Sound Advice

Sound Advice
By Anthony Grimani

The New Hotness
If you’re like me, you always want to be up-to-date on the latest hot topics people are talking about. These days, High Definition TV, DVD, Surround Sound, and Home Theater in general are pretty much at the top of that list. In fact, you may even be considering a home theater yourself. Most people begin their home theater journey by shopping at a local custom electronics dealer, and that’s good. In dealing with a custom electronics installer, though, there’s a topic that won’t always come up. That topic is acoustics, or the way that sound behaves in a home theater after it has left the speakers. Yes, the room itself has a profound impact on what people inside and outside actually hear! If you are thinking about delving into the wonderful world of home theater, you should know some things about acoustics before you take the plunge.

Get…Isolated
A good home theater can produce bass sounds as loud as 115dB, which is comparable to a live rock concert. Unfortunately, normal wall construction only attenuates bass by 10dB. The residual bass in rooms adjoining the theater is 105dB, which is only as loud as…a NYC subway train! Unless you want your home theater to become a bone of contention in your household, you must keep the sound inside the theater. That, my friends, is called Sound Isolation.

In addition to containing sound inside a home theater, sound isolation keeps unwanted or distracting sound out of the theater. A car horn? The maid’s vacuum cleaner? They remind you that you’re in a home theater rather than a starship, an ancient extra-terrestrial burial ground, a sidewalk café in Paris, or anywhere else a filmmaker wants to take you.

Sound isolating a room is difficult. It involves engineered design, as well as special building materials and careful construction. Fortunately, there are many acousticians out there, such as those at my firm, who know how to do sound isolation effectively.

Shh! Do You Hear That?
Even with external noises blocked by sound isolation, there are still things inside a home theater that make noise. The most notorious offenders are refrigerators/ice makers, HVAC systems, and video projectors. Those noise sources, like car horns and vacuum cleaners, distract your attention away from a movie and even mask subtle sonic detail, so something has to be done about them. In acoustics, eliminating all the background noises in a home theater is called Noise Control. Fortunately for you, an acoustician who does sound isolation can probably handle noise control, too.
The Inside Treatment

Now you get to the really fun part of acoustics - how sound behaves inside a home theater. What you actually hear in the room is a combination of the direct sound from the speakers and all the sound reflections from the walls, floor, and ceiling. Acousticians generally classify these reflections using three categories: Discrete Reflections, Echoes, and Decay Time. Discrete Reflections come from one surface (like a wall) and interfere with the direct sound from a speaker by degrading it. Echoes, on the other hand, occur when sound bounces back and forth between two wall surfaces, obscuring details in the sound field. Decay Time is the amount of time required for all sound reflections to die away. In most home theaters, the Decay Time is too long, leading to a confused sound field.

Another major sonic issue is Bass Resonances. Any room can be thought of as a resonating cavity - similar to a very large acoustic guitar. The Bass Resonances that develop in rooms cause certain bass sounds to be much louder and decay much more slowly than others.

Fortunately, acousticians can use special acoustic treatment products to control Discrete Reflections, Echoes, Decay Time, and Bass Resonances very effectively. My company, for example, offers a range of acoustic treatments to suit any aesthetic. Proper shaping and proportioning of a home theater room is frequently used in conjunction with acoustic treatments to further reduce the deleterious effects of reflections and resonances.

Acoustics = Enjoyment
The fact is that truly excellent home theaters are separated from the pack not by the quality of their A/V equipment, but by the acoustics of their rooms. The sonic effect of room acoustics is not subtle; anyone can hear it. It doesn’t take a Grammy-winning recording engineer to hear the difference between a treated and untreated room. Acoustics must be considered for a high-performance home theater, but the trick is that they are the key to the enjoyment of a home theater as well. Performance may not be that important to you, but I’ll wager enjoyment is. Why else would you want to put a theater in your home?

Anthony Grimani is president of Performance Media Industries, a California-based acoustical engineering firm specializing in home theater design and calibration. He can be reached at agrimani@pmiltd.com.