Multiple Small Subs - Geddes Approach

how so? one would think the behavior of waves to be constant but the interference introduced by boundaries would be different,no?

As I said, ONLY in a small room is there a difference. I'll try and explain as simply as possible, but it is not a simple concept.

Consider a wave packets of say 5 cycles at 100 Hz and 1 kHz. The 100 Hz pulse is 50 ms secs long while the 1 kHz one is only 5 ms long. At any location in the small room the LF pulse will smear over each other making individual pulses indistinguishable because of the many reflections that will occur in the room within the 5 cycle time. However, the 1 kHz pulse will be apparent as individual pulses because they are so much shorter and will not overlap.

Hence, the LF sound can only be thought of in the steady state because of this overlap situation, while the 1 kHz wave comes in discrete packets. From a hearing point of view this is a dramatically different situation. The waves are always waves, but the timings relative to the space make them act very differently.
 
Consider a wave packets of say 5 cycles at 100 Hz and 1 kHz. The 100 Hz pulse is 50 ms secs long while the 1 kHz one is only 5 ms long....The waves are always waves, but the timings relative to the space make them act very differently.
That's a really interesting and even provocative notion.

Back to your post of a few days ago diss'ing subjective ratings, I can see how it would influence REW-type data collection (different analyses for high and low freqs) so that the data is more cogent to human perception.

But are there implications for room treatment or speaker design?

B.
 
It's just I've been trained to see strong 1/2 wave cancellations between any two subs, and nearly as strong 1/4 wave cancellations between each individual sub and its set of virtual subs created by its nearby wall/floor/ceiling reflections.
So when I picture adding all those cancellations on top of the room's modes...well, I just can't help but see a big mud bath...of what I've been likening to destructive interference...and I try to minimize the variables..

It is precisely a "mud bath", but in light of what I explained just above, this is inevitable at LFs, and as it turns out, adding more sources makes the mud smoother than it will be with just a single source and the modes. The problem here is that you are taking concepts from HFs and moving them down to LFs in a small room and that doesn't work. 1/2 wave and 1/4 "constellations" simply don't happen when there are multiples of reflections from a single wave pulse. You will only see these effects in a free field where there is at most one or two reflections. When you have dozens it just isn't the same situation.

The effects of room shape on the LF sound field in a small room was the topic of my PhD thesis. It turns out that shape isn't really as significant as one might think, except in cases of extreme symmetry when compared to no symmetry. A cube is very very bad, but virtually all normal shaped rooms will act almost exactly the same. There were two variables of significance: 1) the total volume, as this dictates the modal density and 2) the damping as this dictates the modal overlap. Shape was only an issue when the damping is discrete, i.e. condensed only on one wall for example. In that case the irregular shape made the damping more uniform WRT the modes, while a more symmetric room would have some modes heavily affected while others were not. Simply distributing the damping makes this issue go away. So, for the most part, except in extreme cases all rooms of the same volume and total absorption will behave almost exactly the same.
 
But are there implications for room treatment or speaker design?

B.

There are huge implications!! For example, one wants a lot of LF damping to smooth the LF modes, but not too much at HFs because we want a lot of reflections for "spaciousness".

We want high DI at HFs because we want to minimize the early reflections (to improve imaging,) but maximize the later ones (spaciousness again.) At LFs the directivity is completely irrelevant - even the concept is flawed in the modal region.

We want to measure the direct field at HFs because that dominates what we hear, but at LFs we need to do steady state measurements because at LFs that's what we hear.

There are endless implications to this situation, these are but a few.
 
As I said, ONLY in a small room is there a difference. I'll try and explain as simply as possible, but it is not a simple concept.

Consider a wave packets of say 5 cycles at 100 Hz and 1 kHz. The 100 Hz pulse is 50 ms secs long while the 1 kHz one is only 5 ms long. At any location in the small room the LF pulse will smear over each other making individual pulses indistinguishable because of the many reflections that will occur in the room within the 5 cycle time. However, the 1 kHz pulse will be apparent as individual pulses because they are so much shorter and will not overlap.

Hence, the LF sound can only be thought of in the steady state because of this overlap situation, while the 1 kHz wave comes in discrete packets. From a hearing point of view this is a dramatically different situation. The waves are always waves, but the timings relative to the space make them act very differently.

My listening room is 3.6*3.4*2.6 meters. I thought I heard what you describe in this post today in some electronic music with a sweeping bassnote. The higher frequency part, I could tell the origin of the sound (from the front). The lower frequency part of the bassline I could not tell where it is coming from. Almost as if I, the listener, am located on the baffle of the speaker in stead of listening from a distance if that makes any sense.:confused:
 
I thought of another way of explaining the difference between the HF waves and the LF ones in a small room. It may be more cerebial to those who thought the last explanation was too easy.

The situation is an analog of the difference between Newtonian Mechanics (NM) and Quantum Mechanics (QM). NM applies to large distances (relative to atoms,) where the space and energy are a continuum. Just like HF in a room.

In a small room, just like QM, the LF energies are discrete.

Just like QM -> NM, as the energy density gets ever more closely spaced, the discrete QM merges into the continuum of NM. QM are the LF and NM is the HF. One is discontinuous in energy (QM and LF) and the other continuous energy spectrum (NM and HF.)

In both QM and LF the wave motions are fixed by the modes (Resonances, eigenfrequencies, modal shapes, etc.) - no arbitrary motion is possible.

HF the wave motions can be arbitrary.

One huge advantage that LF acoustics has is that we can spread the modes via damping. To my knowledge no such thing is possible in QM.
 
Last edited:
My listening room is 3.6*3.4*2.6 meters. I thought I heard what you describe in this post today in some electronic music with a sweeping bassnote. The higher frequency part, I could tell the origin of the sound (from the front). The lower frequency part of the bassline I could not tell where it is coming from. Almost as if I, the listener, am located on the baffle of the speaker in stead of listening from a distance if that makes any sense.:confused:

To me, this has more to do with perception than with what I am talking about. But I do believe that what you say is true, although others don't accept it. At LF you can only perceive the totality of the waves hitting you from all directions, so localization is impaired.
 
Damping causes the modal peaks to lower and the width to broaden, just like damping in any other situation. This then causes greater modal overlap and lowers Fs. (The more modes that a signal exites, the more degrees of freedom it has and the closer it gets to s free field situation.) It is possible to get a room such that almost no modes are visible. My room is like this. Only the very lowest mode is obvious. There are still peaks and dips, but to a far lesser extent than an undamped room.

But remember that we don't want high damping at HFs because we want the later reflections to enhance spaciousness. Of course, HF damping is easy and LF damping is hard, so what we want tends to be the hardest thing to get! That's probably why so many rooms are so bad.
 
It is precisely a "mud bath", but in light of what I explained just above, this is inevitable at LFs, and as it turns out, adding more sources makes the mud smoother than it will be with just a single source and the modes. The problem here is that you are taking concepts from HFs and moving them down to LFs in a small room and that doesn't work. 1/2 wave and 1/4 "constellations" simply don't happen when there are multiples of reflections from a single wave pulse. You will only see these effects in a free field where there is at most one or two reflections. When you have dozens it just isn't the same situation.

The effects of room shape on the LF sound field in a small room was the topic of my PhD thesis. It turns out that shape isn't really as significant as one might think, except in cases of extreme symmetry when compared to no symmetry. A cube is very very bad, but virtually all normal shaped rooms will act almost exactly the same. There were two variables of significance: 1) the total volume, as this dictates the modal density and 2) the damping as this dictates the modal overlap. Shape was only an issue when the damping is discrete, i.e. condensed only on one wall for example. In that case the irregular shape made the damping more uniform WRT the modes, while a more symmetric room would have some modes heavily affected while others were not. Simply distributing the damping makes this issue go away. So, for the most part, except in extreme cases all rooms of the same volume and total absorption will behave almost exactly the same.

Thank you for the follow through explanation, and room shape comments.

I'm indeed conditioned to think in free field space...
It's beginning to sound like indoors, with inevitable 'mud' from the gitgo, is a case of "if you can't beat em, join em" :)
 
While I would prefer a large space for bass and you would prefer outdoors, we are both stuck with the fact that a small space in our homes is the only choice that we have. So make the best of it. Learn to think of Bass in a small room as unlike any other situation in sound reproduction. It takes some time to get past the "intuition" that comes from the free field experience, but it is essential to do so because otherwise you will never get the bass right.
 
I am really late to this conversation and somewhat fascinated multiple subs are even a debate.

What clued me in was the wildly different bass responses I received from my left and right speakers due to room placement - I have an L shaped space resulting in the right speaker being near a corner and the left being nearly centered on a 24' wide wall.

I tried moving the left +/- 3' from this position and realized some differences, however, it also affected the overall usefulness of the space.

If I am going to get odd responses from them, there is virtually zero chance of setting up a single sub to cure all my ills is virtually impossible. I then read an article which tested the effectiveness of multiple subs on overall sound quality and, oddly, imaging. Multiple papers indicated there was a benefit.

I have an NAD C658 preamp which includes Dirac. It also has dual subwoofer out that, when two subs are selected, acts as stereo out (not dual summed mono). As most here should know, bass in music has not been summed mono for quite some time.

In my case I built two sealed subs; 12" driver in 1.375cuft cabinets, powered by a Behringer NU1000DSP amp. Though the amp has EQ, DSP, crossover, etc, I am allowing Dirac and the NAD handle all of it for me.

Due to laziness and anticipation of Dirac Bass Management, I have both placed in the same corner 90* to each other. With each cabinet weight 75lb I prefer to wait for Dirac Bass Management to be available when I attempt ideal room placement.

I suspect one will remain in the corner and the other will be roughly 1/3 along the 24' wall. Could be wrong.

I can state the benefits are pretty astounding and there is definitely a difference with two over one.

What I would really like to do is model my listening space but it seems very difficult to do given the odd shape.
 
What I would really like to do is model my listening space but it seems very difficult to do given the odd shape.

Add to that the fact that the equations used in virtually all room modeling programs are incorrect - they assume small damping. So even if you have an easy room to model, it will still be incorrect if the room has a lot of LF damping, as it should.

I have long wanted to write a paper on the differences between the accurate model with damping and the incorrect simplified model. But alas, I think that my writing days are over. I don't have the energy for that kind of deep dive into math.
 
Mine is not a dedicated space and I no longer treat rooms like I did when I had a home studio and helping others with their studios.

When it comes to modelling I was just referring to basic modes that fit in the space and wanting to understand how the room being L shaped affects things overall. The propellerhead in my wants to know all of that, the realist in me recognizes I only need concern myself with the 1 cu meter of space which my head usually occupies.

What I should do is take 8-9 measurement in that 1cu meter space and average in REW - I imagine that would offer a more realistic picture of the response. Or I could be completely wrong.
 
What I should do is take 8-9 measurement in that 1cu meter space and average in REW - I imagine that would offer a more realistic picture of the response. Or I could be completely wrong.

Room measurement theory is a little beyond the scope of this thread, but I'll review a little.

If doing steady state measurements some frequency and spatial averaging is necessary to get a valid result and 8-9 is good (we used 6 in automobiles.) I wrote a paper on this way back in the early 80s' (See JAES) when only steady state was possible. But if you are using gating then this is not the case and two or three measurements should suffice. The gating should be a sliding (variable) window. At LFs however, only steady state is possible, but fortunately the variations across a small space like 1 m are not going to be great so a few measurements should do.
 
Dirac uses 9 measurements in a 1cu meter area to develop corrections for a single listening position. This is displayed as, I assume, and average of all measurements per speaker; Left, Right, Sub1, Sub2, etc.

I imagine, if I wanted to see the corrected results, I should attempt to take 9 measurements using the same measurement locations then average those.

I apologize for derailing the conversation. Initially my post was simply and attempt to share my experience using dual subs. What I did not do is mention just how low I can go while having significant headroom; I can run flat down to roughly 20hz.