The green line represents the first sound that reaches the listener. This Direct Sound has not been reflected off any surface. It’s the first time the sound is heard. In yellow, the Early Reflections are caused by the sound being reflected from the closest walls to the listener and the sound. 

In red, the Late Reflections of sound are caused by other ‘copies’ of the sound that have reflected off multiple walls before reaching the listener’s ears. After so many reflections, a lot of these reflections have lost energy and are quieter. There are so many late reflections that they start to blend together.

It’s a little easier to ‘hear’ the sound of any room you’re in by simply clapping. In different rooms, that short burst of sound will take a longer or shorter time to decay to nothing. The exact same thing is happening to any sound heard in that room – whether it’s people speaking, objects clattering, or sound coming out of speakers.

Measuring Reverb Time

There’s a generally agreed-on way to describe how reverberant a room is. By measuring how long it takes for the energy from a very short, very loud sound to die off, we end up with a measurement of the reverb time of the room in question. Specifically, we measure how long it takes the sound, in seconds, to fall by 60dB.

We’ll talk more later about reducing the reverb time of rooms, but there are more problems with sound in enclosed spaces that we need to know about first.