Acoustic sound control is one of the most misunderstood elements of our industry. The truth is, it’s a lot easier than we imagine and often easier than installing a simple hi-fi system. So why do some contractors shy away from this moneymaking opportunity? Familiarity is likely the number one reason. Many integrators have been installing audio since they were teenagers, but acoustics appears to be some magical mystery that requires conjuring up spirits to get right.
This is not to say that all acoustic projects are easy. Managing bass is often difficult. And designing a world-class concert hall requires a serious level of expertise. What we are talking about here is using acoustic panels to improve intelligibility in a typical home theater, man cave, restaurant or boardroom.
Clap Your Hands!
Yes, clap your hands and if you hear a long, trailing echo you have what is referred to as room chatter. This is caused by sound reflecting off hard surfaces, creating a dense reverberation that makes it difficult to hear the message. We all know how frustrating it is to be in a loud restaurant as you try to converse with someone across the table. The human brain is, in fact, capable of doing so by ignoring unwanted sounds, but this requires tremendous effort, which can be exhausting. When we leave these venues, we usually come to the conclusion that we will never eat there again. This is particularly problematic as we age because our hearing becomes less acute and makes the experience all the more painful.
Absorb the Echo
When sitting across from someone, we can use facial expressions to fill in the blanks and our ears can ignore unwanted information. This is known as the cocktail effect. On the other hand, microphones are not so selective. They pick up whatever energy is in the room and then reproduce it. For more than 100 years, broadcasters have been using absorptive panels to eliminate powerful first and second order reflections that otherwise would make it nearly impossible to hear the orator. The same logic applies to video conferencing and you can be certain that your local movie theater has acoustic treatment on the walls.
Sound penetrates the porous panel causing the inner fibers (semi-rigid glass wool) or membranes (open cell foam) to vibrate, converting excessive acoustic energy into heat. Most acoustic panels come in various thicknesses, 2-inch being the most common. As a general rule the thicker the panel, the lower the frequency it will absorb. The performance of the panel varies depending on density. High-density, 2-inch thick, 6 lb per cubic foot glass-wool panels tend to provide the most even absorption across the spectrum and can be effective down to 75Hz for music playback. Thinner, 1-inch panels usually work well for absorbing energy in the voice range. You should also be concerned about fire safety and local laws. Most commercial buildings or high rises require class-A fire-rated panels to be used for insurance purposes.
How Much Absorption Do I Need?
Place a small acoustic panel in a gymnasium… nothing happens. Put a million panels in the same room and it will sound like an anechoic chamber. Wall coverage follows a bell curve whereby 15% to 30% wall coverage will provide enough absorption to control the echo and bring the reverberant field under control. Depending on the type of room and the application, you may require more or less absorption.
Recording studios and broadcast stations tend to require a lot more absorption than a home listening space. In a studio, the engineer is making surgically accurate decisions on mic placement, equalization, spatial positioning and bass. To hear the subtleties, you have to be very comfortable with your listening space and how well the room will allow the mixes to translate to other rooms.
With home theater or two-channel listening spaces, applying this level of absorption can deaden the room to the point where it is no longer comfortable. You can actually lose the excitement of the performance. A good rule of thumb is 25% absorption on the walls. Start by placing the panels strategically where powerful first-order reflections will collide with the sound coming from the loudspeakers. Then look to the rear wall where panels can be distributed to cut down on some of the front-to-back room chatter.
Low frequencies are omnidirectional, while high frequencies tend to beam. In acoustical terms, high frequencies are easy to manage while bass is the elephant in the room. It is also important to note that absorptive panels will not stop sound, they will merely reduce the energy in the room. Controlling sound from escaping a room requires mass, decoupling and other tricks that we will discuss in future articles.
Bass or low-frequency waves produced by subwoofers or full-range loudspeakers will also echo off walls and create hot spots known as room modes. These occur when two identical waves collide in phase, adding together. When out of phase, the same two waves can create dead spots as they cancel each other out. The solution is simple: add bass traps.
Bass traps attenuate the low frequency energy in the room, thus reducing the cancellations or additions. These come in various sizes and shapes. One of the most effective is a diaphragmatic membrane. Developed by the BBC (famous British broadcaster) these employ a large heavy membrane that moves when powerful bass energy is present, somewhat the same way a microphone diaphragm captures sound. The membrane vibrates, converting sound energy into heat, resulting in the same thermo-dynamic transfer as detailed above. The particularly cool thing about a membrane is that it is not frequency specific. In other words, it is not limited to absorbing certain frequencies like a rigid membrane or Helmholtz resonator. Because the walls act as waveguides–like a large horn–energy tends to gather in corners. This makes corners a particularly effective location to mount bass traps. As with any installation, budgets play a role. You also have to use good old-fashioned common sense to consider the options. Applying acoustic panels into a living room may not pass the “wife acceptance factor” unless you can disguise them as art. But if you have a dedicated listening room or home theater, proper acoustic treatment can elevate the experience so that it better approaches a real theater.