Echo Chamber: Room Acoustics Part 2

In part one of this series, I mentioned that as soon as I walked into my new apartment, I knew I would have to put in some significant acoustic treatments for my system to sound good. How did I know that? One word: echo. You don’t have to be an acoustician to understand how echo works, and how it can completely ruin any possibility of good sound, even with a million dollars worth of equipment. Ever tried having a conversation with someone in a concrete garage? All of those hard, flat, highly reflective surfaces can allow a sound to bounce around for several seconds before it finally decays below the threshold of audibility, making the original sound difficult for the brain to comprehend.

As I mentioned last time, no reflection as in an anechoic chamber isn’t desirable. You want some reflection for music to sound best, not too much, not too little. There’s a Goldilocks zone that acoustic treatments can help you reach, but we’ll get to that later. First, let’s start with the foundation.

One of the biggest problems you often run into when dealing with room acoustics are what are known as room modes, which are naturally occurring resonance frequencies directly related to the length, width, and height of the room. You can find out roughly where they will exist in your own space by plugging the dimensions of your room into a room mode calculator. The numbers won’t be exact because the calculator doesn’t account for things like doors, windows, the density of your walls, etc, but it will at least give you a rough idea of where the modes will be in your space.

Room modes are created when a sound wave travels between two opposite boundaries, for example the left and right side walls or the floor and ceiling. The first modal resonance occurs at the frequency where the distance between the two boundaries is equal to half of a wavelength. A room that’s 16 feet long for example has a mode at 35Hz. Additional modes occur at multiples of 35Hz, because those frequencies also resonate in the same space. A room long enough for a cycle at 35Hz can fit two cycles at 70Hz, three at 105Hz, etc.

Room modes can cause both peaks and nulls in frequency response. When two or more waves meet and are in phase with each other at a specific frequency, you will have a peak in response. When they meet and are out of phase with each other, they cancel and you end up with a null in response. This incidentally is how noise cancelling headphones work. They generate a signal out of phase with the exterior noise, and cancel it out. This is why you can’t fix a null in response simply by increasing the volume at that frequency using EQ. Both the in phase and out of phase signals increase in intensity, and they continue to cancel each other out.

Room modes can’t be eliminated, but their effects can be reduced using bass traps. Bass traps don’t actually “trap” anything, as low frequency audio waves are far longer any any practical bass trap – a 20Hz sound wave for example is 56.6 feet long. What bass traps do instead is reduce the intensity of interfering waves by converting some of that kinetic energy to heat. At least that’s the idea. Unfortunately, things aren’t quite that simple.

Some of the most problematic modes in small to medium sized rooms occur at around 50-65Hz, and a typical 4” thick bass trap is almost completely useless at those frequencies. Why? As I mentioned earlier, bass traps (well, most of them) work by reducing the intensity of interfering waves by transforming their kinetic energy into heat – that is, slowing down the air particles, and they are most effective when the air particles are at maximum velocity. Unfortunately, the velocity of the particles directly at the wall is zero, increasing to maximum at a distance of ¼ of the wavelength. For a problematic 60Hz sound wave, the wavelength is 1125/60, or 18.8 feet. A quarter of that is 4.7 feet. You can therefore see why having just 4” of insulation off the wall might not be very effective, and why a 56.6 foot long 20Hz wave is practically impossible to do anything about.

The 60Hz wave you can do something about however, and there are two ways to go about it using this kind of bass trap. The first is to create an air gap behind the panel, putting it closer to the range where it will be useful at slowing down the air velocity. You can do this by straddling a corner (where bass frequency buildup is most problematic) or simply by having it sit free standing away from the wall. The second way is to go big – really big. Bigger the better. There are a number of large, dedicated corner bass traps on the market, but I chose to go with GIK Acoustics’s Soffit traps, which are a healthy 17” x 17” x 47,” and are very effective down to around 55Hz, with some absorption that rapidly rolls off below that. When it comes to corner bass traps, there is no replacement for displacement, at least when you’re talking about kinetic energy traps. There’s another type of trap that deals with pressure, which I’ll talk about in part three. Stay tuned.