In February we talked about Plasma Display Myths. This month we'll discuss whole house music or distributed audio. Since our radio discussions this month are on this, I thought a little more information might be helpful.
There are numerous approaches to distributed audio but basically they break down to 3 categories:
- Traditional analog sound distribution
- Hybrid analog/digital
- Digital networked
We will discuss each of these in turn.
First, let's point out a few facts:
- All sound is analog when you hear it. We can store it digitally or send it around digitally but when we hear it, it's analog. In the end it has to play through speakers which are motors that move air.
- Room acoustics, how sound behaves, is just as important to distributed sound as it is to theater sound.
- There are many common mistakes.
- Pure analog approaches do not necessarily produce better sound than digital approaches. In many cases the opposite is true.
- Distributed audio can't be done on the cheap.
Second, let's talk about what and what not to do. Some of this is technical, some is practical.
- Volume controls
Many people put in whole house music by running speaker wire from some kind of source, say an amplifier, and then put volume controls in the speaker lines. These controls are usually knobs or sliders on a wall. Bad idea. This is done to save money but it is very hard on equipment and sound quality.
The input from a sound source (radio, cd player, media server…) is usually around one volt. It varies a little depending on how loud the music is. The output from an amplifier, driving speakers, is anywhere from 5 to 20 volts or even more. We can control volume from the source with small changes, or we can control it on the way to the speaker with by throwing away lots of energy.
An amplifier expects to see a certain impedance or resistance from a speaker. When it sends energy directly to a speaker it sees that amount. A speaker-line volume control can only cut down on that energy by throwing some of it away. In other words it can only reduce volume. So the amplifier must always play loud. Loud takes more energy (and it can distort the sound). If the volume control changes the impedance, it can actually draw more energy from the amplifier and make it work too hard. The amplifier will cook - burn out. The volume control tries to keep the resistance the same (impedance-matching) but it doesn't work very well. It creates heat when it throws away energy. Just a few weeks ago, we had to repair two amplifiers and eliminate the volume controls from a system in a restaurant. The sound was distorted and the amplifiers kept heating up. Two of three burned out.
The proper way to control volume is by increasing the source voltage when we want more, not decreasing the output voltage when we want less. This can be done with controls i