My name and writings are being displayed, so I thought I would provide a bit more context. First, I suggest you read Chapter 13 of my book that discusses all of this in more detail than can be provided in a forum. But first, I need to admit to two biases. (1) I believe in the scientific process. Opinions count only if backed up by accurate data. (2) on the particular topic at issue here, bass traps - or more correctly low-frequency absorbers - about 30 years ago I was confronted with a room, my room, that boomed intolerably. The normal acoustical-consultant solution would have been to install low-frequency absorbers. Analysis of my situation revealed that to attenuate the offending resonances would take a significant number of these devices that are large, not attractive, and because the room already existed, could not be built into the structure as might be possible in a new construction. So, my bias: my wife and I like art, paintings and sculpture, so visual aesthetics matter. We also had a great view, and large windows existed where some of the absorbers would most logically be located.
I took it as a challenge to find a more acceptable solution. An analysis of the offending room modes indicated that it might be possible to attenuate them by using two subwoofers in the right locations. This is shown in Figure 13.22 in my book, with before and after steady-state frequency responses and waterfalls. The offending modes were indeed cancelled, leaving only a narrow interference dip, and the "boom" went away, leaving impressively tight bass. I was both pleased with the result and proud to have "discovered" an almost invisible solution to the problem.
But were the modes eliminated? Yes, but only when the sound system was running, and only for the sounds radiated by it. Turn it off, pick up a bass guitar and the offensive modes are back. So, if the room is to be a performance space, either for recreation or a recording studio, the electroacoustic mode canceling is useless. Go buy some bass traps and get on with it.
In my other listening room I had a similar problem, but this one was more easily solved. It was a very large room, 32 feet at its high point - my "classical" music listening room designed to be an acoustically diffuse extension of the stereo sound stage, not to compete with it. For that kind of program it performed superbly, adding spaciousness that two loudspeakers alone cannot deliver. There was one monster resonance at 42 Hz. Again, our extensive view windows, and my antipathy to unattractive objects caused me to think of alternatives to low-frequency absorbers. This situation is described in figure 13.20 where I show what I call "positional equalization" - finding a seating location where the ears were not strongly coupled to the offending standing wave. I show that when it sounded right, the frequency response and waterfalls looked right. The "boom" went away. This was a totally passive solution - no modifications except that my chair ended up in a silly location 5 feet out from the back wall. I put up with this for a while, but when I started to see tracks in the carpet from dragging the chair out for my listening sessions I decided that a better solution was needed. Attentive readers will have noted that I could have moved the loudspeakers instead of myself, but in that situation it was simply not an option.
So, I put the chair back where it logically belonged, plugged a parametric equalizer into the system, matched the center frequency and Q of the resonance and turned it down. The measured steady-state room curve looked a lot like the one with the chair moved, and the bass boom was similarly gone. I show these measurements in Figure 13.21 in my book. Room resonances at low frequencies behave as minimum-phase systems and optimized EQ can repair both the amplitude and time domain problems. Off-the-shelf automated "room EQ" or "room correction" programs may or may not be able to do this properly. Most do not.
But was the mode eliminated? Yes, but only when the sound system was running and only for the sounds radiated by it. The same as in the mode canceling system, but this time also, only for the prime listening location. Other locations in the room took their chances. But, for me, a mostly solitary listener, it was a totally effective solution, and it was totally invisible.
As history has it, in 1991 I moved from the National Research Council and my Canadian home, to southern California to join Harman International as Corporate Vice President of Engineering. In addition to my mainstream duties with the many companies under the corporate umbrella, I set up a small research group, aimed at pushing the envelope of knowledge - there was no product development, but only ideas that might benefit products. I never forgot the neat acoustical solutions just described, but in thinking about it further I reckoned that in small rooms there are only a small number of modes in the subwoofer frequency range - maybe there could be a more generalized solution that could benefit multiple listeners. After I hired Todd Welti we had the means to explore this using his expertise with Matlab. It was not long before he had the first multi-sub solutions. No more than four subs were needed, and two could do a reasonable job - but only in simple rectangular rooms and only for seats in the middle of the room. Figure 13.17 shows some results. This is not what many people think: more subs "filling in" holes. It is pure room mode manipulation, reducing the number of active modes in the room so that there are some areas that are not dominated by booms. The subs must be identical and in specific locations. But simple rectangular rooms are not everywhere.
We needed more, and besides, my new listening room was not a simple rectangle, and again I was not enthusiastic about filling it with bass traps. The expression "necessity is the mother of invention" is very apt. So Todd Welti and another employee Allan Devantier went to work, and came up with something we call Sound Field Management. This is much more complicated, involving transfer-function measurements from each subwoofer location to each specified seating location. An algorithm then chews away at the data and comes up with specifications for the signal processing necessary for the signals being delivered to each of the (usually) four subs. Amplitude, delay and one parametric filter will be specified. The goal is to minimize the bass variations among the specified seats so that equalization, if it is necessary, will be equally effective for all listeners. It works superbly. My room is shown in Figure 13.18 and another in Figure 13.19. There are no bass booms. Kick drums are "tight".
Again, were the room modes eliminated? Yes, but only when the audio system is runnng, and only for the sounds radiated by it. Now, though, we have done it for several seats, and in practice one finds that the bass is comparably good in the intervening spaces - put on a loop of kick drum and bass guitar and walk around the room. The bass response is tight and very consistent, and the funny thing is that the normal sense of a "room" has disappeared. It is not inexpensive, and it is not universally available (pity), but it is a very effective and not visually obtrusive solution.
Would bass traps have delivered a similarly good result? There is no reason to think that they would not have yielded a greatly improved bass response. It would just have been a very different looking room. If the physical volume and visual aesthetics of bass traps are not a problem, they work. It is straightforward physics. If your are building a studio or performance space, they are the only solution. But for sound reproducing rooms, now we have some alternatives. Low-frequency absorption is always a good thing, and if you can find ways to incorporate it, do so. It will make whatever else you do work even better. Something few people realize is that a single layer of drywall on studs (metal or wood) makes an effective low-frequency membrane absorber. Figure 21.7 in my book shows a comparison between one and two layers.
Whatever solution(s) you choose, it is likely that some bass equalization will improve things. Just don't automatically extend the equalization above a few hundred Hz, because that is where the loudspeaker itself takes over and if you have good loudspeakers you might risk making them worse.
Make your choices, it is a free world.
