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Can Cafe' Latte Soothe Room Boom?
Stephen Harner

The secrets out: my recipe for cafe' latte with bottom end punch.

Achieving solid bass, or so-called bottom end punch, is a difficult task. A woofer cone needs to be about ten feet across to efficiently couple with air at the lowest audio frequencies. Fortunately, we can substitute amplifier power for speaker efficiency to help us with that task. Unfortunately, there remains another glaring problem. That is the formation of standing wave resonances in enclosed spaces.

A Guitar String   

A standing wave creates peaks and nulls in the sound pressure in various spots throughout a room. It can be thought of as being like a guitar string, strung between two walls. The plucked string will vibrate up and down (and around), but will not move at the ends because it is fixed to the walls, and the first resonance will have peak motion in the middle. The second resonance, or harmonic, will look like a sine wave, with one half the string rising up while the other half goes down, and then vice versa. So the middle will appear motionless, just like the ends. Thus creating a null at the middle. The next harmonic will look like one and a half sine waves with two nulls at one third and two thirds the length. Higher harmonics will create more nulls across the length of the string.

As it turns out, our ears are sensitive to air pressure, not air velocity (or displacement), so the sound NULLS are actually equivalent to the spots on the string with MAXIMUM displacement, and the sound PEAKS are where the motion NULLS are on the string. In other words, for the second harmonic, the motion nulls are at the ends and at the middle of the string, so the equivalent sound peaks would be at the walls and in the middle of the room.

The sound peak locations coincide with the best woofer locations for optimum resonance excitation, for example at the walls. Bohdan Raczynski presented some very nice charts in his recent article in audioXpress that will help you see what a standing wave room resonance looks like.

Interestingly, the resonance between two walls in a typical room supports not just a single frequency for each harmonic, but a band of frequencies, usually about one third octave wide. Also of interest, at higher harmonics, and higher frequencies, the peaks and nulls will be spaced closer and closer together. Other factors come into play as well, such as tangential and oblique angle resonances. This is explained by F. Alton Everest in a very easy to understand way and his writings on acoustics are highly recommended. I would like to point out that while Everest uses linear frequency response charts to explain room resonances, logarithmic charts would better indicate the closer resonance spacing at higher frequencies.

If loud boomy bass is what you are after, then the standing wave resonances are just what the doctor ordered, as the spatial reinforcement greatly increases bass volume at the peak locations, and you can maximize the bass peaks by placing the subwoofer in the corner. Of course this also requires careful placement of your listening chair to avoid as many null spots as possible. However, if you are a certifiable audiophile and you find uneven bass response unacceptable, then read on.

The Standing Wave Paradox   

Placing a subwoofer in the corner of a shoe box shaped room excites the 3 fundamental standing wave room resonances, boosting the bass at those 3 frequencies, as well as the higher harmonics, at least at certain locations across the room. Each resonance has a bandwidth of about one third octave, so if they are closely spaced, as the higher harmonics are, then they will overlap, considerably smoothing out the peaks. However, the first 3 resonances are usually too close together, or too far apart, and that, along with the inevitable drop off below the lowest resonance, produces a very uneven low frequency response.

The uneven low frequency response is due not only to the empty frequencies between widely spaced resonances and the compounding of closely spaced resonances, but also because of the null spots (as described above) inherent in standing waves. Nevertheless, since all speakers roll off the low frequencies, the room resonances reinforce at least some of those frequencies, producing what is perceived as better bass (hence the paradox).

Surprisingly, the uneven response is difficult to hear because if some bass notes are loud and others are soft, one assumes that the musician played them that way. The Bose Wave radio is astounding to most people because bass frequencies around 80 Hz are reproduced quite loudly, but what they don't know is that there is a precipitous drop-off in bass response below 80 Hz. They simply don't notice the absense of very low notes. Also, the human ear is less sensitive to variations in the level of low frequencies versus higher frequencies.

Another factor to consider is frequency shifting during reverberant decay caused by the room resonances. As stated before, the room resonance actually supports a band of frequencies about one third octave wide. This can be thought of as the Q of the resonance and the bandwidth is affected by acoustical absorption. If a note is played at a frequency 1 Hz greater than the peak room resonance frequency, it will still be reinforced by the room resonance, but as soon as the note stops playing, the sound field decays via reverberation and the frequency is pulled 1 Hz lower by the room resonance. Thus the reverberant decay shifts, or de-tunes, musical notes.

The Near Field   

Now if the room resonances could somehow be eliminated, then we could compensate for the weakened bass by using large woofers and large amplifiers! Alas, the room resonances are part of the laws of physics and cannot be completely eliminated (not in this universe). Many people have the mistaken impression that splaying the walls will eliminate room resonances. Not so! That will only change the Q, which results in a wider resonance bandwidth, albeit with a lower central peak. Some suggest that bass traps are the answer, but they have no effect on the null spots inherent in standing waves, and, since absorption is proportional to surface area, a bass trap would have to be quite large to significantly reduce the peaks. However, we might be able to overwhelm the room resonances with brute force by listening in the near field of the woofer.

Many sound engineers have tried the obvious solution of placing the subwoofer very close to the listener's head, (within the near field). This works but is not aesthetically pleasing and not very practical. Our collective spouses I am sure would furrow their brows in disgust here as they picture large subwoofers hung from the ceiling in the middle of their family rooms.

A better solution is to place the subwoofer very close to the listener's body (and down low) in a coffee table!

After all, the lowest frequencies are more felt than heard. I am not at all sure that I can hear 20 cycles per second, but I can sure feel the vibrations and I can see my pants leg flapping when I stand in front of a massive subwoofer. Because the coffee table is typically in the center of the room, the 2 lowest room resonances are not excited by a woofer placed at this location (right in the nulls of the length and width of the room) while the floor/ceiling resonance would still be excited (as the woofer is close to the floor). Unfortunately this position will effectively raise the inevitable low frequency roll off inherent in all woofer systems by not exciting the two lowest room resonances, and the accompanying bass reinforcement. Nonetheless, here is our chance to compensate with large woofers and large amplifiers (you knew I was wanting to do that anyway).

So the subwoofer will need a large bass boost to compensate at the lowest frequencies, and while we are at it, we could even go lower, below 20 Hz, down into the nether reaches, which may not be heard, but they can be felt. This requires a high output subwoofer that can handle a lot of power down ultra low, which could cause problems with vented subwoofers. Although the subject of sealed versus vented woofer boxes is quite controversial, I thought I might put my own two cents in and stir the pot a bit anyway.

The Cheater Tube   

Most commercially available subwoofers use vented enclosures with port tubes, what I call "cheater tubes". The sole reason for venting a speaker enclosure is to boost a small portion of its low bass response. From a physics standpoint, the boost is a resonant peak. Vented enclosures are nothing but Helmholtz resonators. Of course, Helmholtz resonators can be musical, as evidenced by the acoustic guitar, but they may not be accurate. You may lose something in the trade off for the extra boost, and that is the ability to punch out fast transient peaks accurately.

Martin Colloms disagrees, saying that de-tuned, over-damped bass reflex alignments sound as quick and agile as you could wish for. I must admit, it is difficult to hear any real difference between well engineered sealed and vented boxes. And Collums makes a valid point, that "Unless price is no object, every designer must fix on a realistic target for an affordable bass driver...". So if you can match the system bass Q with your room by utilizing a tuned (vented) enclosure, thus producing a flat low frequency response, then why not? Ah, but you could also opt for a more expensive driver in a sealed box, along with equalization, which would achieve similar results without the driver unloading below the box cut off.

Bass guitar players in general do not always agree that sealed boxes are best, but if you watch some music videos you might see that the most popular bass amplifier for professionals is an Ampeg with eight 10" drivers in a sealed enclosure. Think of a bass drum: a large membrane moving outward as the drum beater hits it. This produces a sharp transient pressure pulse. Can you see a Helmholtz resonator reproducing a bass drum accurately? It seems to do a pretty good job, but I am not convinced, and I must recommend sealed boxes.


Central room placement of subwoofers can help minimize the problems with standing waves, although it will not completely eliminate them. For practicality, the woofer can be mounted in a coffee table, an end table, or a raised platform under the seating. For multiple listeners, use several woofers. If everyone is within three or four feet of a woofer, the near field may not dominate, but it will certainly mitigate the room resonances and help smooth things out.

To sum up my recipe for cafe' latte with bottom end punch:
1. Drag that coffee table sitting in your family room out to the workshop.
2. Mount a sealed subwoofer box underneath it.
3. Put one or, even better, two woofers in it.
4. Then drag it back into the family room.
5. Hook it up to a powerful amp with an equalizer used for low frequency boosting.

Sit down and plop your legs onto the coffee table (maybe with a cafe' latte), and prepare for the most beautiful, tight, punchy bass you have ever heard, and get the added thrill of the table vibrations on your legs!

References and related reading   

1. Everest, F. Alton, "The Master Handbook of Acoustics", Second Edition, Tab Books, 1989

2. Sherwin, James S., "Certified Bass for the Certifiable", Audio magazine, January, 1990

3. Greiner, R. A., "The Lowdown on Subwoofers", Audio magazine, August, 1993

4. Raczynski, Bohdan, "Subwoofer Placement in Nonrectangular Rooms", audioXpress magazine, September, 2002

5. Colloms, Martin, "Bass and the Room", audioXpress magazine, October, 2002

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