I'd like to ask the forum what the primary differences are in sound, performance, and application of sealed powered subwoofers vs vented either bottom vented, rear, etc. B&W makes most if not all of their current line of powered subs sealed. Yet I see other manufacturers offer vented subs. What is the difference? Do the sealed subs produce a higher quality tighter controlled bass vs a more sloppy reverberating type of LFE out of the vented types? Thanks.
Hawksford is an academic [as am I] but the serious work in sound reproduction is mostly being done at the practical rather than the theoretical end.
Stan,
It is a pleasure to have an academic researcher in audio engineering on these forums - what aspect of audio are you currently researching?
Dr. Floyd Toole's work was almost entirely practical - his research demonstrated strong correlations between loudspeaker measurements and listener preferences. He has written an excellent book - Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms.
My reference noise for reproduction in a subwoofer is not natural acoustic bass but the noise that comes from a 10ton metal stamping press at the KTP plant here in my state. Thats the real everyday authority. Who cares about the sound of real unamplified instruments anyway. The group delay of an acoustic bass or cello must be really high because they are naturally ported and are also mechanically coupled to ground very poorly just like most speakers. Tom
There are university courses on every conceivable subject including creationist geology. I prefer the work of accomplished engineers such as Martin Colloms, Ben Duncan or Malcolm Hawksford who are actually engaged in the design of audio equipment. Those who can , do; those who can't, teach has some validity here. If it were so easy to quantify the requirements for accurate sound reproduction why is it so hard to do and why does good equipment sound so different? Good designers will tell you that theory is important but it is only the starting point. An over reliance on measurement is one of the root causes of most bad sound. Hawksford is an academic [as am I] but the serious work in sound reproduction is mostly being done at the practical rather than the theoretical end.
Then you might enjoy reading up and learning a bit more on the engineering of audio equipment. There are University accredited courses in the physics of applied acoustics and electrical engineering. It is not all smoke,mirrors and voodoo, as some of the journalists would have everyone believing.
I have never heard of Floyd Toole . In just what way is frequency response predictive? I will refure you to the review of the Spendor SP1 at WWW.regonaudio.com. " Many speakers look good in the anechoic test chamber or on the computer analyzer. Few of them sound good at home. And if sounding good means producing an audible facsimile of the input for a real listener in a real listening room, then most speakers do a rather poor job indeed. ". He adds: "Of course, one might hope to verify such an impression through measurement, but no one has ever been quite sure what to measure in order to evaluate performance in real living rooms. (Most of what has been passed off as "scientific" analysis has been either too crude or too biased to be of any real use or validity". Your general thesis that measurement alone is definitive of sound quality I had thought to have been abandoned years ago. Your further corollary expressed in a previous exchange that, while you had never heard the equipment I was using , logic would tell you what they sound like I find to be breathtaking in it's naïvety.
Shadorne, check out Nelson pass's 11/1/08 artical on his website entitled " Audio, Distortion and Feedback." I think he's right when he says that preference for 2nd or 3rd order harmonic distortion is listener dependent. IMHO it's not so simple to classify lower order distortion as uniformly preferable. - Jim
Thanks for the link - interesting reading. In simple terms, GedLee is saying that higher order harmonic distortion is much worse than low order distortion. This means 2nd order is better than 3rd order ...etc. etc. This jives with everything we know - Class A amps sound better at low volumes - IMD distortion is the worst - and that the "masking" effect means that we may not hear nearby frequencies to a fundamental as easily as we might hear a 9th harmonic (BAD).
This matches what Ralph has said so many times on these forums...high order odd harmonics are bad - even in relatively much smaller amounts. To take your example above, 30% second order harmonic distortion (barely or not quite audible) may be akin or equivalent to 0.3% distortion in the 9th harmonic. In that sense, an amplifier with THD of 1% all in the 9th harmonic would likely sound much worse than an amplifier with 10% THD but all in the 2nd harmonic.
One could jump on this and say that all measurements are meaningless, however, one must reflect that if an amplifier has a measured THD at full power of less than 0.004% (vanishingly small) then it will likely sound good anyway - irrespective of a GedLee higher weighting to the higher order harmonic distortion (as, be it low order or higher order, the distortion is simply very small).
Perhaps the problem (what listeners observe) begins when you hook up an amplifier to a complex load and make the poor amp send bucketloads of current to drive the woofer and then mere milli-amps to drive the delicate little tweeter. When the rubber hits the road (in the real world and not a lab test) the amp find itself being asked to perform two rather diametrically opposing tasks: extreme butterfly wings delicacy and elephant brute force. A case where IMD distortion seems inherently likely - so why does the industry stick so vehemently to this design approach? And why is GedLee largely ignored in manufacturer spec sheets?
Stanwal, I almost agree with your statement about the failure of measurements to correlate with subjective preference. I think it's fair to say that the audio industry has been measuring distortions that are easy to measure instead of measuring distortions that correlate well with human hearing. However, there has been progress made in this area recently which you are probably unaware of.
If you have access to the Audio Engineering Society's library, you might want to take a look at "Auditory Perception of Nonlinear Distortion - Theory" and "Auditory Perception of Nonlinear Distortion", both by Earl Geddes and Lydia Lee. Therein you will find described a rather complex metric that correlates well with subjective perception. The industry has largely ignored it, aside from this article from Audio Express magazine:
NO measurement or design principal has ever been shown to give a close correlation with sound quality.
I agree in the sense that no single measurement does - it is usually a combination of measurements and design principals that correlate to sound quality. Often there is more than one way to skin a cat.
BTW - The student who ran these subwoofer tests in a parking lot has been hired by Genelec - to work in their R&D department. He will have access to anechoic chambers now. I suspect the parking lot is just a way to get raw baseline comparable measurements under controlled conditions - for sure these need to be interpreted carefully. As Duke points out - a roll off is probably more desirable than a flat response to 20 Hz - due to the in room wall boundary boost effect.
Genelec is not well known to audio consumers but they have a strong following in the music recording business.
I got around to looking on the HT Shack site. I had somehow supposed this to be connected to Radio Shack. I find that it is run by Home Theater Mag. , to which I subscribe but seldom read. I had always thought it axiomatic that the requirements for home theater bass were different from, and less stringent than , music reproduction. Sub reviews by mags like Hi Fi News and Hi Fi Plus state this explicitly. I will be glad to provide citations. I was surprised to see subs apparently being tested in the middle of a parking lot. At first I thought they might be preparing an article on Hi Fi for the Homeless but then surmised that they were doing free air testing of the woofers. This mirrors the testing procedures of 30 years ago when anechoic chambers were used to test speakers. I have not seen this done in years as it became apparent that to take a speaker out of a room was to take away most of the value of the measurement. Since the REL are explicitly designed to be placed close to a wall I cannot see what their performance in a boundary less environment can tell us. The room is the most important component in any system, removing a speaker from a room does not provide an equal playing field; it will favor those designed to be placed well out into a room. In a good test of a component the measurement is done AFTER the listening test in order to explain what the panel has heard. As often as not the measurements contradict what the panel has determined, i.e., the product with the least distortion or greatest frequency range is seldom the best. This does not mean some mystic power is at work but that we have NEVER succeeded in constructing a set of rules which will tell us what makes a component sound good. Stan Curtis, who has spent a lifetime designing amps, is currently writing a series of articles about his career ,describing much of it a process of forgetting theory and trusting experience. For example, that point to point wiring is better than circuit boards and that higher quality parts make a sonic difference even when the measurement of their performance is the same. Peter Walker often remarked that he could design an amp that would look good on every measurement but which would render familiar tunes unrecognizable. 40 years ago I had memorized the Stereo Review and High Fidelity performance graphs of about every amp on the market [not hard in those days]. It was a total waste of time. A friend recently chided me for not paying enough attention to the theory behind a product we both use and like. I told him I didn't care if it worked by channeling Angels from heaven if it sounded good. The only way to determine if you will like the sound of a component is either to listen to it in your own home [by far the best] or read a test by a person or group whose ears you trust of the item in question actually in use in a home environment. NO measurement or design principal has ever been shown to give a close correlation with sound quality.
Shadorne - my bad! You are absolutely right; sealed boxes have LESS group delay than vented boxes. I was thinking that, but carelessly typed the exact opposite.
That first sentence should read:
"A sealed box will have less group delay than a vented box..."
My compliments on your restraint in not calling me the idiot I certainly appeared to be.
Um, I've sold just about every kind of sub on the market over the years, and owned a variety of both. I'd say that, overall, the more musically accurate subs I've come across where "sealed" designs. But then I've had some excellent ported designs. I would say it depends, and that you can get excellent perfomers on both sides. Sealed usually offeres a tighter more accurate response, seams like, while efficiency and output definitely seem to go to "the port". Still, I've found I can get good results with both. But, if I had to, in a music system, I'd likely lean to sealed. In an HT system, where all out output mattered, or in an undersized application in a larger room, I'd look at ported. Depends. My favorite music/ht sub thus far has been the Paradigm Servo 15, hands down, in the past. But, I've had some very expensive sealed MTS infinity subs that were stellar as well. So, a lot has to do with design considerations, I'm sure. I'm no expert speaker builder though...just an observer - lol. Um, does this help? I can't tell.
I am confused now because you say group delay is worse on sealed? You can clearly see on the links I gave how blocking the port reduces group delay - so how do you explain that? (My understanding is that the port radiates one full cycle out of phase at the tuned frequency - inevitably this implies poor group delay at the tuned frequency and can be seen on most plots on HT SHack website)
My guess is that kick drum sounds better on a sealed sub is probably again the higher group delay and possibly port ringing effects. (Port ringing may even affect mid bass quality)
Stan,
I agree with you that measurements are not everything. However I approach things differently from most - I try to start with good measurements to begin with and then listen for what sounds best to my tastes. It kind of weeds out stuff which simply sounds good but is actually adding coloration.
Have you ever had a pair of RELs in your house or did you dismiss them on the basis of some specification?
No. However I did not dismiss them - if you did go to the HT Shack website you will notice they have EXCELLENT extremely low group delay. Lord appears to design these deliberately with a very low Q and therefore they are not as efficient (play as loud with low distortion) as others but are probably much more musical sounding than many of the subs that were tested - especially on a kick drum. I suspect the REL's would work well with low Q speakers like ATC but I have not tried myself - so this is simply speculation based on the measurements. They also use Volt drivers that have a good reputation. I did not dismiss REL.
Shadorne, I don't go to web sites to compare my subs to others. I listen to them. Have you ever had a pair of RELs in your house or did you dismiss them on the basis of some specification? Those of us who have been in audio a long time have learned that there is no spec sheet or single measurement that will tell you anything definitive about the performance of any component.
A sealed box will have more group delay than a vented box because it has a 4th order rolloff below system resonance, rather than the approximately 2nd order rolloff of a venteed box. However, the audibility of group delay at low frequencies has not been not firmly established, and recent studies indicate that it is marginal at best with music rather than test tones.
On the other hand, frequency response has been well established to be audible. That's why I focus on the frequency response rather than the group delay or system damping.
The reason for the above two observations lies in the human hearing mechanism. Briefly, at low frequencies the ear is very poor at resolving timing, and much better at resolving intensity.
On another subject, note that the woofer is powered throughout its stroke; it doesn't rely upon the airspring in the cabinet to restore it to rest position.
A relevant comment by Earl Geddes: "Remember that the damping and the frequency response are one and the same thing. If I correct the frequency response then I am simultaneously correcting the damping."
The implication of Earl's statement is that we can focus on the in-room frequency response curve, and when we get it right we have also gotten the system damping right. Focusing on the system damping isn't wrong, as it will theoretically lead to the same result, but in my opinion it's easier to correct the frequency response.
Once while in an exerimental mode I built roughly comparable sealed and vented subwoofer enclosures. The enclosures were the same size and almost the same efficiency. The sealed box used a 10" woofer with a Qtc = .50, and the vented box used a 6.5" woofer and was tuned to give a roughly room-gain-complementary response.
Subjectively, the vented box went deeper and would play louder before audible distortion set in (the latter surprised me). The sealed box had better impact in kickdrum, while the vented box sounded more natural on other bass instruments.
Suspecting that the difference on kickdrum might be related to the larger, more powerful woofer in the sealed box, I switched to an 8" woofer (with a magnet system comparable to the 10" unit) for the vented box. This substantially narrowed the gap on kickdrum but did not eliminate it entirely. By now the two boxes were also approximately the same cost, as the 8" woofer plus vent was close to the same cost as the 10" woofer.
I'm not saying this was a definitive series of tests, and my blind listening panel was pretty small (one person, whose assessments I agreed with). I came away with the conclusion that the cost-no-object approach would be an equalized sealed box with very large-displacement woofers, but that the more cost-effective approach was the low-tuned vented box (which in both cases souned better on everything other than kickdrum).
To issue broad decelerations that one type of design is better than another is almost always wrong.
It is simple physics. Sealed enclosures tend to produce tight, accurate bass with a flatter frequency response curve. They are also generally the enclosure of choice when looking for sound quality. Ported designs are more efficient and give you more SPL output - they are generally the choice for higher output when sound quality is less important. I explained why above but I'll try further. More technically speaking it has to do with system Q....a ported design tends to have much higher Q (underdamped) than compared to a sealed design, again this is physics - the sealed box acts like a shock absorber and dampens the movement of the cone - this makes the woofer stop quickly when the power is removed whilst a ported design will just waffle around even when power is removed. (Of course you have very sloppy ported subwoofers with port tuned at 40 Hz and much better sounding ones like the one I showed in the link above which can be tuned at 10, 15 or 20 Hz. Nevertheless, in a sub, the ported designs are always sloppier (higher Q) than sealed - although a 10 HZ tune will have much better sound quality than a 20 Hz tune)
You cannot ruin the lower midrange with a sub if you cross it over properly and run it at the proper level
I guess it depends on your perspective. Most subwoofers typically add 20% harmonic distortion anyway. There are some measurements on REL subwoofers on the HT Shack website - so you can compare their performance to other subwoofers.
Full flared ports reduce the noise problems with ported designs. I would say both can be made into fine subwoofers. Still for the ultimate in bass for music or HT a bass horn, massive OB design or bass tower sounds best. But they are large costly why most folks purchase affordable subwoofer designs. To me the total design of a subwoofer is flawed from the start designed more for WAF and profits. Less for ultimate perfromance. I would say most loudspeaker designs also fall into this.
You cannot ruin the lower midrange with a sub if you cross it over properly and run it at the proper level. Many people expect the sub to take the place of the woofer in their main system. I use them as a SUB woofer, to add bass below the level of the woofer in the main speaker. I do not run the signal from the preamp through the sub back to the main amp. I have 4 pairs of subs , 3 are ported and they all work well. My RELs are ported and I do not know of a sealed box woofer near their price that I would consider their equal. To issue broad decelerations that one type of design is better than another is almost always wrong. Over the last 30 years I have had a number of excellent speakers and most, maybe all, were ported.
I believe that the room plays a dominant role in the bass region, and one of the things that rooms tend to do is boost the very deep bass due to boundary reinforcement. A sealed box sub generally gives better synergy with this room-acoustic effect because of its typically higher-but-more-gradual rolloff characteristic. A comparable vented box sub usually goes deeper, but the room's inherent bass-boost often results in excess deep bass energy and a bloated-sounding bass characteristic.
There are exceptions to these generalizations, so I recommend evaluating subs on a case-by-case basis.
Do the sealed subs produce a higher quality tighter controlled bass vs a more sloppy reverberating type of LFE out of the vented types?
Yes - just like most speakers - when a vent is used to extend the bass of a small box it increases the group delay and you lose accuracy. What most people are not aware of is that this ruins the lower midrange because a ported sub will "mask" other sounds that you would normally be able to hear. (this can affect sounds several octaves higher than the sub and it is due to the way hearing works)
Compare the same sub from HT Shack Subwoofer Tests sealed versus ported.
Although the ported sub plays louder with lower distortion it goes on making noise (rings resonantly for up to three extra cycles). Note how the spectral decay plot has a camel hump - be wary of any sub or speaker that does this - it will tend to give you one note bass. (unfortunately it is the most popular design as it sounds impressive and fools people into a purchase based on a stereo showroom listening test versus more accurate speakers/subs)
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