New build for a small mastering/music production room.

Myth #1: Tuned bass traps are best because they can target lower frequencies than broadband traps.

It's true that broadband bass traps are less effective below around 80 Hz, and tuned traps can absorb to lower frequencies with less depth. But audio rooms need substantial absorption at all low frequencies. Once you have enough tuned traps to sufficiently tame the room modes, there's not enough space for the broadband traps needed to absorb the remaining low frequencies. Further, rooms have three primary modes—one each for length, width, and height. So that's three frequencies to absorb. But that's just the fundamental modes. There are also higher-order modes at multiples of each fundamental. So if the main length mode is 30 Hz (corresponding to 19 feet), there are also modes at 60 Hz, 90 Hz, and 120 Hz. Now we have four frequencies to deal with just for the length dimension!

Another problem with tuned bass traps is they're more difficult to design and build, especially for a room under construction whose response can't yet be measured. Software such as my Graphical Mode Calculator can predict mode frequencies from dimensions, but these calculations can be off by five percent or more due to construction differences. Mode calculations assume the room is a perfect rectangle, and that all six boundaries are infinitely rigid and thus reflect 100 percent of the sound. But most rooms are built with drywall and wood floors that flex slightly and thus absorb, with windows and doors and minor variances that further confound mode predictions. When the room construction absorbs bass, the resonant frequencies are lowered. So building a trap tuned to 60 Hz will be less efficient if the resonance ends up at 56 Hz once the room is completed.

Therefore, broadband bass traps are the most useful type for recording studios and hi-fi listening rooms. Peaks and nulls can occur at all frequencies, not only frequencies related to the room's dimensions. Although most loudspeakers send sound forward, they become omnidirectional at low frequencies. A loudspeaker 34 inches away from a wall has less output at 100 Hz because 1/4 wavelength (90 degrees) for 100 Hz is 34 inches. Then one octave higher at 200 Hz that same distance creates a peak as large as 6 dB. The distance and frequency are related because it takes time for the sound to reach the wall and return. As you can see in Figure 2, a distance of 1/4 wavelength is the same as 90 degrees of phase shift. The sound travels 90 degrees to the wall, then 90 degrees back, arriving 180 degrees out of phase. This causes the waves to cancel and create a null. At 200 Hz the distance is 1/2 wavelength so the round trip is 360 degrees (in phase again) which creates a peak. Additional peaks and nulls occur at related frequencies with another null at 300 Hz, another peak at 400 Hz, and so forth at 100 Hz multiples. Nearby frequencies are also affected a lesser amount.

This phenomenon is called Speaker / Boundary Interference Response (SBIR) and is explained more fully on my Frequency-Distance Calculator. Again, SBIR peaks and nulls are related to distance, and have nothing to do with room dimensions and modes. Since loudspeakers are often moved for various reasons, trying to reduce SBIR using bass traps tuned to a specific frequency is futile. Further, SBIR occurs with all sound sources positioned near a boundary, not just loudspeakers. It also occurs at predictable distances from sound receivers (your ears or a microphone). Depending on how many sound sources and receivers are in the room, and how far each is from every room surface, there are likely dozens of individual problem frequencies.

The rear wall behind listeners is another place broadband bass trapping is needed. If the seating is closer than about ten feet from that wall, the reflections are considered “early,” which creates peaks and nulls related to that distance rather than sound like an echo. So absorption on that wall is always useful. Let's say a listener's head is six feet in front of the wall. That yields a deep null at 47 Hz, a peak at 94 Hz, another null at 141 Hz, another peak at 188 Hz, and so forth at 47 Hz multiples. Now, if you move the couch back one foot closer to the wall the frequencies instead start at 57 Hz and continue at 57 Hz multiples. Since people sometimes move their furniture, this again shows why broadband bass trapping (and broadband absorption generally) is best for most rooms.