In Search Of The Audio Abode---The Hi-fi House Letters part 2
Deep bass & small rooms
Editor: I started reading J. Gordon Holt's article "In Search of the Audio Abode" in the April issue but got hung up partway into it when he started talking about how low bass requires a large listening room. His 8' by 10' by 13' compact little room---he claims---"won't support deep bass worth a damn!"
Well, gee, Gordon, where have you been? Where is the editor? I thought that old wives' tale had been put to rest years ago. I realize that you people live at the other end of nowhere (probably because you like a more laid-back lifestyle, no traffic, and low housing costs), but all you would need to do is listen to a good auto sound system and you would begin to question the old idea that the largest dimension of a room must be a half wavelength at the lowest frequency which you want to reproduce. I know that many audiophiles thumb their noses at auto sound, but the fact is that I have put together auto sound systems that would put many very expensive so-called audiophile home systems to shame (except, of course, in imaging), and not only in the bass!! However, we will limit our discussion here to bass.
A couple of 8" woofers on the rear deck of a car will usually have---without any equalization---flat response down to 25 or 20Hz and below, even though the woofer may have a free-air resonance of only 35 or 40Hz. In fact, excellent bass in a car is the easiest thing to come by---just the opposite of in the home, and usually much smoother, provided you know what you are doing. How is it possible to have excellent bass in a small room, and why don't you have to do much equalizing? Hopefully the following discussion will clarify the situation.
In an article that I believe was recently referred to by John Atkinson in Stereophile, there was experimental evidence directly countering Gordon's comments. The article was in the Journal of the Audio Engineering Society, Vol.36 No.6, June 1988, titled "Subwoofer Performance for Accurate Reproduction of Music," by Fielder and Benjamin. In this article, the authors describe measuring nine different combinations of subwoofer placements in five different rooms. The result (shown in their fig.4) was a rising response beginning at about 30Hz and increasing at about 12dB per octave with decreasing frequency. No room dimensions were given. However, I'm sure they were typical rooms available to them or us. Remember that this is an average response and that your room can be guaranteed to be different and more erratic. My own experience basically confirms Fielder and Benjamin's work.
To gain an understanding of the mechanisms involved, the first thing we need to realize is that there are a number of transition regions as a function of the frequency where the effects of the room on the frequency response of the loudspeaker system differ. For the purposes of this discussion, only the first transition at the low-frequency end of the spectrum is of interest. This transition region occurs---you guessed it---in the vicinity of the frequency at which the room dimensions equal one half a wavelength (Gordon's 550/room dimension). Below this transition region we can think in terms of pressurization of the room; above, we must consider pressure waves and hence standing waves.
Remember, our ear responds to pressure. The natural trend, starting much above this transition region and going downward in frequency, is toward a rising response continuing well below the transition region as compared to the anechoic response. The main difference in the region above the transition as compared to below is that you can guarantee that the response will be very uneven in general due to the peaks and dips caused by the standing-wave resonance.
Immediately below the frequency of the lowest room resonance there will be a dip back to a nice, smoothly rising response curve. In this region the room looks like a lumped acoustical element (the dimensions are small compared with a wavelength), and therefore the pressure tends toward uniform everywhere, like that in a balloon. This is clearly the most desirable region to work in, as there are no wild peaks and dips. Unfortunately this nice situation is only available to those of us who have very small listening rooms and/or good sound systems in our cars. If you have a very small listening room, the peak-dip problem due to standing waves gets pushed to a higher frequency. In certain respects it is easier to deal with the problem at higher frequencies because of effective absorptive materials and speaker directivity.
I verified these theories with some measurements of the inside of a 5"-diameter tube of various lengths from 3' on down to 6", and on the inside of a large woofer box of dimensions of 20" by 22" by 30". The speaker used to drive the tube and box was a wide-range 4" Audax driver mounted in a 4" by 7" by 9" box and slightly equalized to give a very flat response from 80Hz to about 10kHz. All the results can be scaled to "room sizes." The tube simulates a one-dimensional room (one that has standing waves in only one dimension) so that the true trend can be seen. The box could simulate a normal (although poorly dimensioned) room of, say, 10' by 11' by 15'.
Fig.1 shows the results for the tube of 2' length. The standing-wave resonances are clearly evident, as well as the continuing rise below the last peak. These peaks would be considerably modified in a three-dimensional case and, in addition, where the microphone was away from the back "wall."
Fig.1 Standing waves in cylindrical tube, 24" long, 5" diameter (dashed line corrected for drive-unit rolloff)
Fig.2 shows a number of measurements on the box as well as the anechoic response of the speaker used on the same scale for reference. These curves were obtained using a 1/3-octave warble sweep which effectively averages the response to more closely approximate what we would hear. 200Hz would be the approximate transition region for this "room." The top curve shows the response with no damping (fiberglass) in the box. Notice the very erratic response, and also the fact that the response does not roll off below the transition frequency. Notice the considerable smoothing of the curve that results from the addition of damping (fiberglass).
Fig.2 Anechoic response of 4" drive-unit plus standing waves in test box (lower dashed line, anechoic drive-unit response at 18"; dash/dot line, drive-unit on back on concrete firing upward at 18"; dotted line, in box with 50% internal walls covered with 0.5" fiberglass; solid line, in box with no fiberglass; upper dashed line, corrected for drive-unit rolloff)
It must be remembered that this is a scaled situation and the fiberglass would not be as effective at lower frequencies unless it was also of proportionately greater thickness. Other factors would also enter the picture. However, we can clearly see that the response below the transition frequency does not plummet. Again, however, many things will affect the real-life response, such as resonant walls and walls that the sound effectively goes right through. If the sound goes right through the wall, then you are back down to the anechoic response, or close to it. The low-frequency boost that occurs in the typical room is, by the way, the reason that speaker systems that have lots of low-frequency response are often so problematical (boomy, over-rich, uneven bass). A little equalization will do wonders.
The room/speaker interface is by far the most complex and therefore the most important link in the chain to high-quality sound, but relatively little has been done with it because of its complexity and because of a lack of understanding, but mostly, I believe, because of the difficulty of controlling it. There are a number of tools available to help the room/speaker interface: room selection, speaker/listener placement, new construction, bass traps, passive and active absorbers, diffusors, and equalizers. But most audiophiles cannot take advantage of the majority of these tools for various reasons.
The most practical answer to many of the problems of the room (and many speakers) is equalization. Unfortunately, most audiophiles have a very misinformed view of equalization, so the vast majority of them will continue to go on never achieving the full potential of the equipment that they have. Few systems, no matter how good, will have a truly good acoustic response in the typical listening room without equalization. David Wilson is the epitome of that realization with his equalized WAMM system.
Most equalizers, graphic or otherwise, do not degrade the sound (provided they are not audibly noisy), but rather their improper application will. They will, of course, change the sound as they should. For example, if a speaker sounds too aggressive and tizzy and we correct the problem with an equalizer (the problem is usually due to excessive and/or peaky upper midrange), we will subjectively lose detail. However, the detail was excessive to begin with but we often find we like it. And so what do we do---we blame the equalizer for losing detail. Most of the time, however, it is the misapplication that gives equalization a bad name. It cannot be used without test equipment and lots of experience.
I've stated many times that there is no system that I can't improve with equalization. Don't misunderstand me. Equalization is not the perfect answer to acoustical problems. Rather, it is by far the simplest and least expensive, and in many cases the only answer, to quality sound in many situations. Equalization, for example, will not cure an imaging problem due to live walls in close proximity to the speakers or a badly asymmetrical speaker location (although it can make many speaker systems sound quite acceptable even in asymmetrical situations). Nor will having a room with all dimensions greater than 30' guarantee you smooth response, as you will still have the standing-wave resonances to contend with.
If you do everything possible within the restrictions of your wife, decorator, budget, room, etc. and then apply equalization, you will have an excellent sound system. It will effectively cure the problems due to frequency aberrations in all your equipment, including many phono cartridges, tube amps, speakers, and rooms, all in one fell swoop!!---John Koval, Santa Ana, CA