Why do subs sound bloated or slow?

Hi Bryon,

That's what I figured, but hey!? :o)

Thanks a bunch!

Telescope - The speed at which electricity travels through a wire is so fast that it makes differences in cable lengths inaudible, since the high speed makes any differences in latency (elapsed time) vanishingly small.

So you have nothing to worry about.

Bryon

Was wondering with all this talk about time alignment
of subs with the mains, are their any concerns say if
your running your sub & speakers both off of the speaker
level connections of your amp, and the speaker cables are 2 feet longer than the sub speaker cables, are there any issues
with alignment this way?? Probably a dumb question but!?!?!?

04-21-11: Dbphd
One of the most reliable phenomena in psychoacoustics is what is know as the masking level difference (MLD). Present a mid-freuquncy sinusoid in correlated noise to both ears and adjust the level until it becomes inaudible; flip the phase of the sinusoid in one ear, and the signal pops up as much as 15 dB, depending on frequency.

Dbphd - My understanding is that BMLD is a measure of the difference between the thresholds of detection for…

1. Auditory stimuli in which the signal and noise are the same phase and level.

...and…

2. Auditory stimuli in which the signal and noise are different in phase and/or level.

It’s unclear to me how the existence of BMLD affects my observations about subwoofer time alignment. Maybe you can elaborate.

At the time, the data suggested the auditory system doesn't preserve timing as such up the neural chain, but may convey such information by the more central areas excited.

Again, it’s unclear to me how this bears on my observations about subwoofer time alignment. I’m not saying it doesn’t. I just don’t see the connection. Maybe you can say more. Having said that, I do have a few thoughts...

As you are probably aware, there has been a great deal of neuroscientific research over the past 20 years using fMRI and PET scans. Much of that research has been directed at correlating various brain regions with perceptual, linguistic, and motor abilities/deficits. A significant amount of that research has been devoted to auditory perception, including the brain regions that correlate with temporal processing and temporal resolution.

I’m under the impression that the neuroscientific research on temporal resolution has discovered a number of neural correlates for auditory temporal resolution, corroborating earlier measurements of temporal resolution obtained through psychoacoutic experiments (e.g. gap detection).

Here is an excerpt from an article in the journal Cerebral Cortex:

We used positron emission tomography to examine the response of human auditory cortex to spectral and temporal variation…Results indicated that (i) the core auditory cortex in both hemispheres responded to temporal variation, while the anterior superior temporal areas bilaterally responded to the spectral variation; and (ii) responses to the temporal features were weighted towards the left, while responses to the spectral features were weighted towards the right. These findings confirm the specialization of the left-hemisphere auditory cortex for rapid temporal processing, and indicate that core areas are especially involved in these processes.

Here is an excerpt from a paper by researchers at UCI and University of Toronto:

Our findings of M100 modulation by the shortest gap (2 ms) tested are also in good accord with animal studies of auditory cortical temporal acuity, where gap detection thresholds have been measured using electrophysiological methods to record activity in single or cluster units. A key result of those studies is that the firing patterns of neurons in auditory cortex reflect minimum detectable gap thresholds that are similar in scale (at 2-10 ms) to thresholds measured psychophysically in human [4, 5, 13]. Our MEG findings reported here provide evidence for a similar level of temporal resolution to brief (2ms) discontinuities in sounds in the synchronized neural response of tens of thousands of neurons in secondary auditory cortical fields [10], reflecting neural response properties at the population level in auditory cortex.

Research like this leaves me with the impression that human temporal resolution is highly sensitive, not only when measured psychoacoustically, but also when measured neurologically.

Of course, it does not *necessarily* follow that subwoofer time alignment is audible. But it does seem to indicate that my statements about subwoofer time alignment are not *disproved* by the current state of neuroscientific research.

Bryon

I rarely post on tweaks, but I just discovered one that is relevant to this thread: I placed 3 Black Diamond Racing Jumbo Pucks under my sub, which is resting on a three inch maple platform. This, in effect, replaced the soft feet of the sub with extremely hard feet. The result: An audible increase in bass articulation.

FWIW.

Bryon

I agree with Marty that, for the greatest control over subwoofer integration, you will need some way of measuring and correcting the frequency response of the system. There are both hardware and software approaches to measurement/correction. Both can yield excellent results. Personally, I use a software approach to measurement, namely Room EQ Wizard, and a hardware approach to correction, namely Meridian Room Correction.

IME, the ability to measure and correct the system's frequency response dramatically increases your control over sub integration. But if your system doesn't allow for this, the techniques I described in my previous post will get you a good part of the way toward excellent bass.

Good luck.

Bryon

Personally, I'd recommend an RTA unit, preferably with on-board PEq.

Adjust the PEq and phase for flat on-axis FR at and around your usual listening position. There may be some variation as you move the mic, but -in my case- there was little since my room is set up for solo listening and "window" is narrow.

Screw around a bit with the PEq bands (i.e. use them as tone controls) and/or placement - changing one variable at a time - and try to note all correlations between audible results and the shape of the RTA's FR output. If you have control of the sub's Q, adjust that to different settings as well.

Eventually, you'll figure out what the best sound "looks" like on the readout, and you'll be able to re-locate the subs and dial in a close replica of your preferred response very quickly. Tweak from there by ear.

For me, the best SQ is yielded by flat (or flat to +/- 35hz with slighly and gently rising output below) with subwoofer Q set for medium damping. This has been pretty much true, regardless of where the subs have been physically placed.

I find the SQ remarkably consistent (and remarkably good) when using my Rythmiks/NHT and Velo SMS-1 when dialed in with this technique, which basically holds on-axis FR constant. I would, however, be the first to acknowledge that, for other listeners, differences may be more audible.

Marty

BIF - IME, the two simplest and most effective techniques for subwoofer placement that require no equipment other than a SPL meter are:

1. Optimizing frequency response by placing the sub at the listening position, then walking around the room and listening for the location where the bass is *consistent* across low frequencies. You can do this by ear with music, or you can use a SPL meter and low frequency test tones. Once you find the location in the room that has the smoothest frequency response, you place the sub in that location. Now the bass response at the listening position should be in pretty good shape.

2. Optimizing time alignment by maximizing constructive interference at the crossover frequency. This technique was described by Cbw in his post on 1/27:

You flip the polarity of either your mains or your subs (whichever is easier), then play a tone at the crossover frequency (e.g., and 80 Hz tone for an 80 Hz crossover). You then adjust the delay (or sub position) to *minimize* the SPL at the listening position. (You're effectively maximizing destructive interference.) When you're done, you flip the polarity back to normal, and you should be very close to time aligned.

Both of these techniques are simple and highly effective. Unfortunately, IME, they don't always give you the same answer about the best location for the sub (which is the point I've been trying to make throughout the thread). So, if I were you, I would start with the first technique to find a general location for the sub, then use the second technique to refine the placement.

Hope that helps.

Bryon

Marty and CBW: Interesting posts. On Jan 23rd, I mentioned above:

"I . . . run two channel fronts and I have a Paradigm Signature Servo subwoofer. This Paradigm sub has its own Class D amp to power the woofer and three controls knobs to control phase, cut-off and loudness.

Because I only have one sub, I also bought a Paradigm X-30 crossover unit. The X-30 has the same controls and also allows me to combine left and right channels without shorting my pre-amp Mains. I feed my main fronts off the preamp's Main 1 output to the power amp. I run my fronts straight out. I feed the X-30 off of the pre amp's Main 2. output As stated, I use the X-30 to combine the two channels into one channel for the subwoofer.

My front speakers are Paradigm Signature 8 v2. The fronts roll off at roughy 35-40HZ. Of course room placement also affects base contour. However, as a general matter, I cross the sub over at 40-50HZ. Based on my ear, that sounds about right. Too high a cross-over and the bass sounds boomy. Too low, and bass is thin.

I also adjust phase by ear. I listen for the best sounding bass response. Loudness often varies by source material, but as I got used to working with the sub controls, I don't move the adjustment controls around too much."

Does this sound like a reasonable approach to adjusting the subwoofer? Also, as mentioned, I can adjust phase from 0 to 180 degrees. My sub is placed about 6 inches behind the fronts. For some reason, it seems like a 40 60 degree phase adjsutment sounds best to my ear.

Would you guys explain the relationship of (a) phase degree adjustments (i.e., 0 to 180 degree) to (b) coplanar placement and (c) time coherence??? My sense is that in a perfect world, if the sub and mains are placed equidistant on a fron plane with the mains from the listener, there is no need for a phase adjustment. However, if the sub is moved forward or behind the mains, or placed along a different wall, then a phase adjustment is needed.

One other complication: As stated, I can adjust phase, cut-off and loudness from BOTH the sub and the X-30 control box. Because it is more convenient to use the X-30 control box, I set the sub controls at zero degrees for phase; 3/4 gain for loudness; and 180 HZ (max) for crossover cut-off. However, as stated, I use the X-30 box to control the foregoing adjustments. Phase: 30-40 degrees; Crossver 35-40 HZ; Loudness -- whatever sounds about right, less is generally more. Does this approach make sense to you??

Thanks for your advice. BIF

Cbw,

Good point re: time coherence and co-planar siting, and an interesting test excersize that makes some sense. Never tried it, but - if I get the energy - I will.

I spent well over a year screwing around with my subs daily (during a long early retirement/extended unemployment stretch) which allowed for pretty much unlimited indulgement (other than wife imposed constraints) in such activities. Now that I'm working again, time constraints are a limit, but the idea is appealling and I hope to give it a go at some point.

Marty, you mention in your response that you had your speakers/subs coplanar. That might not actually result in optimal time alignment depending on a number of factors. In particular, if there is EQ on the subs, that can delay the signal by anywhere from one to several milliseconds. That means your subs might need to be one to several feet closer to your listening position than your speakers for proper time alignment. If you have a means to delay your mains, that's easiest, but otherwise you're stuck moving the subs. There may be other sources of delay, too. But even if there is no delay, the subs may need to be a bit forward or back from the mains. In my case, the optimal position put the subs an inch or so in front of my mains.

Some people may already know this, but there's a neat trick for getting your time alignment close: You flip the polarity of either your mains or your subs (whichever is easier), then play a tone at the crossover frequency (e.g., and 80 Hz tone for an 80 Hz crossover). You then adjust the delay (or sub position) to *minimize* the SPL at the listening position. (You're effectively maximizing destructive interference.) When you're done, you flip the polarity back to normal, and you should be very close to time aligned. From there you can listen to a music passage that spans the crossover frequency and move the subs an inch or so at a time and see what sounds best. If you've moved your subs a lot from their original position, you should probably re-EQ before this last step.

It's a fun Saturday project, even if you can't possibly leave the subs in their final position. And you can let your ears tell you if time alignment matters for your system. In my case, it was a huge difference. As Marty says, YMMV.

Bryon,

Certainly no need to apologize - particularly since there is a clean chain of logic to your theory. I've just found that, in my experience, varying the listener to sub distance doesn't produce any audible issues TO MY EAR, provided that FR is appropriately EQ'd. Of course, that last bit about MY EAR always requires the YMMV disclaimer.
It is always possible that other listeners are more sensitive to the phenomenon than I am, so my responses should be treated as a single data point in the discussion, and that is the purpose for which they were intended.

Marty

My bottom line is that when I optimize my subs for smoothest FR in a co-planar arrangement it sounds no better (or worse) than when the subs are FR optimized for placement 2 1/2 feet behind the mains...I'm pretty confident (can't be 100% sure, though) that no one would characterize the bass that I'm currently producing as either bloated or slow... So, my experience just runs counter to your theory.

I don't doubt your experiences, Marty, and I thank you for sharing them. FWIW, the theory I proposed in the OP wasn't intended to be an exhaustive account of the variables that determine good subwoofer integration. I have no doubt that there are many things I have failed to consider. In fact, that was the principal reason I started the thread - to explore some of those things with people who, like me, use subs in their 2 channel systems.

And FWIW, I'm not trying to convert anyone to a theory that I only tentatively believe. I apologize if I appear that way.

Bryon

Instruments that produce very low frequency sound are "bloated and slow". A 30 foot organ pipe, for example, takes a while to develop its full output. The typical subwoofer is "faster" than the sounds it reproduces, which is as it should be.

My bottom line is that when I optimize my subs for smoothest FR in a co-planar arrangement it sounds no better (or worse) than when the subs are FR optimized for placement 2 1/2 feet behind the mains. It does, however, require significantly more EQ to get to the same place, so I stuck with the flush to the wall sub placement.

I'm pretty confident (can't be 100% sure, though) that no one would characterize the bass that I'm currently producing as either bloated or slow - and I do have room (via the adjustable Q on the Rythmiks) to skinny it down further if it did sound that way to someone.

So, my experience just runs counter to your theory. OTOH, while I'm extremely happy with the character, extention, and integration of the bass I'm getting from my subs, any of you folks might adjudge it differently.

The ol' YMMV.

Marty

Is there a moratorium on serious conversation?

Bryon

This is getting way too serious.

01-25-11: Cbw723
Bryon, while I agree with much of what you say, I think room interactions are secondary. The perceived coherence of the signal is going to be primarily determined by the direct waves from the speaker and sub to the ear.

I'm not sure I agree with this point. I agree with Josh358 that the "perceived coherence" of the signal is significantly affected both by the direct sound and the indirect sound, and my suspicion is that the indirect sound is even more relevant, since the indirect sound is mediated by room modes, which are highly audible. But this is not a major source of disagreement. What IS a major source of disagreement is the importance (or lack thereof) of transient response at low frequencies, and hence the importance (or lack thereof) of time alignment at low frequencies.

It just occurred to me that there is a plausible argument about the importance of time alignment at low frequencies that has nothing to do with with improving transient response, but only with improving frequency response, namely:

Time alignment between the sub and the mains improves low frequency response by minimizing the patterns of constructive and destructive interference around the crossover frequency.

An explanation of why this is so can be read here (see the second section entitled "Timing is Everything"). For reasons of improving frequency response, some manufacturers of subs explicitly advocate the time alignment of the sub with the mains. For example, Rythmik Audio says:

In order to get the most out of your subwoofer, it is critical that it is correctly integrated with the rest of the system...

...The simple method is to compensate by changing the speaker distance setting on your receiver. Bass management in HT receivers has a speaker distance adjustment which process the signal on digital domain. If one puts distance of the sub x feet further away than its physical distance relative to other speakers, the HT receiver will put out the signal to the sub x/1000 sec before it puts out signals to other channels. That essentially puts a negative delay on the sub which can be used to reduces the "phase lag" on the sub and therefore reduces the phase difference between the sub and the front speakers. This trick enables us to use the speaker distance as a tool for phase adjustment between subwoofer and front speakers.

The point of all this is that there may be compelling reasons to time align a sub with the mains that have nothing to do with transient response, but only with frequency response.

Bryon

What room interaction does, I think, is produce boomy, one-note bass that is distorted in both the time and frequency domains. Equalization can help with the frequency domain problem, and at the same time, reduce the amplitude of the smear. But room treatment or dynamic equalization is required to reduce its duration.

To make matters worse, bass modes can shift the pitch of the note.

Still, that doesn't really explain why vented subs typically sound mushier than acoustic suspension ones, and acoustic suspension woofers mushier than planars. Cone breakup and nonlinear distortion may have something to do with this, the harmonic distortion on a subwoofer can approach 10%. Cabinet resonances could also play a role. The role of group delay in a vented sub is perhaps more controversial, since as someone pointed out small amounts of group delay don't seem to be audible on musical material, and also since the delay caused by resonances is much greater than the delay introduced by the vent.

Finally, many subs, particularly ported ones, aren't designed for flat response. They're designed to make the loudest thump possible to please people who want to reproduce explosions and dinosaur stampedes. Or to produce lots of bass on the cheap for people who don't understand the difference between loud and deep.

Still, at the end of the day, I haven't seen a truly rigorous explanation of the phenomenon of perceived woofer speed.

Shadorne, the flaw in your logic is that a movement of my head is not the same thing as an equivalent movement of the sub. I'm arguing that it is important to get the correct time alignment between the subs and the mains, thus the important parameter is the position of the subs *relative* to the mains. Small movements of the sub relative to the mains will affect how the sound combines in their region of overlapping frequencies. If the speaker and the sub are close together (or on the same axis, anyway), even large movements of my head will not affect their relative distances to my ears (only the absolute distance), and thus will not change their time alignment.

This is, in fact, an argument for keeping your subs close to your mains: if the sub is placed off-axis, then small movements of the head will affect the relative distance of the mains and the subs to the ears, altering the coherence of the signal.

Bryon, while I agree with much of what you say, I think room interactions are secondary. The perceived coherence of the signal is going to be primarily determined by the direct waves from the speaker and sub to the ear. Room interactions may be stimulated to a greater or lesser degree depending on the coherence of the signal but, as you point out later in your post, the effects of small changes in position or delay don't seem to show up prominently in plots of frequency response.

Not that this benefits this discussion, but I feel the need to thorow out a peeve of mine when discussing "subs" - this supposed magical frequency of 80 Hz. IMHO if you are using 80 Hz as a crossover point you're already off to a bad start with regard to integration. Again, IMHO it is better to cross over the sub at 70% of the -3 dB point of your main speakers and tweak from there. Then worry about level and phase. The wavelengths of sub bass frequencies vs. room size are too mismatched to worry about time alignment. Check out the design of the Vandy 5 line. Richard give a good explanation (and he's fanatical about time and phase alignment).

01-25-11: Martykl
Small movements of the source of deep bass can audibly affect the perceived sound, but once again I'd be inclined to attribute this to room interactions (rather than increased group delay)...

Marty - You, I, and Cbw are all in agreement that small changes in sub placement can result in significant audible changes at the listening position. Where we differ is what we attribute those audible changes to. You attribute them to differences in room interactions, by which you may or may not mean frequency response. Cbw and I attribute them to differences in time alignment, and hence transient response.

Cbw suggested a reason to doubt whether the audible differences at the listening position resulting from small changes in sub placement are attributable to room interactions:

Marty, I don't think the effect I described can be reduced to room interaction because, as I said in my earlier post, I can achieve the same result by adjusting the delay of the mains relative to the subs without physically moving anything.

I have experienced the exact same thing. That is to say, I can produce significant audible changes at the listening position by changing the digital delay on my sub, without altering sub placement.

Having said that, I have to acknowledge that changing the delay on the sub DOES change the "room interactions" insofar as it affects the patterns of constructive and destructive interference between the sub and the mains, which are mediated by the size and shape of THE ROOM. So changing the delay on the sub does change its room interactions. Hence it might be reasonable to conclude that differences in room interactions explain the audible changes at the listening position.

But even if this is true, it still leaves open the question of whether those audible changes at the listening position are attributable to frequency response or transient response. The reason is because constructive and destructive interference, while being forms of "room interaction," alter BOTH the frequency response AND the transient response of the system. Because of this, concluding that the audible changes at the listening position are attributable to room interactions does not definitively answer the question of whether those audible changes are attributable to differences in frequency response or differences in transient response or both.

In my view, the way to determine whether the audible changes at the listening position resulting from small changes in sub placement are attributable to frequency response or transient response is hold one constant, alter the other, and listen for changes. In my reply to Audiokinesis above, I reported just that kind of finding. That is to say, I have had many experiences in which I have moved the sub on the z axis by a small amount and produced significant audible changes at the listening position, EVEN WHEN the measurable frequency response remained the same for both sub placements. It was precisely those experiences that led me to the conclusion that the audible changes at the listening position were attributable to changes in transient response.

Bryon

Cbw723,

You need to investigate further. If you can hear the difference caused by a time delay of 1 inch (roughly 0.1 msec) in a subwoofer then there is something seriously wrong. 1 inch is less than the distance between your ears - it is of the order of a very slight movement of your head.

Do you have a tool to ensure your head is within 1 inch of teh optimal positioning while listening?

Quite simply the highest frequencies of a subwoofer are normally around 100 Hz (max) before there is a steep roll off and this corresponds to a wavelength of around 10 feet.

Your observation suggests that your subwoofer is putting out significant sound at above 1 KHz - for example at 3 Khz a 1 inch movement can make a large difference on a pure tone (although it would very unlikely to be audible with music).

Scientists seem to agree that 1 to 2 milliseconds is the threshold of hearing a delay in sound - and this will occur at upper midrange frequencies and not at subwoofer frequencies which are omnidirectional to the ear/brain.

In essence, what you report suggests something else is going on.

01-25-11: Martykl
There's an element of "When did you stop beating your wife?" to this discussion. It starts with the presumption of guilt (for subwoofers) and asks for proof of innocence.

Hi Marty - I regret the title that I chose for this thread for a variety of reasons, including the one you mention. It is was not my intention to imply that subs ALWAYS sound bloated or slow, as I mentioned in a post the same day I started the thread:

In the OP, I wasn't trying to suggest that all subs sound bloated or slow. I was trying to identify some reasons, WHEN subs do sound bloated or slow, WHY that is the case.

The principal reason I identified in the OP is a conclusion that I arrived at after extensive experimentation in my system and others, namely that optimizing frequency response often comes at the expense of optimizing transient response, since efforts to optimize frequency response usually include non-coplanar placement or EQ, both of which disturb the sub's time alignment with the mains, and hence the transient response of the system.

The use of the of the terms "bloated" and "slow" were not intended to be derisive, but rather descriptive. It seems to me that room modes often result in non-flat frequency response that can be described as "bloated," and that time misaligned subs often sound out of sync with the mains, which can be described as "slow." Having said that, I quite agree that bloated or slow bass is not a problem unique to systems with subs.

But what IS unique to systems with subs, IMO, is the "zero sum game" that results from scenarios in which efforts to optimize the system's frequency response disturb the system's transient response, and vice versa. The reason this is unique to systems with subs is that, for systems without subs, the time alignment of the system is determined almost entirely by the manufacturer, not the end user, for the simple reason that the woofers are physically attached to the same cabinet as the other drivers. For systems with subs, frequency response and transient response (for bass) are independently controllable parameters. Hence systems with subs have unique challenges, including, IME, the challenge of optimizing both frequency response and transient response. That was the central idea in the OP, and the idea I've been advancing throughout the thread.

Part of the confusion here might have been avoided if A'gon didn't limit my initial post to a little over 100 words. I have no idea why that is done, and I have seen many other threads in which the OP was far more than 100 words.

FWIW, I use a sub in my 2 channel system, and it sounds neither "bloated" nor "slow." But it took me literally years of periodic experimentation to arrive at that point. The reason, I believe, is the "zero sum game" I've been trying to describe.

Bryon

Number one reason why subs, or for that matter any speaker producing frequencies below 80 Hz, sound bloated is the room. Adding a second sub will substantially help in reducing room mode peaks and nulls. That's why some people swear by using a full range pair instead of satellites and sub. Corner placement of a single sub is a guaranteed formula for exciting all your room modes to the max. It's impossible to hear whether or not the sub is doing its job correctly when the room is booming.

Marty, I don't think the effect I described can be reduced to room interaction because, as I said in my earlier post, I can achieve the same result by adjusting the delay of the mains relative to the subs without physically moving anything.

I should note that I didn't mean to be dismissive of Bryon's initial observation which (I assume) is paraphrased as:

"If you're gonna fix the room induced FR problems by separating the source of bass from the rest of the spectrum and moving it closer to the wall (in the form of a sub), you're gonna induce time domain issues."

Assuming that this was the original point (and I apologize if I've mischaracterized it), the issue I have lies not in the general idea, but in the use of the terms "slow", "bloated", and "zero sum".

Even if you concede that gains in FR come at the expense of issue in the time domain (and those using even cheap HTRs needn't necessarily concede this as these units generally compensate electrically for the difference in physical distance), that does not mean that these issues:

A: will be perceived as slow and bloated for time domain problems

and/or

B: will audibly cancel each other out (zero sum).

Indeed, I personally find the former orders of magnitude more audibly troubling than the latter. And I also believe that any audible effects in the time domain can be largely mitigated through careful set-up. In sum, I'd say that, in a well set-up subwoofer system, the benefits in FR obliterate any costs in the time domain.

But, as noted, that is me, personally, and I understand that others may reach a different conclusion.

Marty

Stringreen, sub level is certainly important for proper integration, but turning down the level on a misaligned sub won't solve the problem, it will just make it less noticeable (along with the bass).

Shadorne, I disagree "that you can't perceive accurately what is happening to the bass response except by what it 'masks' in the higher frequencies." If you consider a single frequency around the crossover frequency (say, 80 Hz), it is being played by both the mains and the sub(s). Those outputs will either sum properly, or not, and you can easily hear very small changes in delay. (As an experiment, try playing a constant tone and turning the phase knob on a properly-aligned sub while watching an SPL meter.)

But real signals are the sum of a wide range of frequencies, and the overlap between subs and mains is also across a range of frequencies. Properly aligned, the waveforms will sum together properly, but when misaligned they will smear out the low-frequency information. The effect is plainly audible, and can't be attributed solely to masking.

One more thought re: cbw's observation that his subs are sentitive to movements of 1 inch. That would be a little extreme IME, but his point is taken. Small movements of the source of deep bass can audibly affect the perceived sound, but once again I'd be inclined to attribute this to room interactions (rather than increased group delay) and note that it, too, is not unique to subwoofers.

Marty

"There's an element of "When did you stop beating your wife?" to this discussion. It starts with the presumption of guilt (for subwoofers) and asks for proof of innocence."

Yes, good point!

Subs are not more problematic in general than most other ingredients used to produce good sound.

They can be done well or not just as easily.

There's an element of "When did you stop beating your wife?" to this discussion. It starts with the presumption of guilt (for subwoofers) and asks for proof of innocence.

I can (and have) set up subs that sound somewhat overdamped - producing a lean sounding tonal balance (to my ear) despite flat on-axis measurements at/around the listening position on my RTA unit. My Rythmik subs allow for adjustable Q and I have backed off the most highly damped setting for just this reason. I have also heard many extended/full range speakers that sound underdamped, bloated and "slow".

The question of properly damped bass isn't unique to subs. As to group delay issues attendent to placement, endless screwing around with varying placement of subs relative to mains leads me to believe that it's a non-issue, but that would have to be qualified for related variables, including:

Crossover frequency and slope. IF these issues are actually audible at higher frequencies (and, as a practical matter, I have my doubts) , a lower crossover frequency and steeper slope would largely determine how audible they are in a given system. It would also allow increasingly greater separation as the crossover point drops and/or the sub's high cut slope is steepened. IOW, generalizations would be misleading.

The relative damping of the subs and mains at/near the x-over point. IME, there is a pretty good chance that the (sealed) sub is more highly damped than the main speaker (if not matched with care) and the qualitative result of this mismatch is hard to predict. Incidentally, this one is -again- not unique to subs. Many a 3 way full range speaker can exhibit problems in the lower crossover region.

and

The sensitivity of a given listener, among others.

Bottom line, it will be VERY difficult to prove inocence (or guilt) using the math cited here, but much easier using your ears.

Just MHO, as usual.

Marty

You shouldn't really "hear" the sub. If you do, they are too LOUD!!!

Certainly group delay and frequency response are intertwined.

I suspect that "masking" is what makes the effects audible. What I mean is that you can't perceive accurately what is happening to the bass response except by what it "masks" in the higher frequencies.

If a 20 Hz signal suffers a group delay of say 30 msecs - what happens to the harmonic distortion associated with that sound...are they also delayed by 30 msecs?

If the harmonic distortion is significant and delayed by 30 msecs then you may have a very audible 80 Hz harmonic that is audibly delayed by 30 feet. I cannot help but suspect that this harmonic will make the bass sound "slow", "bloated" or "muddy" by masking higher frequencies.

In essence, just consider the entire harmonic distortion from low frequencies (below 50 Hz) all delayed by the group delay - this smears out the bass response and masks transients to the ear.

I agree that a hump in the frequency response will ALSO cause masking by emphasizing certain frequencies by as much as 5 dB more than they should be and making the balance of sound such that you do not properly hear the higher frequencies (sound of the high frequency hit from the stick hitting the drum head).

Sound Engineers know that they can change the "speed" or attack of bass notes by altering the higher frequencies - emphasis is added for greater attack and de-emphasis to make the sound more laid back - in my opinion they are playing with teh amount of bass "masking" when they do this

Crossing over to full range speakers....at too high a frequency, is guaranteed to give you that bloated feeling.

I currently have my subs next to my mains, slightly forward. They are sensitive to movements of one inch. I can play a passage and it will sound right, but if I move the subs an inch forward or back, and it's out, and stays out if I move it farther. In previous setups, I've been able to introduce a delay to the mains relative to the subs (in Pure Music, for example), and I had the same results: the coherence of the sound was sensitive to delays that were a fraction of a millisecond.

So time alignment is crucial, and I don't think the subs will ever be truly integrated in a system until that is achieved. I think once it is tackled, EQ is available to deal with the frequency response of the room. That's my current approach, and I don't think I've sacrificed much, except that there are subwoofers sitting in not-inconspicuous locations in my living room.

"Many subs have group delays of up to or more than 1 cycle. Of course this is audible."

I don't remember the exact group delay figures used in the study I read, but they were consistent with what one would expect from a well-designed vented system. Using digital signal processing, group delay was isolated from frequency response (don't ask me how they did it). On test signals group delay was slightly audible, but on music it was statistically inaudible. The audible effects normally attributed to group delay were replicated by a frequency response curve mimicing the high group-delay system, even if the group delay is eliminated by DSP. So while large group delay correlates very well with perceived "slowness" in the bass region, it is not the actual cause.

01-24-11: Shadorne
Spatial issues are not that important in the LF as we are talking only a few milliseconds with waveforms that are 50 milliseconds long at 20 Hz.

Shadorne - I agree that, at 20Hz, which is a wavelength of over 50 feet, and which corresponds to a time scale of around 50 ms, a few milliseconds of time misalignment is probably no big deal. But what about at 80Hz, which is a wavelength of about 14 feet, and which corresponds to a time scale of around 14 ms?

Let's say the delay introduced by EQ is something like 2-4 ms, and that the delay introduced by placing a sub in the corner, when the mains are out in the room, is something like 3-6 ms. In systems that combine corner placement with EQ, you might have a delay between 5-10 ms. All of these numbers are an appreciable fraction of 14, the number of milliseconds that corresponds to a wavelength of 14 feet and a frequency of 80Hz.

IMO, this math illustrates that the use of EQ and/or the use of corner sub placement could introduce delays that result in significant time misalignments. As to whether those time misalignments are audible, I can only say that, from my experiences with sub setups, differences of LESS THAN A FOOT are easily audible, even in circumstances where there is little or no difference in frequency response.

Transient response is important but spatial positioning of a sub is not the big deal you make it out to be - as long as sub and speakers are within a few feet of being the same distance to the listener then no big deal.

You may be right that I am exaggerating the importance of time alignment at low frequencies, though it is not my intent to exaggerate. My view about the importance of time alignment at low frequencies resulted from about 2 years of periodic experimentation with placement, EQ, and delay. But I suppose the view I'm advancing might be considered unconventional. Though, if it is unconventional, then why do several different manufacturers of full range speakers make an effort to time align their drivers, INCLUDING the woofers?

It seems to me that the importance (or lack thereof) of time alignment at low frequencies essentially depends on the human limits of temporal resolution at low frequencies. The limits of temporal resolution has been studied in depth, where estimates place it on the order of magnitude of MICROseconds, with some estimates placing it as low as 5 μs. But these studies tend to measure higher frequencies. I do not know the human limits of temporal resolution at lower frequencies. If they are anything even vaguely approaching those of higher frequencies, then it would lend credence to the idea that time alignment is audible at low frequencies, making time alignment an important consideration in subwoofer setup.

Bryon

The answer is simple. Its because people stick there ultra powerful subs in a corner, which throws massive boom, and is often far away from the mains as well. I have been saying the same thing here, and nobody seams to get it. Put the sub under the center channel, between the mains.....put your electronics in the corner! If you do this all the problems are gone. In the older days subs stunk IMO, even the good ones cant compare to subs now. Back then corner placement was about added db gain(they still never blended right then either). These days, putting these huge, powerful subs in the corner is rediculous. The ONLY way I would try a corner placement is if you were running the sub under 40HZ. Otherwise it never sounds right to me.

Why dont you put your main speakers against a wall, or corner? -Bloated, muddy, heavy bass, right? So why do it to your sub? I ran my sub at 100HZ with my Magnepan 1.6's and in the center position you would NEVER have known it was coming from the mains. Phase/distance issues are GONE!

I dont like any subs sound placed out of center.

Spatial issues are not that important in the LF as we are talking only a few milliseconds with waveforms that are 50 milliseconds long at 20 Hz. A PEQ will delay only a few milliseconds also so no big deal.

Group delay can be a lot more than a few milliseconds and can be a problem as it can add delays that are equivalent to moving your sub 30 feet....

Transient response is important but spatial positioning of a sub is not the big deal you make it out to be - as long as sub and speakers are within a few feet of being the same distance to the listener then no big deal. Of course this argument only holds for very low frequencies and a mere 2 milliseconds can be important at 1 KHz

01-23-11: Plato
If the subs are not located in the same horizontal plane as the main speakers (and close to them) as is usually the case when they are placed in corners or close to the front wall, then unless you have subs with fully variable phase controls of at least 0 to 180 degrees, you'll need to do extensive trial and error to get the phase relationship correct between the subs and the sats. And yes, that's pretty much as hard as it sounds.

Hi Frank - I agree with you that finding the optimal phase setting is a major challenge of subwoofer setup. But I would add that, IME, phase adjustment alone is often not an adequate way to time align the sub with the mains. The reason was stated by Cbw723:

I'm not sure the automatic setup system work any better. They are primarily EQ programs, and do nothing for the transient response. And, frankly, it would be very hard for them to fix transient response because that requires playing the mains and sub(s) at the same time and (usually, because of sub placement and processing delays) delaying the MAINS. The phase control on a sub won't do this either and, to the extent that it is useful at all, can only work when the sub is closer to the listener than the mains.

I agree with Cbw723 that neither phase adjustment nor EQ (whether manual or automated) will adequately address the time alignment problems created by placing the sub farther from the listener than the mains, which is probably the most common setup in the typical audiophile listening room. The reason phase adjustment cannot adequately address the time alignment problems of this setup is that, by placing the sub farther from the listener than the mains, the speakers that need to be delayed for proper time alignment are the MAINS, and NOT the sub. Obviously, the phase adjustment on the sub can do nothing for this. The common use of digital EQ on the sub just makes the time alignment problem worse (even though it can dramatically help frequency response), because it introduces a processing latency that further delays the sub's output relative to the mains.

IME, for setups in which the sub is farther from the listener than the mains, the only way to time align the system is to be able to DELAY THE MAINS. But, judging from the systems here on A'gon, very few audiophile systems have this capability. For systems that do not have the capability of delaying the mains, time aligning the sub with the mains requires that the sub be placed more or less coplanar with the mains. But placing the sub coplanar with the mains might not result in the best frequency response. Which brings me back to my initial observation in the OP that...

Under many circumstances, optimizing frequency response and optimizing transient response is a zero sum game.

IMO, the way to defeat the zero sum nature of this game is to:

1. Place the sub(s) to get the best frequency response (varies from room to room) and fix transient response problems with DELAY. This assumes you can independently delay the sub(s) and the mains, which as I mentioned above, doesn't seem to be a common capability in audiophile systems.

-OR-

2. Place the sub(s) to get the best transient response (i.e. roughly coplanar with the mains) and fix frequency response problems with EQ. But to the extent that the EQ introduces processing latency, you will have to move the sub(s) CLOSER to the listener than the mains. Again, this doesn't seem to be a common arrangement in audiophile systems.

-OR-

3. In light of (1) and (2), I have recently come to the conclusion that the most effective way to optimize both frequency response and transient response is to be able to independently control BOTH THE EQ AND THE DELAY of both the sub(s) and the mains. That allows you to correct for room modes (better frequency response) and time align the various speakers (better transient response).

Of course, I could be wrong about this. And...

This all this assumes that the time alignment of the sub(s) with the mains affects the transient response of the system in ways that are (a) audible, and (b) not reducible to changes in frequency response. Duke (Audiokinesis) has expressed doubts about that assumption, raising the question: What are the limits to the temporal resolution of human hearing at low frequencies? As I understand it, that is a subject about which there is some controversy.

Bryon

As said earlier the crossover point. The crossover point is the main and possibly the only reason for a sub to sound bloated. If your mains go down to 50 and your sub is crossed over at 80 with a very slow slope of 6db you could have a problem. Also having the sub too close to a corner can accentuate humps in the subs frequency response.

Transient response is not irrelevant. Many subs have group delays of up to or more than 1 cycle. Of course this is audible. The other factor intrinsically tied to transient response is the resonance.

An underdamped design will keep oscillating after the signal has stopped - this is adding sound after the sound has in fact stopped. Most subs are designed that way. No wonder it sounds bloated.

A good design will be overdamped or critically damped. These will not sound bloated.

01-23-11: Audiokinesis
Both the bloat and the slowness you describe are primarily frequency response problems; at low frequencies the ear has very poor resolution in the time domain so it doesn't really pick up the transient response in and of itself.

This is precisely what I was hoping to discuss.

Duke - As I mentioned, my theory about frequency response and transient response tending to be a zero sum game is just that - a theory. Which is a way of acknowledging that I very well may be wrong. Having said that, the theory that optimizing frequency response often comes at the expense of transient response, or vice versa, explains many of my experiences with subwoofer setups using frequency response software. Of course, there may be other explanations for my experiences.

IME, small differences in sub placement on the z axis of the room often result in significant audible changes in bass response, EVEN WHEN there are little or no measurable differences in frequency response. Since the audible changes don't seem attributable to differences in frequency response, I eventually concluded that they were attributable to differences in transient response, or more precisely, differences in the system's time alignment, where 'alignment' is understood in terms of the arrival times of the system's various drivers at the listening position. That conclusion is consistent with the audible changes being a function of z axis placement.

Encouraging that conclusion is the impression that, for some z axis sub placements, the bass sounds "in sync," whereas for others, it sounds "out of sync." Again, this is sometimes the case even when two placements have a similar or identical measurable frequency response.

On the basis of these experiences, I concluded that, when two different z axis sub placements measure similarly in frequency response, but one of them sounds significantly better than the other, the reason is because the better-sounding placement has a more ideal time alignment with the other speakers in the system.

Again, I recognize that this conclusion could be a mistake.

Bryon

"Why do subs sound bloated or slow?"

I think it's hard to optimize a sub set-up by ear. That approach requires more time, work and patience than most people are willing to invest. If more people used the available tools - real time room analysis - their sub set-ups would sound a helluva lot better, on the whole. If you add room correction to the mix, I suspect that the average set-up would be awfully impressive, with little discussion of "bloated" or "slow".

Just MHO based on personal experience.

Marty

I agree that most people don't know how to set up subs properly, but I'm not sure the automatic setup system work any better. They are primarily EQ programs, and do nothing for the transient response. And, frankly, it would be very hard for them to fix transient response because that requires playing the mains and sub(s) at the same time and (usually, because of sub placement and processing delays) delaying the MAINS. The phase control on a sub won't do this either and, to the extent that it is useful at all, can only work when the sub is closer to the listener than the mains.

All of what I'm saying applies to music only. For home theater, it doesn't matter a whole lot if your sub is a full cycle behind the mains. You just need it to be in phase at the crossover point and EQ'ed to avoid the room modes, and it will sound pretty good.

Duke, you make a great point. The room acoustics play a huge part in getting good bass and unfortunately the majority of rooms were not designed with any thought to good sound reproduction. Many rooms are just plain horrible for proper bass reproduction.

That said, mapping the room response and selecting a subwoofer with the best characteristics for that particular room will make more difference (in many cases) than simply throwing more money into a sub thinking that just by buying an expensive product your results should be much better (not always the case...).

I will also add that I've had better luck matching subs to mains that delve below 50Hz on their own, which could be a coincidence but it's been my experience.

i couldn,t live with my magnepan mmgs without dual 10inch velodybne subs. i hate boomy bloated sound. it takes some work and adjustment to get everything the way you want it. good questions and answers.

"Under many circumstances, optimizing frequency response and optimizing transient response is a zero sum game. In other words, getting one right usually means you get the other wrong."

Strongly disagree. When you get one right, you get the other one right at the same time.

Both the bloat and the slowness you describe are primarily frequency response problems; at low frequencies the ear has very poor resolution in the time domain so it doesn't really pick up the transient response in and of itself. The ear cannot even detect the presence of bass energy from less than a singe cycle, and it takes several cycles to determine the pitch. There is a correlation between transient response and sound quality at low frequencies, but that is because of the kind of frequency resposne curve that inherently accompanies good transient response. More on this later.

Now the ear is quite good at hearing the sort of frequency response anomalies rooms typically impose on subwoofer systems.

Given the wavelengths of bass energy and the time it takes for the ear to hear bass, by the time you hear it, the room has already very significantly altered the native frequency response of the sub. You have two primary mechanisms going on (really just different manifestations of the same mechanism): Severe peaks and dips imposed by room interaction, and a general trend of boosting the bass as we go down in frequency (room gain).

One effective solution is to use multiple subs. Each sub will interact with the room differently, and the sum of the two dissimilar peak-and-dip patterns will be much smoother than one sub all by itself. If you pay attention to internet posts where someone has gone from using one sub to using two subs, they invariably report more natural-sounding bass. So I believe that one of the reasons a lot of people prefer the bass of a good pair of speakers over satellite+sub bass is that the latter doesn't benefit from the smoothing we get from having multiple bass sources... unless of course you use more than one sub.

The observation that low-Q sealed subs sound tighter and more natural than vented subs goes back to room gain. Typical room gain is +3 dB per octave below 100 Hz. A low-Q sealed box rolls off at -6 dB per octave below system resonance, so combined with room gain that gives good extension without over-emphasizing the low bass. On the other hand a vented box is probably "flat" down to a much lower frequency before room gain is factored in. Suppose our vented box is "flat" to 25 hz; after room gain, it's now +6 dB at 25 Hz! That will definitely sound heavy and sluggish. As proof that it's the frequency response and not the transient response that matters, take both subs outdoors where there is no room gain, and the vented sub will sound tight and solid but the low-Q sealed sub will sound thin and anemic.

So to recap, audible bass anomalies arise primarily from frequency response problems, which in turn are primarily caused by room interaction issues.

Duke
dealer/manufacturer

Bifwynne, thanks for your insights... at last someone gets what I was saying.

I have one sub set up in a two-channel system with Magnepans and it works quite well and another in my Home Theater (previously mentioned).

They both integrate very well and add some punch and weight on the very bottom which is all they are meant to do.

It's great that sub manufacturers are finally finding ways to do the necessary adjustments automatically because most audiophiles simply don't understand how to make the appropriate adjustments. I think that's why so many people complain about bloating and poor transient response and so have negative opinions based on their own experience (and failure to get it right).

I agree with Plato. I also run two channel fronts and I have a Paradigm Signature Servo subwoofer. This Paradigm sub has its own Class D amp to power the woofer and three controls knobs to control phase, cut-off and loudness.

Because I only have one sub, I also bought a Paradigm X-30 crossover unit. The X-30 has the same controls and also allows me to combine left and right channels without shorting my pre-amp Mains. I feed my main fronts off the preamp's Main 1 output to the power amp. I run my fronts straight out. I feed the X-30 off of the pre amp's Main 2. output As stated, I use the X-30 to combine the two channels into one channel for the subwoofer.

My front speakers are Paradigm Signature 8 v2. The fronts roll off at roughy 35-40HZ. Of course room placement also affects base contour. However, as a general matter, I cross the sub over at 40-50HZ. Based on my ear, that sounds about right. Too high a cross-over and the bass sounds boomy. Too low, and bass is thin.

I also adjust phase by ear. I listen for the best sounding bass response. Loudness often varies by source material, but as I got used to working with the sub controls, I don't move the adjustment controls around too much.

I understand that Paradigm's current subwoofer line-up offers a bass adjustment kit which uses a small mike to sample bass contour, phase and loudness information directly into the woofer's self contained power amp. The sub's amp does all the work and makes the necessary adjsutments. That's about as good as it gets.

Anyway, I hope this helps. For those of you whose subwoofer doesn't allow cross over, phase and loudness adjustments, I suggest buying a cross over control box like the Paradigm X-30. It really works. BIF