Acoustics and reverberation time

Hi everyone,

We often talk about room modes or first reflections but few talk about a major reason for treating a room: reverb time.

Reverberation time is the amount of time an audio signal stays active in a room. An anechoic test chamber has no reverb, since only the source signal can be heard. We often measure reverb time with the measure RT60. That is, how long in time until the signal has decayed by 60 dB. We divide this into multiple bands so we can evaluate room treatment in mid-bass, mid, treble separately, but there is also the question of how smoothly it decays. Signals should decay randomly but smoothly. Peaks indicate an echo. Multiple peaks a slap echo. Too little reverb and you end up with a very dry sounding room, with no ambience.

What does this affect? It is like a TV or computer monitor’s pixel. Imagine the screen having a memory of all the previous pixels, like the screen starts to turn grey or blue based on what you saw a few frames ago. So, it blurs the signal. It also colors the signal. A room with excess mid/treble reverb can make every speaker seem like it lacks bass when the reality is that there is too much mid/treble in the room. A side effect of this is that speakers sound harsh when you turn up the volume. Of course, this is subjective, as you can overload a speaker, but when you are using relatively little power and the sound quality changes, it is often excess treble reverb time.

One curious experiment which will make you a believer in reverb time is to treat bare wooden floors between or behind the speakers with pillows or blankets. Why does this help the mid/treble? Well, reverb time. :)

Perhaps now we can imagine why diffusion works. Instead of being pure absorbers, they scatter the sound. They help maintain the reverb time but prevent these coherent, regular reflections. So when we are looking at room treatment we are attempting a combination of many traits. Controlling early reflections, and maintaining a diffuse, rapidly decaying (but not too rapidly) sound field, in addition to managing room modes.

At the gross level, your room acoustics are tone controls. You are playing with the mid/treble balance, and at the finest levels they are helping to localize sounds and provide an enjoyable playing field for your music.

This should also help you understand somewhat why equalizer solutions, including Digital Signal Processing (DSP) based like automatic room correction or DIRAC, etc. can only work up to a point, and why having good room treatment widens the sweet spot, and makes these tools work over a broader physical area.