Bass frequencies are the most difficult to tame in a small room because the wavelengths are long, which requires thick absorbers called bass traps. The before/after frequency response was measured in the same room as Figure 1 and shows the room’s low-frequency response before and after adding bass traps. (See Figure 2.) You can see three severe nulls around 64Hz, 84Hz and 140Hz in the red before trace, as well as additional nulls at higher bass frequencies.
Most rooms have many such nulls in the range below 300Hz or 400Hz, but some people fear that adding bass traps will reduce the amount of bass even further. In truth, bass traps reduce peaks and also raise nulls, so they make the response flatter, rather than add or remove bass. In many rooms, the main problem is deep nulls caused by reflections from the wall behind you combining out of phase with the direct sound from the loudspeakers. Therefore, adding bass traps increases the perceived level of bass. But in some rooms, especially those that are square- or cube-shaped, peaks can dominate the response. In that case, adding bass traps reduces the peaks, again making the response closer to flat. Whether peaks or nulls are the larger problem also depends on where in the room you listen.
Because deep nulls cause you to hear less bass than is really present, you’ll tend to add too much bass to compensate. As you can see, the finest loudspeakers in the world are of little value if your room skews everything you hear this badly. When bass traps are added to a room, the low-frequency response also changes less around the room. The most effective place for bass traps is in corners where bass waves tend to gather, though other locations are also viable. Note that rectangle rooms have 12 corners: four where each wall meets another wall, four where each wall meets the ceiling and four more where each wall meets the floor. After treating as many corners as is practical, the front and rear walls are good candidates for even more bass traps. When bass traps are added, the response not only becomes flatter, but also tighter and clearer because the decay times are reduced. (See Figures 3 and 4.)
It’s impossible to make any small room perfectly flat, so the more bass traps you add, the closer you’ll get. It’s that simple. The only trade-off is how good you want versus how much effort and expense you’ll endure. The response and ringing in these graphs is about as good as can be expected in a small room, short of lining every single inch of room surface with extremely thick absorption.
Finally, it’s worth noting that our ears perceive low frequencies as being omnidirectional. With content below about 100Hz, it’s difficult, if not impossible, to tell where the sound is coming from. Some people think that years of acoustical research are wrong, and people can perceive bass direction at very low frequencies. But I’m convinced the real issue is buzzing and rattling from a woofer or subwoofer, or port noise from a reflex enclosure, or maybe vibration from a nearby window. If you can hear where a subwoofer is placed, and the crossover is 100Hz or lower, then something else is going on.
—Ethan Winer owns RealTraps, an acoustic treatment company in New Milford, CT. This article is excerpted in part from Winer’s new book, The Audio Expert, from Focal Press. More information is available at www.ethanwiner.com/book.htm.