The following article is an excerpt from Shure Notes / Musician, Issue #48 (September 1, 2011).
Ever think about what good sounds really means? You’re probably thinking good material, excellent musicianship, and the right equipment, but it’s really a little more scientific than that.
Most problems in live performance are directly related to fidelity, intelligibility, and loudness. If one or more of these basic measures of sound quality isn’t right, then your audience can’t really hear the music you’ve worked so hard to perfect. Let’s look at them, one by one.
Is it true? This is mostly determined by the overall frequency response of the sound arriving at the listener’s ear. It must have sufficient frequency range and uniformity to product realistic and accurate speech and music. All parts of the audio chain contribute to it: a limitation in any individual component will limit the fidelity of the entire system.
Is it understandable? This is a function of the overall signal-to-noise ratio and the direct-to-reverberant sound ratio at the listener’s ear. All this really means is that the “signal” — which is the desired sound from the sound system — must be at least 20 decibels louder than the noise and reverberation level at the listener’s ear to be intelligible.
What makes a room “live” or “dead”? Here’s where direct-to-reverberant ratios comes in. It’s determined by the acoustics of the room and the direction of the loudspeakers. Reverberation time is the length of time that a sound persists even after the sound source has stopped. A high level of reverberant sound interferes with the intelligibility of the sound since your audience won’t be able to determine where one sound stopped and another started. On the other hand, a very low level of reverberant sound can create a lifeless acoustic environment: a dead room.
Most musicians find this concept the easiest to understand and apply: optimum volume levels must be achieved without unwanted distortion or feedback. A sound system used by a rock-and-roll band demands that attention be paid to Potential Acoustic Gain; in other words, the amount of amplification that can be delivered before the screeching howl of feedback occurs. The position of microphones and loudspeakers — as well as room acoustics — all play a role.