Multiple subs vs. bass traps

[audiokinesis]

I have not tried the multiple subwoofer method from Geddes, but did have stereo subs, mono subs, in my system.

Assuming the subs are widely and intelligently distributed, two subs will be approximately twice as smooth in-room as one, and four subs will be approximately twice as smooth as two. (From a perceptual standpoint, smooth bass = "fast" bass.) In other words, I think you only got about halfway there with stereo subs, and maybe less if they were along the same wall.

...how we can then perceive bass in headphones?

Sorry I wasn't clear; it's a matter of time, not room size. For example, three wavelengths at 50 Hz takes 60 ms regardless of room size.

I have to say that this comment make me perplex: how high do you use your subwoofer? Yes, subwoofer really do flatten the FR, but what about the bass frequencies not covered by subwoofers?

Right now I have four subs asymmetrically distributed, and one of them is wired in reverse polarity (they are all driven by a common amplifier). If I recall correctly, three of them are sealed and one is ported (they have tightly-pluggable ports).

Above the region where the subs are active, I use highly directional mains plus a secondary set of speakers optimized to a) make a fairly late-onset, spectrally-correct contribution to the reverberant field and b) smooth the upper bass/lower midrange region. The details are beyond the scope of a thread on a subwoofer forum.

At the listening position, you dont want any early reflection at any frequencies. you dont want mids nor highs early reflection but you do want and need secondary reflections. You dont want to allow any early reflections at the listening position that are at least not attenuated around -10db at 15ms.

I agree with your general principles and goals here, and have focused my own efforts more on the speaker design side of things than on the room treatment side, as speaker design is part of my job. Ime the spectral content, time delay, intensity, and arrival direction of the reflections all matter, and much moreso than is generally appreciated.

 

[Mindsource]

Assuming the subs are widely and intelligently distributed, two subs will be approximately twice as smooth in-room as one, and four subs will be approximately twice as smooth as two. (From a perceptual standpoint, smooth bass = "fast" bass.) In other words, I think you only got about halfway there with stereo subs, and maybe less if they were along the same wall.

According to Geddes, two subs is a huge improvement from one. Three is still a significant improvement from two. But four subs and you are well into the territory of diminishing returns.

To anyone unclear of what's being explained, honestly, I posted that video because it is absolutely pertinent to the topic here. Dr. Geddes received his PhD in acoustics based on the work he did on this subject, and that video puts his findings into an easy to understand presentation.

Here it is again
https://www.youtube.com/watch?v=SCWL-zusyqw

Watch it.

 

[RHosch]

youknowyou,

You've been banging on and one about people not understanding room acoustics but the entire foundation for your posts in this thread is simply incorrect.

Domestic sized rooms at low frequencies behave as minimum phase systems. You obviously haven't run across this crucial fact before. Go now and look it up and stop repeating incorrect assumptions.

In the context of this discussion, it means that frequency and time domains at low frequencies in our rooms are inseparably linked. Alter the frequency response, and you change the time domain response. Yes, lowering a peak with eq ABSOLUTELY DOES reduce time domain ringing at that frequency.

The problem with a single sub and eq is that you can only approach a good frequency response (with correspondingly good time domain behavior) for a single or very limited seating positions. By using multiple subs, you excite modes more evenly, excite some dominant modes less, and have a smoother frequency response to start with. EQ applied to multiple subs multiplies its effectiveness, and if they are well placed, both frequency response AND time domain (ringing) can be tamed over a broader seating area.

Passive treatments have the advantage of working at all points in the room, and the disadvantagenof being huge at low frequencies.

The proper order of design for low frequency is
1. Multiple subs placed correctly
2. Add whatever treatments you practically can
3. EQ the MINUMUM PHASE SYSTEM to further refine both frequency response AND RINGING BEHAVIOR

 

[youknowyou]

youknowyou,

You've been banging on and one about people not understanding room acoustics but the entire foundation for your posts in this thread is simply incorrect.

Domestic sized rooms at low frequencies behave as minimum phase systems. You obviously haven't run across this crucial fact before. Go now and look it up and stop repeating incorrect assumptions.

In the context of this discussion, it means that frequency and time domains at low frequencies in our rooms are inseparably linked. Alter the frequency response, and you change the time domain response. Yes, lowering a peak with eq ABSOLUTELY DOES reduce time domain ringing at that frequency.

The problem with a single sub and eq is that you can only approach a good frequency response (with correspondingly good time domain behavior) for a single or very limited seating positions. By using multiple subs, you excite modes more evenly, excite some dominant modes less, and have a smoother frequency response to start with. EQ applied to multiple subs multiplies its effectiveness, and if they are well placed, both frequency response AND time domain (ringing) can be tamed over a broader seating area.

Passive treatments have the advantage of working at all points in the room, and the disadvantagenof being huge at low frequencies.

The proper order of design for low frequency is
1. Multiple subs placed correctly
2. Add whatever treatments you practically can
3. EQ the MINUMUM PHASE SYSTEM to further refine both frequency response AND RINGING BEHAVIOR

we kind of agree lol. but I think you missed evidence here. Smoothing the FR with subwoofer do not really smooth the time domain and LF resonance. Lf resonance is very slightly improved with multiple subwoofer VS bass traps.
So please dont say I dont understand that multiple sub will help time domain response, but it wont come close to the effect of proper absorption.

Yes, lowering a peak with eq ABSOLUTELY DOES reduce time domain ringing at that frequency.
problem is, at only ONE location. move your head 30 cm and the eq may create more problem that it solved.

<<Whilst EQ can be applied to minimum phase regions of a response with predictable results, we need to remember that the measurement is only valid for the microphone location at which it was made, and as frequency increases the response changes more rapidly as the microphone moves. >>

Domestic sized rooms at low frequencies behave as minimum phase systems. You obviously haven't run across this crucial fact before. Go now and look it up and stop repeating incorrect assumptions.


<<Modes and even reflections are for the most part minimum phase, however the summed effect of many modal resonances along different axes/paths and of multiple reflections is usually not minimum phase, though only strongly so in small frequency bands.
The summation of multiple reflections is often not minimum phase, and a reflection which is larger than the original signal (e.g. due to focussing from a surface) is not minimum phase, but any single echo or reflection not larger than the original (the usual case since surfaces inflict loss) is minimum phase, as are the modal resonances along any particular path. >>

https://www.gearslutz.com/board/5691998-post34.html
https://www.gearslutz.com/board/studio-building-acoustics/504980-minimum-phase-room-response.html

 

[audiokinesis]

According to Geddes, two subs is a huge improvement from one. Three is still a significant improvement from two. But four subs and you are well into the territory of diminishing returns.

This is true in the specific case of the proprietary subwoofer equalization service that Earl offers to his customers. In the more general case of increasing the number of subs without the benefit of Earl's "special sauce", here is a quote that is relevant. I've included the whole thing because he addresses the topic of reverberation in the bass region which is applicable to the present discussion, and have bolded the part about the improvement we get by going from one to four subs:

Bass problems in small rooms is a subject that I have spent my life studying. Its solution poses some very real problems some of which have solutions, like you have pointed out, but others I have not found yet.

As far as Greisinger's, and several other papers go, the idea of several subs around the room gets to the problem of spatial variation of the LF sound field.

At a single point in space, one can EQ the response, of course, but that has been shown to actually make the problem worse at other points, so it is not a global solution. To some, like myself, it is not a solution at all. The spatial variations, and to a certain extent the frequency response variations, will go down (get smoother) as 1/N, where N is the number of independent sources. In your example above the main channels having two woofers and then two subs goes a long ways towards a smoother response, about 1/4 the variation or four times better than one sub.

I have done this for years, and even have a large center channel speaker giving me five LF sources in my small theater. This yields a very smooth spatial and frequency response variation at LF. Results can be seen at my web site (GedLee LLC) - see the "Small Room" Power Point presentation.

But alas, this solution does not address the subjective aspects of LF. That's a whole other ball game.

In a small room, we all know that the modal density is low. This yields large peaks and dips in the response, which with multiple sources gets smoother, but the limiting factor is still the room configuration. The only way to smooth this limiting room response is to use a large amount of LF damping. This is the solution that I recommend. But, and here is the downside, as a result, the reverberation at these lower frequencies goes way down. There is essentially no LF reverberation field at all in this kind room.

Some might say that this is a good thing, and I would probably have said that some years back, except for a recent experiment that I did that convinced me otherwise. Now at HF, there is no doubt that a strong reverberant field is a good thing, given the right kind of sources, etc. (see my white paper, same web site).

I had a second pair of Summas that I needed to store, so I put them in my living room - may as well set them up! Now this room is pretty big, (> 15 foot ceiling, very open space, etc.) and VERY live (wooden floor stone fireplace, leather furniture). The thing that struck me was the impressive perception of the bass. The highs sounded about the same as the theater, as would be expected since the room is less of a dominate factor at these frequencies, especially with Summas, but the bass was dramtically better.

I specifically used the word "perception" since the actual measured response in this room was not notably different than in my theater. THIS intrigued me immensly, especially since I had been studying this problem for decades.

Now, I had always known that bass sounded very good in an auditorium. Some years back, my brother, a bass player, and I once talked about this at a concert. He asked me why the bass always sounded so good in a big room. I have hought about this a lot over the years. I could explain the physics of the problem well enough, but the psychophysics of the percetion was not at all clear to me.

This question and my recent experinces have led me to the conclusion that it is the large reverberant field in a large room that yields that full bass sound without it being overly oppressive. This makes eminent sense since our perception of loudness and pitch is strongly influenced by the duration of a signal. A very short bass note is very difficult to sense the pitch and it's actual loudness seems negligible. We can raise the volume to overcome the loudness problem (ala Fletcher Munson), but this still leaves the pitch difficult to determine. This is basic psychoacoustics.

In a big room with a lot of LF reverberation the bass is clean and clear with no need for incresed output to give a perception of loudness. This then is the difference between the large room and the small room - the reverberation at LF. You see its the modes that carry the energy in the reverberant field and the lack of modes creates an inevitable lack of reverberant energy.

So why not make the small room reverberant. First, this can only be done to a limited extent for the modal reason given above, and second a small reverberant room will have a large spatial and frequency variation in the sound field that cannot be removed with EQ or any other technique that I know of except adding damping. But then the damping kills the reverberant field and we are left with that muddy (non-clear or pitch perception difficult) bass that we are trying to avoid.

So you see, IMO, there is no real solution to this problem. We will have to take the tradeoffs one way or the other, but, for a small room, we will always have to live with poor bass of one kind or another.

I am working on a concept of making the subs reverberant with the hope of creating a simulated reverberant field in the small room. But this is an ellaborate and dificult to impliment solution to this critical problem. Only time will tell. I'll let you know if I manage to solve this problem.

Just my 2 cents - or more like 90 cents.

Earl Geddes

My understanding is that the reason adding more subs only smooths the frequency response variation "to a certain extent" is that, at some point, the number of subs becomes high enough that they can no longer be spaced far enough apart to meet the "independent source" criteria.

The above quote points out a possible theoretical problem with the CABS approach, which is intended to virtually eliminate reverberation in the bass region, assuming it works as claimed. If Earl is correct in that the bass reverberation we get in a large room plays a major role in its subjective superiority over bass in a small room, then from a perceptual standpoint removing bass reverberation from our small rooms may not be the best solution.

Duke

 

[RHosch]

we kind of agree lol. but I think you missed evidence here. Smoothing the FR with subwoofer do not really smooth the time domain and LF resonance.

It does. Toole has studied this and published results, as have others.

problem is, at only ONE location. move your head 30 cm and the eq may create more problem that it solved.

I addressed that. Its why multiple subs are useful. Or gobs of treatments.

<<Modes and even reflections are for the most part minimum phase, however the summed effect of many modal resonances along different axes/paths and of multiple reflections is usually not minimum phase, though only strongly so in small frequency

The gearslutz reference is incorrect. Summation of minimum phase phenomena is minimum phase.

Spectral reflections at higher frequencies are not minimum phase, thus the issues with FIR room correction.

 

[youknowyou]

It does. Toole has studied this and published results, as have others.

it doesnt come close to proper absorption. yes subs helps, but absorption is much better to reduce LF resonance.
I must admit I'm far from understanding how we perceive LF in rooms and time domain of LF, but my experience tells me that treating those resonance with bass traps made a amazing difference in SQ. Something my stereo subs didnt even come close to do.

<<The summation of multiple reflections is often not minimum phase, and a reflection which is larger than the original signal (e.g. due to focussing from a surface) is not minimum phase, but any single echo or reflection not larger than the original (the usual case since surfaces inflict loss) is minimum phase, as are the modal resonances along any particular path."
so this is incorrect?

if you read this thread, it seem quite clear the this is true
https://www.gearslutz.com/board/studio-building-acoustics/504980-minimum-phase-room-response.html

 

[RHosch]

Any time the discussion moves to spectral reflections from individual surfaces (the focusing reference from the gearslutz quote), you have moved high enough in frequency that we are no longer discussing subwoofers in normal sized rooms.

So the quote is technically correct, but contextually wrong.

 

[Jack Caldwell]

The...(Earl Geddes)..... quote points out a possible theoretical problem with the CABS approach, which is intended to virtually eliminate reverberation in the bass region, assuming it works as claimed. Duke

Hmm, I think I might disagree on a couple of fine points in what Earl concluded.

My limited experiments with the mini-DBA/CABS, although not necessarily analogous to the full monty version of CABS, did give me a starting point to compare from, and lack of reverberant qualities was not something I noticed.
If they were there on the recording, the system did just great!

1) When the recording was not "dry" and contained extended LF reverberant information, such as what is experienced in a large symphony hall, the subjective experience was quite close to a feeling like the room had "expanded" to the size of the original recording venue.

2) Overly "dry" recordings sounded very much like... overly "dry" with insufficient reverberation. Please note: the same "dry" recordings also sounded overly dry on headphones.

3) The good recordings did not sound dry or lacking in reverberation, if anything they sounded very spacious. Like with the headphones, only it "felt" much better as we also feel these frequencies in our bodies

If Earl is correct in that the bass reverberation we get in a large room plays a major role in its subjective superiority over bass in a small room, then from a perceptual standpoint removing bass reverberation from our small rooms may not be the best solution.

Duke

Earl is probably right on target when it comes to playing a double bass or massed bass strings in a room vs a concert hall. The acoustic of the venue will play a determining factor in the perceived quality of the bass information. A live double bass in small room just doesn't sound like it does in a big concert hall.

However, when it comes to recordings, even in large symphony halls, sometimes we wind up with overly "dry" ones.
With those, I would endorse Earl's notion, and state that something needs correcting, but I would prefer to FIRST eliminate the room from the equation and add proper reverb back in as a second step, e.g.

A) Remove the "bad" colorations and room reverberant information from the room via implementation of a full DBA/CABS system.

B) Implement a "hall reverberation" DSP unit to feed the subwoofer channels ....remember the Yamaha DSP-1? I do! Even fondly on occasion...

c) JUDICIOUSLY add in extra reverberation for the subwoofer channels... and maybe for the mains also.... yes, I can hear the howls already... but when the recording is already too dry... well, then what's the harm?


In sum, I don't think adding reverberation is going fix the bass in a small room unless that small room doesn't sound like a room at all, and that's where I think the DBA/CABS approach might be the cat's meow.

Oh and I did like the note about 10 posts back about being able to do a "virtual CABS" using only the front array and DSP in Acourate... maybe adding in a variable reverb control would make it even snazzier!

 

[mitchba]

...
Yes, lowering a peak with eq ABSOLUTELY DOES reduce time domain ringing at that frequency.

problem is, at only ONE location. move your head 30 cm and the eq may create more problem that it solved.
...

Here are 6 measures of my eq'd right speaker across a 6' x 2' grid area at the listening position using REW's frequency dependent windowing (FDW) and psychoacoustic smoothing. These display settings closely match what our ears hear:

The smooth response in the low end is corrected via a FIR filter that does not boost any ringing and has a long correction window to average the response across a wide sweet spot. At the higher frequencies, the correction window is short and the fr smoothness across the listening area is mainly due to using constant directivity waveguides. I don't have or need 1 or more subs to achieve smooth bass at the listening position. That's what the DSP software does. I have minimal acoustic treatments used for 1st reflection points.

Most modern acoustic analysis and DSP correction systems now use FDW based on the acoustics research by jj Johnston and his report on: AES PNW Meeting Report - Acoustic and Psychoacoustic Issues in Room Correction The presentation is worth reading as it explains what our ears hear in small room acoustics and jj is an authority on psychoacoustics if one looks up his bio.

Modern 64bit precision audio DSP software provide acoustic analysis and transparent correction both in the time and frequency domains:

I point this out as when discussing "eq" as most think in the frequency domain only. With modern audio DSP software, that's not the case.

There is more to getting good sound out of any speaker/room interface than just focusing on (so called) bass traps or multiple subs. Smooth bass across a wide sweet spot at the listening position can be achieved without either.

Speaker/room integration is a system with may facets to look after. You may wish to look at a system specification like: https://tech.ebu.ch/docs/tech/tech3276.pdf

 

[SpeakerBob]

There's one thing I'm not getting about the 'EQ fixes ringing' thing. -

Say you have a mode ringing at 40Hz and it is +6dB above the rest of the curve and you then EQ it down 6dB for a flat response.

The ringing does reduce somewhat as it will be -6dB at whatever ms compared to before.

However, if you compare 40Hz @ 90dB pre EQ and post EQ the ringing will be exactly the same ? So EQ has not had any effect on the ringing, whereas bass trapping would show shorter decay times at 40Hz @ 90dB compared to 40Hz @ 90dB in the same room without treatment ?

When I look at the waterfall charts even after EQ the ringing frequencies still have big 'bumps' compared to the rest of the plot so the ringing is still there. It hasn't been fixed.

 

[youknowyou]

There's one thing I'm not getting about the 'EQ fixes ringing' thing. -

Say you have a mode ringing at 40Hz and it is +6dB above the rest of the curve and you then EQ it down 6dB for a flat response.

The ringing does reduce somewhat as it will be -6dB at whatever ms compared to before.

However, if you compare 40Hz @ 90dB pre EQ and post EQ the ringing will be exactly the same ? So EQ has not had any effect on the ringing, whereas bass trapping would show shorter decay times at 40Hz @ 90dB compared to 40Hz @ 90dB in the same room without treatment ?

When I look at the waterfall charts even after EQ the ringing frequencies still have big 'bumps' compared to the rest of the plot so the ringing is still there. It hasn't been fixed.

yes, but it seems that this is simply ignored.

 

[SpeakerBob]

These plots confused me too - How can you EQ 40Hz up by a couple of dB and the ringing drops considerably ?

 

[youknowyou]

These plots confused me too - How can you EQ 40Hz up by a couple of dB and the ringing drops considerably ?

same can be said for subwoofers. graphs dont show significant reduction of ringing (decay) with subwoofers when not in DBA arrangement.

but the argument/theory is that the ear is very poor at LF to detect ringing.

My answer is that adding bass traps which reduced significantly decay at LF, in my room, also very significantly improved the SQ of my bass.
So in my experience, in practice, the ear must be quite sensible to LF decay and any system would greatly benefit from reduction of LF resonance.

 

[RHosch]

Say you have a mode ringing at 40Hz and it is +6dB above the rest of the curve and you then EQ it down 6dB for a flat response.

The ringing does reduce somewhat as it will be -6dB at whatever ms compared to before.

However, if you compare 40Hz @ 90dB pre EQ and post EQ the ringing will be exactly the same ? So EQ has not had any effect on the ringing, whereas bass trapping would show shorter decay times at 40Hz @ 90dB compared to 40Hz @ 90dB in the same room without treatment ?

You're not getting it because it isn't exactly intuitive. Buy reducing the peak via eq DOES reduce the decay time. It DOES address ringing. But any reduction of a peak at one location may be deepening a null at a distant position, so what you can do for large seating areas using just one sub and an eq is limited. What you can accomplish with multiple subs and comprehensive eq is impressive.

Don't cling to the assumption that frequency adjustments can't alter time behavior - ringing, decay time, whatever you want to call it. It CAN and does at low frequencies in domestic sized rooms because of the physics of how long waves in small boxes behave.

Look up minimum phase. That simple definition is a good starting point. That's what subs in our rooms are.

BTW, traditional IIR filters (eq) are minimum phase. Minimum phase corrections are adequate and appropriate to minimum phase errors. The guy above who used FIR filters in the subwoofer region is misguided. He has intentionally addressed frequency response problems WITHOUT addressing ringing in the time domain. Backwards approach for subs, debatable but not without merit for higher frequencies.

 

[SpeakerBob]

You're not getting it because it isn't exactly intuitive. Buy reducing the peak via eq DOES reduce the decay time. It DOES address ringing.
QUOTE]

The bit where you're shouting 'DOES reduce the decay time' I actually stated that myself in the post you quoted - " The ringing does reduce somewhat as it will be -6dB at whatever ms compared to before."

My point was that even if you EQ a peak down to make a flat response, the specific frequency that you EQ'd will still be ringing away compared to the rest of the passband.

So I guess I'm still not getting it - how can EQ fix the ringing ? Can EQ make 40Hz @ 90dB decay quicker in a room ?

 

[Mindsource]

You are eq'ing for flat steady state response.

 

[SpeakerBob]

To make it easier -

If I play a 40Hz tone at 90dB and press stop and it takes x amount of time for it to decay 40dB,..

What settings on my DCX2496 will make it decay 40dB quicker ?

 

[SpeakerBob]

But any reduction of a peak at one location may be deepening a null at a distant position,

That sounds illogical to me - reducing the energy going into the peaks will also reduce the energy going into the corresponding nulls resulting in less deep nulls.

 

[RHosch]

Poor wording on my part. It doesn't increase relative depth of the null, but by bringing down the peak at that frequency the null also decreases in amplitude. You haven't made it any better.


And I guess I don't know what definition you are using for ringing, because it clearly isn't exactly the same after eq.