The sloped baffle is to align the voice coils.... perfect voice coil alignment is the big draw of a point source driver. (proper coax mount)
Tim |
Mofimadness answered correctly
About time alignment.As far as phasing... it is simply having your individual drivers operating
In unison at all frequencies... As you change crossover points and slopes, phasing changes between drivers.... Correct time and phase alignment is what seperates many ok speakers
From world class speakers... These factors are why so many audiophiles like sigle drivers or point source.... I hope this helps, Tim |
Hi guys, I don't want to pretend to be The Expert on this subject, but I've built hundreds of speakers and have worked and designed along side some of the best back in my Marcof/SpeakerCraft days... so,
"but only if in combination with 1st order crossovers as used by Thiel and Vandersteen, revealing a coherent time-aligned step response"
1st order helps, but there is no speaker that is in complete phase at all frequencies... When you roll a speaker hard, it throws the driver out of phase just by the nature of steep slopes...
Remember a speaker rolls in and out of phase up and down the frequency ladder, once you choose your initial crossover point, you need to remeasure the drivers phasing up and down the scale with your first crossover in place , then choose another point that gets absolute phase at the crossover point. You may see a speaker that has a 350 & 3750 crossover point and you know that the tweeter can easily be crossed at 2500 or so, as long as the mid is smooth out to a given frequency, the designer may choose to take the frequency where absolute phase comes back into place. Also Impedance compensation helps control phasing a also. For that reason, I prefer impedance compensation, normally a driver sounds better when it needs compensation and compensation is used.
"I recall reading that Focal, Harman and Paradigm have anechoic testing chambers, staffs of engineers and physical facilities that presumably enables these companies to make rational and informed choices when designing and building speakers. I also recall reading that these companies manufacture in-house their own drivers".
Anechoic chambers help tremendously in measuring driver frequency and the effects that driver modifications have on a drivers frequency response, but Impedance & phasing are measurements that can be done without having a critical listening environment, I'm not trying to say that a great listening environment is not important, only that impedance and phasing can be handled without an anechoic chamber. Again, time alignment & correct phasing are keys to an imaging champ. I've kept it as simple as possible, I hope this helps,
Tim |
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 |
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 |
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 |
"If an 18kHz sound left its' source and a 30Hz sound wave left its source at the same time. would they both get to the listener at the same time?"
Yep, its not the speed of frequency, its the speed of sound. All frequencies travel at around 1000 ft per second... I'd have to look it up to be exact, but it also varies by sea level. The difference is how many times a wave will hit you. |
I have never heard Roy's speakers, but I would love to, I've never seen any speaker that was plus or minus just a few degrees throughout its frequency range... It would probably be easier to read a book than try to get all the info from a forum, so much skipped over, so many half truths... very difficult... We have only really discussed baffle step compensation... I have tried several times to electrically time align a woofer rather that the tweeter. It can be done, but throws so many other things out of wack that I've never really been successful.
One thing that I would say... it is possible to get a great sounding speaker without very good alignment, but most speakers that "just aren't right" do not have good alignment characteristics.... Any speaker that has very good alignment characteristics, you will find very listenable, it may not be your cup of tea, but it will do most things well. At least that is my experience.
Tim |
Bombaywalla, sounds_real_audio didn't ask if the were on a sloped front or a flat front, He simply asked, If they leave the source at the same time, would they end up at the listener at the same time...
I believe his real question is "do all frequencies move at the same speed"
I'm sorry if I mis understood the question, but as it was proposed, the answer is Yes.... I'm not try to start an argument, only to head off confusion.
Tim |
"I still wonder out loud whether speaker inductance as a function of frequency response in fact remains constant within the speaker's pass band. Indeed ... even if speaker inductance remains constant as a function of frequency, wouldn't that also impact phase coherency?"
Speaker inductance is one of the main factors that we measure to build a impedance/phase correction circuitry. The more that impedance and phase can be controlled, the easier it is on amplifiers (especially tubed).. Tubes can handle inductive loads reasonably well, but crap on themselves trying to drive capacitive loads.
Here is a thread where Al does a good job of explaining it...
http://forum.audiogon.com/cgi-bin/fr.pl?htech&1377551562&read&3&zzlMesch&& |
Hi Psag,
What you are quoting makes absolute sense... If you pull the crossovers and make everything perfectly phase and time aligned along with perfect frequency response, then the only difference is sound between speakers is the materials themselves...ie, how does the box sound, what does a Kevlar cone vs a paper or aluminum cone sound like etc.... So you are hearing first hand, (by correction) how important a flat response along with phase and time alignment can be. |
"So the question then becomes: Doesn't the presence of that inductive component of the driver impedance (especially in the case of the tweeter) cause a deviation from first order 6 db/octave behavior? And if so, to a degree that may audibly compromise phase and time coherence? And if so, is that or can that be compensated for in other aspects of the speaker's design?"
ABSOLUTELY, I had stated earlier that a simple cap seldom produces a 6db slope for that exact reason, no speaker that I've seen shows purely resistive(other than a ribbon). In the crossover, we can only lower impedance of a driver with compensation circuitry. A simple pad in series on a tweeter will raise impedance, but also cause you to need a new crossover. All this is the difference between electrical vs acoustical crossovers, cause & effect. 6, 12, 18 or 24 db per octave crossovers on paper, often end up being larger slopes because of the natural inductance, impedance and capacitance of a driver that must be taken into consideration in the design itself. In my experience without impedance compensation work, a driver with 6 db filters are still TYPICALLY 10 to 20 degrees out of phase... Any speaker that is with + or - 15 degrees of phase for its useable response curves in my mind IS phase coherent. |
Well, I know you guys are looking to Roy, but to add just a bit to what he said... The idea of z-compensation is to control rising impedances. You can add resistance to tweeters to make higher impedances with some success as long as you figure the added impedance in your crossover, but z comp or zobel controls the amount of rise of impedance. The main importance is that your speaker is crossed at a given frequency at a given impedance (say 2k @ 8 ohms)... impedance swings can throw that out of whack... so it is possible for your impedance to actually vary as it plays. Next z comp also makes a consistent load to your amplifier. Also, everytime I have used Zcomp, it improved phasing also. I hope this helps, Tim |
