Room Acoustics

Room Acoustic and Sound Treatment Basics

An important consideration for your home theater is the acoustics that can be improved through sound treatments. Whether to and exactly how to treat a room to get better sound is a science that has so many variables that it would take volumes of study to really understand it in depth. To understand how your room affects the sound coming from your speakers you have to understand a bit about acoustics and how sound travels through the room.

Sound emanates in waves from the speakers. The waves bounce off dense materials such as wood paneling, glass, mirrors, even picture frames with glass or plastic coverings. These reflective materials reflect sound waves back into the room. Absorption occurs with porous materials such as foam, couches, carpeting, or acoustic panels that are designed to absorb sound.

The problem with too much acoustic reflection in a room is that sound waves bouncing around too much will emphasize the treble or mid-highs (the sound waves that alternate the fastest, effectively producing the most waves), creating bright sounding audio. The other danger is cancellation: sound waves 180 degrees out of phase that hit each other en route to the audience. The phase relation will cause the sound waves to simply cancel each other out because they are the equal and opposite polarity of the same frequency and hit the audience at the same time.

Cancellations create holes in the frequency response of your audio. So, too much acoustical reflection should be controlled by speaker placement. Aim speakers at the audience in a way that will limit their sound waves encountering the walls prior to the sound waves reaching the audience. This sounds more complicated than it is. It just means that if you imagine sound emanating in a cone from the speaker, all the cones of sound should converge on the audience at the same time before reaching a wall. Naturally sound will hit walls and floors and there will be some reflections.

But not all reflections are bad. Before you go overboard with acoustic treatments and try to completely limit acoustic reflections you should also understand that you dont want your audio room to become an anechoic chamber. An anechoic chamber is a room treated so as to be completely free of echo. Theyre used as labs for testing speakers but theyre not good places to listen to your music. Most people actually enjoy a bit reflection -- this is described as giving the sound a lively quality which basically means the middle highs are accentuated slightly, producing the effect of more detail in voices and certain instruments. Acoustic reflections can also augment the sound from certain bass tones and can even promote the rumble effect we all love from movie soundtracks. So its a fine line between some reflections and some dampening.

A word on timing: Earlier we looked at your cones of sound (emanating from your speakers) reaching the audience at the same time. As you know, this is impossible in the real world as room shapes and sizes will prohibit this optimal setup. Fortunately, most Home Theater receivers have timing settings where you can impose micro-second delays from any or all of the speakers. Use this feature to correct the room acoustics for the distances. Usually your surround speakers will be in closer proximity to your audiences ears than your front and center speakers. This means they should get enough delay to allow the front and center speakers to catch up. The result of not doing so may not seem so severe -- in fact sometimes a bit of delay to can make the room seem larger. But the real danger is cancellations. If there is a 1 microsecond (for instance) from the time a sound hits your ears from the surround channel and the time the same sound hits your ear from the front speaker, a certain frequency will be completely cancelled and several other frequencies may also be adversely affected. In our example, whatever frequency takes 1 microsecond to complete half a cycle will be cancelled, and other frequencies divisible by the same number may also be affected.