Two things get mixed in here. "Fast bass" is a subjective term meaning tight, controlled and accurate. There would be no problem if not for this getting mixed up with myths and half truths. Some people mean it literally - apparently little woofers are faster!
Mark hit the nail on the head:
I think the same is true with so many things many audiophiles are sure about because they heard it! You have to be careful how you link a subjective impression to an actual cause.
People put a sub into a room and it doesn't sound as "fast" as their full range mains. Their conclusion is that the sub is slow and subs in general are slow. I'm using those words in their subjective sense, but you might think of it as accurate vs boomy. So what is actually happening? It's probably more a matter of integration and room acoustics. When they run the sub, it ends up at a higher level. The sub merely puts a magnifying glass on all the problems in the room. Added to that, more challenges are added with a sub.
So when fast bass comes up, I think half the time we need to just recongise the use of a subjective word, and and other times we're dealing with audiophile mythology.
As far as decay is concerned, I think the room decay is far more interesting than the driver itself. Here is one interesting test. Take a nearfield measurement of a sub or woofer with REW. You can then see both response and decay plots. Now measure in the listening position. They decay is then determined by the room. Very quickly you realise the room is the limiting factor. Little woofers, big woofers - they are just a means to get there.
Mark hit the nail on the head:
I think the same is true with so many things many audiophiles are sure about because they heard it! You have to be careful how you link a subjective impression to an actual cause.
People put a sub into a room and it doesn't sound as "fast" as their full range mains. Their conclusion is that the sub is slow and subs in general are slow. I'm using those words in their subjective sense, but you might think of it as accurate vs boomy. So what is actually happening? It's probably more a matter of integration and room acoustics. When they run the sub, it ends up at a higher level. The sub merely puts a magnifying glass on all the problems in the room. Added to that, more challenges are added with a sub.
So when fast bass comes up, I think half the time we need to just recongise the use of a subjective word, and and other times we're dealing with audiophile mythology.
As far as decay is concerned, I think the room decay is far more interesting than the driver itself. Here is one interesting test. Take a nearfield measurement of a sub or woofer with REW. You can then see both response and decay plots. Now measure in the listening position. They decay is then determined by the room. Very quickly you realise the room is the limiting factor. Little woofers, big woofers - they are just a means to get there.
True, it can be used as a subjective term. And, yes, I suspect room resonances are often what is heard. Whether it's an objective property of the subwoofer or a subjective quality, what I object to is the idea that it is some property of the sub that is separate from its magnitude/phase response, and independent of the room reponse as well.
That generally comports to my experience. I think the order is right, too: room modes, integration, and then raw sub performance. For example, a system with this in-situ frequency response (5-point spatial average) sounded "slow" and "incoherent" to me:
(single subwoofer in a front corner)
The dominant factor, I think, was that 60ish Hz spike in the response.
However, a system with this in-situ frequency response (also a 5-point spatial average) sounds "locked-in" and "tight."
(three subwoofers, including the corner sub with response shown above, set up using Geddes' methods, and three low-Q, low-level global parametric EQ cuts.)
But I do think there's one more factor that hasn't been mentioned: driver inductance. In my mostly uncontrolled* subjective experience with subwoofers, I've found a pretty strong correlation between low inductance and preferred sound. Subwoofers with high ratios of Le/Re (and especially high Le/Re and no Faraday shielding in the motor) sound off to me even when the upper bass inductive hump is EQ'ed out. The adjectives that have come into my mind listening to various such subs in various systems at various times include "thick," "plodding," "incoherent," or "hazy." (I am assuming an upper cutoff of something in the 80-150Hz range.)
So, back to Todd, Mark, and anyone else who wishes to chime in, what say you as to the role of inductance in subwoofer subjective sound quality?
*I have attempted one blind, EQ'ed-to-the-best-of-my-ability comparison of bass units in closed boxes, but all of the drivers had low inductance. I failed to reliably distinguish an ultralow-inductance underhung-coil driver (Aurasound NS12-794-4A, ~0.3mH/3.0?), from a low inductance split-opposed coil (Differential Drive) driver (JBL W15GTi, ~0.9mH/3.2?), or a low inductance XBL^2 motor driver (Exodus Audio Maelstrom-X Mk. I, ~0.87mH/3.1?). Levels were limited to what could reasonably be expected of the NS12. Otherwise I'm pretty sure a good 25+ mm xmax 18 would sound better than a good ~20mm xmax 15 or a good ~15mm xmax 12! But those are all what I'd consider elite-level woofers, and all have clean performance to well above a subwoofer's typical passband, so perhaps no difference should have been expected. But I was still surprised.
regarding those inductance questions, and indeed your graph, what importance is there to the low pass frequency?
have often wondered if 'they all sound the same when eq'd the same', tho not in relation to inductance (nothing I have ever thought about) but more box Q. Do you have any experience on that question?
In that vel graph, you don't run your subs up to 200 do you? ie it is a combined sub and woofer response?
have often wondered if 'they all sound the same when eq'd the same', tho not in relation to inductance (nothing I have ever thought about) but more box Q. Do you have any experience on that question?
In that vel graph, you don't run your subs up to 200 do you? ie it is a combined sub and woofer response?
Terry, not sure if you are familiar with it but there was a little article written by Dan Wiggins of Adire audio that suggested inductance was an indicator of the "speed" of a driver, rather than moving mass.
Hi paul, if that is the one debunking the fast and slow sub 'myth', then yeah I have read it. Doesn't mean I understood it tho! haha
nah, was just jumping on the bit where DS said 'could not tell them apart when eq'd'..I have wondered since eq has become so common if it also kinda threw out the box volume and Q that used to be calculated???
As an example, I am pretty sure my 18's are in a small box, but I don't bother too much as I just make them do what I want if you follow.
So that is the question, say a given driver in two very different sized boxes (ie very different Q), eq'd the same at the LP, what perceived difference??
I would say that I'm in the 'sounds the same if eq'ed the same' camp. As far as I know, the effects (talking small signal domain) are all minimum phase. Possible exception to that might be in the combined response in the crossover region, where theoritically you can get a non-min-phase response. I frankly suspect that most the effects are due to pre-existing biases. It is very common to have pre-exisitng biases, then when you do hear a difference (due to any number of possible causes), one tends to interpret the difference in a way that supports the pre-existing bias.
Hopefull I haven't offended anyone, and I'm certainly open to be proven wrong.
Hopefull I haven't offended anyone, and I'm certainly open to be proven wrong.
It strikes me as pretty important. Many subs can't play cleanly high enough to really help randomize modes in the modal region.
For a decade, until a couple years ago, I used to build everything to low (.5-.6) Q. Now I don't care. I build a sub to fit the space in which I want to put it, and EQ to taste. (With good parametric EQ having gotten so cheap - a miniDSP-in-a-Box is just $160 shipped to the U.S. for a tiny DSP that has enough flexibility to do basically whatever one wants to sculpt the level/delay-phase/FR of 4 subs. - there's no reason for anyone serious about music reproduction NOT to use it or another of the modern DSP's down low.
I changed my ways after setting up a system for a friend that sounded shockingly good despite the high Q of every sub (it was a JBL W15GTi in about 2.5 cubic feet in the front corner and two closed "too small" boxes with either Peerless SLS10's or SLS12's, arrayed per Geddes, and the Behringer parametric EQ for level/delay/FR shaping.)
I did a crude test (not blind, because I only had one woofer and one box) with the sub I was then using in my nearfield/computer system. (JBL W15GTi in a ~4.5 cubic foot closed box, powered and EQ'ed by a Crown XTi2000 amp. Harman, represent!
) Basically, I stuck books inside the cabinet, measured and EQ'ed for flat response, listened, and repeated until it sounded off. (My expectation was that it would take maybe 3 books!) IIRC, It wasn't until the box got somewhere below 2 cubic feet, with a modeled Q of well over 1, that it first sounded bad. And even then, I'm not sure if it was the driver, or the amp running out of steam down low.I'm not quite sure how representative that test is, though. If you've seen a JBL W15GTi you'll know that there are few if any woofers more throughly designed to combat power compression, between its Differential Drive motor with split 3" voicecoils and all that heatsinking. Intuitively, the problem with getting bass in small boxes is heat, because box size determines low-end efficiency.
The subs are lowpassed at 120Hz. (You can see the start mains' floor bounce, which is centered at ~240Hz.) The mains in that particular system are very small, so they are also highpassed at 120Hz. (I do think running mains full-range to provide three more pressure sources in the modal region is a good idea, it has improved things for me before, and I will do it in the future.)
thanks Todd, and I don't see how anyone could be offended! It is/was a question I personally don't know the answer to, and in any case i just eq it to taste and leave it at that.
Thanks DS. The one 'flaw' I have found with using the vel graph (on a mates system, have no other experience than that) was that it only went up to 200. I don't recall the exact details, but it was giving us 'funny' results, and it was only till we ran an REW sweep (that went past the cutoff point of the vel graph) that we could see what was going on.
Anyway, that limit of 200 in that particular circumstance was giving us grief. Interesting tidbit is all.
Thanks for the rest of the answer.
Thanks DS. The one 'flaw' I have found with using the vel graph (on a mates system, have no other experience than that) was that it only went up to 200. I don't recall the exact details, but it was giving us 'funny' results, and it was only till we ran an REW sweep (that went past the cutoff point of the vel graph) that we could see what was going on.
Anyway, that limit of 200 in that particular circumstance was giving us grief. Interesting tidbit is all.
Thanks for the rest of the answer.
A bit too much maths for my liking! I never really accepted or rejected the idea, just put it down to one theory on the subject. That one sits in my "I don't know box." It's a pretty big box and it has a habit of becoming bigger as I learn more.
Importance of box Q. That is an interesting one. Another one for the "I don't know" box. One thing I tested in the past the relates to Q is open baffle vs sealed subs. I found that the old Stryke (now acoustic elegance) AV12 sounded much better in a dipole open baffle H frame. It was tighter, admittedly only went down to 30 Hz, but enough for music. Open baffle subs are low Q because the Q isn't raised by the box, so it comes down to the Q of the driver in free air. Now later I tested Rythmik servo subs in sealed vs open baffle. With EQ to match I could not pick the difference. The Rythmik sub has a Q of 0.3 in a sealed box the right size. It is one of the traits of the servo, because the driver has a Q of 0.5 which is quite high, but the servo adds damping. It's a little unproven hypothesis of mine that the low Q of the servo sub has a part to play in sounding the same as an open baffle version. I could be wrong about that, it might not be a cause and effect relationship - you have to be a bit more rigorous to find that out. In this case it doesn't matter, I had no reason to stick with the open baffle sub, the sealed box hits with a much bigger stick!
Can't see anything that might offend.
Back on the topic of Q, Brian Ding at Rythmik has done some measurements of the effects of damping. The idea being that the Q of the system represents not just the roll-off and transient response characteristics but also the ability of the driver to resist the impact of internal dynamic box pressure. In other words, sound waves inside the box are trying to get out of the box, pressing on the walls and the driver. People focus a lot on making the box solid to resist that, but often don't think about the driver itself. It is made to be compliant, to respond to the motor and move! The box pressure is not static like we often think, but it is made up of sound waves that work against the driver. Could it be that this is more significant than the box itself? That the easiest path for bass to get out is through re-radiating through moving the driver in a way that isn't intended? This is similar to driver EMF where the damping factor of the amp resists the driver and amp getting caught in a feedback loop. Damping factor prevents it happening. Likewise, the system Q prevents the driver re-radiating the bass from within the box, but it's not black and white. Some will leak through by modifying the motion of the cone. The question is not how much if it happens, because that is beyond question - it can't not occur. The question is how significant it is, how audible? And does it change significantly where the Q is changed?
Those are rhetorical questions, unless someone knows of research or has investigated it.
Higher-Q systems tend to "ring" more, causing muddy sound. The servo controls the ringing to yield a "faster" design. My discussions with Brian lead me to believe we are in agreement about any aspects of subwoofer design (thus leading to my pair of Rythmik's). IIRC, the speaker system has mechanical and acoustic Q, at least so says my old grad acoustics text. It gets complicated since we have the sonic pressure waves and effect of air pressurization in the box... About as much as I can recall, not having piddled with speaker design in years. I have noted the general level of DIY folk's knowledge has risen immensely since the 70's and 80's when I last played at speaker design. Proves how little I know about that aspect of audio. - Don
Qts (total Q of a driver) is related to Qms (mechanical) and Qes (electrical). The older Rythmik drivers had a Qts of 0.5 which is high, but somehow the servo adds extra damping. Also the box size combined with driver Qts would normally create the system Q. Normally you would size a box for a Q of 0.707 and if you want a critically damped sealed box you make it bigger so you get 0.5. Infinite baffle subs are very low Q and some believe that is part of their sound quality.
All this can be measured, the tricky part is relating things to what you actually hear, there are always so many variables and they are not always easy to control.
I suppose you could take a driver, put it in different sized boxes then measure its time domain performance nearfield. You could then put it in a room and look at decay plots. Does the room then mask all differences? Most of the time, I'd say probably. When people split hairs over 30ms more group delay in an actual room, that gets interesting. That is nothing compared to modal ringing.
All this can be measured, the tricky part is relating things to what you actually hear, there are always so many variables and they are not always easy to control.
I suppose you could take a driver, put it in different sized boxes then measure its time domain performance nearfield. You could then put it in a room and look at decay plots. Does the room then mask all differences? Most of the time, I'd say probably. When people split hairs over 30ms more group delay in an actual room, that gets interesting. That is nothing compared to modal ringing.
Ah yes, infinity is often difficult to achieve in practice...
Thank you for all your comments, Paul, very helpful! As for group delay, I think the problem there is more one of first arrival time rather than all the mess that happens later. At roughly 1 ms per foot, a 30 ms offset could move the apparent location of your sub quite a bit... I agree the ringing is likely what distinguishes good and bad subs (and rooms). I've decided that on Wednesdays I will not debate the audibility of things.
Thank you for all your comments, Paul, very helpful! As for group delay, I think the problem there is more one of first arrival time rather than all the mess that happens later. At roughly 1 ms per foot, a 30 ms offset could move the apparent location of your sub quite a bit... I agree the ringing is likely what distinguishes good and bad subs (and rooms). I've decided that on Wednesdays I will not debate the audibility of things.
Mark Seaton
WBF Technical Expert (Speaker & Acoustics)
I applaud you and wish more were willing to acknowledge the unknowns or uncertainties. Generally the more you study any subject, the more factors you come to question or grow suspect of. That doesn't mean you can't have various hypotheses, but hypothesis does not equal fact. Many enthusiasts would be amazed or mortified by how many things product designers and other professionals wonder about and would like to look further into. The lazier consumers want absolutes that only exist at the lips of a gifted salesman.
I would suggest that simple system Q, even for a sealed box, is insufficient to define the system and frequency response, especially with modern components and systems as we use them. Looking just at Q will lead you to skim past many other important factors.
My own opinion is that so far as the audibility of differences in subwoofers themselves, ie within the same listening environment, come down to the exact frequency response, changes in response with level, distortion audibility and the change in distortion with level. To be clear, that can make for a nearly infinite range of possible combinations, some being rather similar in sound with others being dramatically different.
Reflected energy inside the box only is a discrete issue when the reflections are discrete, which requires the distances to be fractions to whole wavelengths (this is a concern at midbass and midrange frequencies in full range speakers). Once the box is less than about 1/8th wavelength in dimension, you're just modulating the pressure. The reactionary pressure of the air inside the box is the exact behavior we are counting on. Until boxes get rather large, discrete reflections simply don't fit inside the box.
Heavy discussion of driver Q is interesting if you don't have any electronic response adjustments to use on the subwoofer. Once you allow for adjustments electronically, the response shapes become infinite, and even without we often forget that most modern woofers have significant enough inductance to significantly and audibly skew the shape of the frequency response.
Once we get back to the reality of a real room, we have to remember that if you don't have any energy down low, you can't excite any offensive room modes. Similarly, adding too much low frequency energy to a system can most certainly produce subjective descriptions of heavy, sluggish, muddied or inarticulate.
^^^ Good point about the room. I keep seeing the trees and not the forest by focusing on just the sub, when in a room other things may dominate the (character, or timbre of the) sound. - Don
p.s. Bring on the math!
p.s. Bring on the math!
I applaud you and wish more were willing to acknowledge the unknowns or uncertainties. Generally the more you study any subject, the more factors you come to question or grow suspect of. That doesn't mean you can't have various hypotheses, but hypothesis does not equal fact. Many enthusiasts would be amazed or mortified by how many things product designers and other professionals wonder about and would like to look further into. The lazier consumers want absolutes that only exist at the lips of a gifted salesman.
+1 googol ...
Frank
From what I am reading here, Infinite baffle subwoofers looks more and more appealing (always was to me) .. Implementation is a problem for me right now ...
One of the most serious problems with IB subwoofers is that they are kinda hard to relocate for better bass performance. No joke, one of my friends with a massive IB sub with 4 very robust 18 inchers ended up with a major hole @ 60 Hz at exactly his favorite listening position.
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