Bass Traps

or

How to Cheat the Laws of Physics

The standard answer to managing bass issues is to put bass traps in the corners of your room. Let’s step back a minute to analyze this. What is the problem? The problem is too much bass (boomy sound) and/or too little bass (thin sound) at specific frequencies. What’s the cause? Room reflections meeting up with the original waves either constructively, summing to make them twice as loud as produced by the speaker, or destructively, canceling the sound. How can we control this? Obvious, get rid of the reflections. There are three ways to absorb sound waves; resistive absorption, fibrous material that traps air converting the acoustic energy into heat; mechanical absorption, a membrane that moves with the wave to absorb its energy (like a speaker driver in reverse, or the shock absorber on a car); acoustical absorption, a resonant cavity to dissipate the energy (like a ported enclosure in reverse). Mid to high frequencies are easy to absorb with fibrous materials, carpet, foam, fabrics. Low frequencies can be absorbed too, except they require huge amounts of resistive material, not millimeters, not centimeters, but much more. The longer wavelengths require thicker and denser materials. For effective control of bass frequencies below 100 Hz, wavelengths longer than 4 meters (13 feet) resistive materials would take up meters of space—not very practical. Corner bass traps have nowhere near the needed absorption capacity necessary unless they are very large. Other types of bass traps are designed to mechanically dampen. They need to be large enough to absorb a sufficient amount of the low frequencies, essentially covering an entire wall. In addition, the interior of the trap must have enough resistive material to dampen the internal energy. One hundred millimeter (4″) mechanical traps are ineffective below the upper bass region, about 150-200 Hz. That’s not good enough to manage the trouble area below 100 Hz. Acoustic absorbers, Helmhotz resonators, need to be carefully tuned for each room and for each of the problem frequencies in the room. You can’t buy an off the shelf resonant absorption unit. Another non-acoustical technique for managing bass is equalization. It can help with boomy bass by reducing the output of the source, but even with that, because summing and nulling varies from point to point within a room, can only fix the problem at one listening position. And no amount of EQ can overcome a null. Bass problems come down to room size. Rooms with dimensions less than a full wavelength will always have problem frequencies. Eliminating the problem requires a VERY large room with each dimension greater than 15 meters (50 ft) to accommodate the longest waves, or a room large enough to accommodate the custom engineered products built specifically for your custom engineered room. Translation : $$$$$.

What to do? There is no easy answer. The best results for me have been three fold. First, keep the subwoofers away from the corners—just the opposite of the most common recommendation. Corner placement guarantees every room mode is excited to its fullest, which does result in the loudest bass, but also the most uneven, spotty bass response. Second, use two subwoofers. Multiple sources create more, yet conflicting, interference which helps to produce a more even room response. Third, use EQ below 100 Hz on the strongest peaks. Simply leveling off the peaks helps to even out the in-room bass response. Unless you can enlarge your room, the best and the only real solutions use the laws of physics rather than futile attempts at cheating them. And that’s worth repeating, you can’t cheat—the laws of physics always win.

Learn more at [Parallel Audio]