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Bass problems in small rooms

This is 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 (www.gedlee.com) - 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 negligable. We can raise the volume to overcome theloudness problem (ala Fletcher Munson), but this still leaves the pitch difficult to asses. 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

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