dialing in by ear alone, at the lp, is sufficient. I can tell you that in practice that process is far from complicated and can be done in a couple minutes by anyone. It just may seem impossible to grasp for you at the moment because (understandably) you don't have any way to vary the control over your time alignment one way or the other so there is no way for you to realize its effects at this point. But, try to visualize it in terms of what you may already have some ordinary experience with. Try this: turn on a TV in one room of your home and then turn on another TV in a different room, but turn that one up loud enough to hear it while you're in the other room listening to the 1rst TV. Make sure they are both on the same channel. You can't help but notice the substantial time delay - like a rather serious echo, but in a nutshell, that's really all there is to time alignment. You've likely heard this effect countless times before without thinking about it at places like car dealerships, fire halls, etc...places where a PA system is using more than one loudspeaker in different places outside (usually on the other side of the building), creating the very noticeable amount of delay. Now, the big difference (apart from the sheer amount of time in the delay) is the fact that, like with the TV example, all of the frequencies overlap...hence the echo effect, naturally. But, with HiFi speakers (except to a degree at the xover regions, which we'll ignore here) the frequency groups are largely separated in their delay. What this means is that (with any conventional, flat baffle arrangement, anyway) the highs reach your ears first, then the mids and then the bass. Basically it means that the highs are simply being projected out into the room and are imaged a little closer to you (in soundstage distance) than the mids, which are in turn projected a little farther into the room than the bass...it simply becomes apparent to you that the higher frequency band seems physically imaged a little closer to you. Time alignment resets this effect, moving the imaging of the higher band back into place with the frequency band below and gets all the instruments and voices that much more correctly positioned from top to bottom in the frequency range...a very nice and more realistic or natural presentation - definitely worthwhile. Once you've ever heard the effect for yourself it takes all of about 5 seconds to solve the mystery as to how and why it might be useful, but, with speakers if you visualize the difference in terms of the distance of the soundstage image from you with each driver's operating range, then you just may at some point be able to recognize just how most speakers have somewhat less than ideal time alignment. And some will seem a little worse or better than others. If you use subs, however, it is always recommendable to take time alignment into consideration along with the other factors concerning placement, but I don't believe perfect time and phase alignment has ever been done in any speaker and I may never see it in my lifetime. But, triamping in this way for me is giving me the opportunity to pretty much 'fix' the time-alignment problem, which won't hurt me at all. Apart from building your own speakers though, it seems there are not too many designs that have actually tackled the problem, as near as I can tell. Until more makers become convinced there is a market for time-aligned cabinets (expensive because it's so labor intensive), then there may be only a handful of alternatives for us. Other problems for manufacturers include that in designing cabinets for true time alignment (meaning driver offset at something close to the correct distance) they start getting into a world of cabinet diffractions and reflections - not easy enough to successfully solve, particularly in any aesthetic sense. It's not horribly critical to get the distance precisely right, but the closer the better up to a point...like trying to position both your speakers the same - it might be quite good to get them within, say, a 1/4" of each other, but under an eighth of an inch??..at some point there ceases to be an improvement, but then again, with time alignment you would notice generally a better sense of scale and improvements in the macro areas of the soundstage...while the soundstage details would remain largely unchanged. Hope this helps. |
^Again good step and square wave response are indications of wave form fidelity (time and phase). Again, as to it's importance, well that's up to debate, with opposing opinions prevalent on both sides. I will say when I first began seriously auditioning speakers and without any real technical knowledge I was consistently drawn to those speakers that unbeknownst to me at the time shared those design properties, and those speakers designs continue to favorably impress. Others don't always seem to share the same sensitivity and/or priorities that speakers with these design priorities share. Only by listening to them yourself will you know for sure if it's important to you. Oh, btw, they usually share the similar quality of steady impedance loads, a subject of which I'm confident you will recall being discussed previously here on Audiogon. |
Ok ... if Tim or Bombaywalla catch this, here the ultimate Q. How can one tell whether a speaker is time and phase coherent? Critical listening? Reviewer comments? Bench test?
If critical listening is that important, the real challenge for us is, as many have written, that it is not easy to meaningfully audition speakers. So what's a person to do?
I'll ask again, how important is time and phase coherence? FWIW, ... really more as an FYI, ... Paradigm's web site states that its "speakers have phase coherent crossovers designed so that the summed output of the drivers is completely and accurately rejoined." Is that hype? It is true at all frequencies? Dunno
Thanks again. |
I'll be off the net for a couple of days, so I'll end my part of this thread here. Simple 6 db slopes do not tell the story at all, Most would be amazed that when you put a simple cap on a tweeter expecting a 6db slope, many times you might find 9 or 10 db slopes.... There is a difference between electrical vs acoustic slopes... On my MTM speakers that I referenced... I may be using 12/18 slopes, but the finished acoustic slopes are 24db per octave. Speaker design with new drivers from scratch requires measurements, you can get amazingly close with some of the better software however... Then lastly to throw another wrench in the fire... you can easily get time alignment between a tweeter & woofer without being phase aligned, but you can never achieve TRUE phase alignment between a tweeter & woofer without time alignment. I haven't read either of the threads referenced above, but hopefully a quality speaker manufacturer out there has explain it better.
I was only trying to help, Tim |
06-21-14: Bifwynne
Bombaywalla, I have a follow up question. How are small speaker manufacturers able to design speakers without the benefit of the R&D budget, engineers, and testing facilities that some of the larger manufacturers have at their disposal. Bifwynne, as Timlub wrote earlier, I don't think it's necessary to have an extensive R&D budget, scores of engineers, anechoic chambers & mutli-million $ machines to make a good sounding speaker *iffffff* the manuf really knows how to design a speaker. The companies that you stated need this to make-up for their lack of knowledge of speaker design. Focal has gotten better over the years. when I 1st heard their Electra series speakers, they sounded like sh**. Their ultra-expensive speakers have a big wow factor but nothing more. Those R&D budget, scores of engineers, anechoic chambers & mutli-million $ machines are good marketing hype items that sells. ;-) of course, my opinion based on listening to many of the cited manuf's speakers. If one really knows what one is doing then it is possible to select & buy speaker drivers made by OEM speaker driver manuf that are superlative in their specs & performance. The next thing to do is to design a speaker integrating those drivers into a cabinet such that sum of the two is greater than each part. Al and Ralph .... where are you??? :)
Bifwynne You know Bifwynne, it is entirely possible that Al & Ralph cannot help us here. Raplh is an amplifier expert & the fine points of speaker design might not be in his 'quiver of arrows'. Same for Al. Unlike explaining electronics, one cannot explain way speaker design unless one has dealt with this complex task. Yeah, everything is a compromise - engineering is an applied science. By its definition it is supposed to make compromises to bring about a solution. The key is: which engineer makes the best compromise? I believe that the 3rd link I stated clarified your earlier question re. the difference between time coherent & phase coherent. The sloped baffle gives you a time alignment of drivers but does not tell anything about the time or phase coherence of the speaker. Another thing I learnt was that just because a speaker has a 1st-order x-over does not mean that the speaker is time or phase coherent. I found that a lot of speaker manuf hide behind their using a 1st order x-over. I found that many such speakers used a 1st order x-over but their drivers were some higher order (such a 2nd or 3rd) which means that the driver performance rolled off with a higher order well before the 1st-order x-over kicked in. So, in effect, such a speaker is a 2nd-order or 3rd-order speaker & not a 1st-order speaker. To be truly a 1st-order speaker the speaker has to be electrically & mechanically a 1st order speaker meaning to say that the driver's performance needs to extend beyond the x-over point so that the roll-off is being done only by the electrical x-over network. it's complex material that cannot be absorbed in 1 reading. I've those threads printed off & re-read them from time to time to refresh my understanding. Each time I extract new info from them. Like Unsound wrote earlier the 1st thread & I think the other 2 as well are some of the very best threads to have appeared on A'gon. |
A good step response and square wave response can be a significant indication of wave form fidelity, things that any other piece in the system should readably be capable of.
Much of the need for correct listening position for the appreciation of such design principles is in regard to distance from the speakers for proper driver integration, typically such designs have something of a wide horizontal dispersion, sometimes there is concern for the listeners vertical position. Many of us do serious listening while sitting somewhat centered between the speakers. Such designs at least offer a very good semblance of wave form fidelity at some position as opposed to never providing any semblance of wave form fidelity from any position. You don't have to sit with your head in a vice to get the virtues of such designs! |
Hi Bif,
Well, you've got a bit of confusion going on. A sloped front is used to align the voice coils on the tweeter/woofer or tweeter/mid/woofer. The idea is to get all frequencies starting on the same plane... Using an easy example is a 2 way... A woofer may be 6 to 8 inches deep and it may be 5 inches or to where the voice coil meets the spider & magnet gap (that is your alignment point).... The tweeter however may be less than 2 inches deep. On the sloped front, you can move the tweeters voice coil backwards in alignment to the woofer's voice coil by moving it up the slope.... Guys, I am on solid ground here, you can ask, who ever you want, I may not be known like Al or Ralph and don't come close to their electronics knowledge, but Speakers, I've got a good handle on.
There are tons of other issues that we haven't discussed. Moving the tweeter too far from the woofer (Depending on crossover frequency) can cause all kinds of problems with smearing, lobing and other dispersion issues. I was only trying to handle the original question of a sloped baffle and phasing.
I've had great luck using 6/12 crossovers on 2 ways @ 2500hz and find most of the time (depending on driver) that the speaker is in phase at the crossover point with those slopes. I am currently using an MTM, crossed at 1700 htz with 12/18 slopes, it is phase coherent at that frequency and time aligned in the crossover and images as well as anything that I've sat in front of.
Simple 6 db slopes cause less problems to deal with, I've used them with great success also, but its not the only way.... I hope this helps, Tim |
Tim, although I don't understand the science, I recall reading that a sloped baffle is one component of achieving phase coherence. The proper cross-over order (1st, 2nd, and so forth), plus maintaining a specific listening position, plus, setting the cross-over frequencies at the right points, and so on and so forth also contribute to maintaining optimal phase coherence.
In addition, assuming that the drivers are in proper phase alignment in the overlap region of the cross over points, presumably there should not be phase cancellation or augmentation which will mess up the frequency response. That is frequency response should remain flat. But what about maintaining the proper harmonic structure of complex musical passages?
What other benefits are gained by locking one's head in a head vice in order to maintain the precise listening position, which presumably will enable the wave fronts from the various drivers to combine in an optimal phase coherent fashion at the listening position.
So ... how much of this is hype? What other characteristics make for a good sounding speaker? I used to think I had a handle on this this issue ... but no more.
BIF
Al and Ralph ... are you catching any of this?? Throw me a line. I'm sinking in techno quick sand. |
Simply putting the voice coils in the same plane does guarantee a cohesive and coherent sound.
Drivers that are placed close together are just as important for a cohesive sound.
It is equally important to have a bass driver that is as fast as the midrange otherwise you end up with a bass that seems disconnected. I hear this in some very expensive speakers. |
