Introduction

Because the human ear is used to hearing sounds within their acoustical contexts (ie a combination of direct and refelected sound) it is important for us to use reverberation in our recordings. Therefore, a music recordist needs to know ...

How sound behaves and reflects in different environments.
The theories of reverberation.

What is reverberation?

In a room a pattern of echoes will build up shortly after a source has begun to generate sound. Echoes are discrete (individually distinct) sound reflections. Reverberation then, is the combined effect of many discrete echoes. It is the most complex time-domain effect and places a sound within the context of its space.

Direct sound

Direct sound is that which arrives at the listeners ear direct from the source .

Reflected sound (echoes)

Reflected sound is that which arrives at the listeners ear after bouncing off a surface or object.

Ambient sound

In all acoustical spaces (rooms) there will be a mixture of direct sound and reflected sound (also called the diffuse or reverberant sound). This mixture is called the Ambient sound.

Near field

Close to the source the direct sound will dominate. This area is called the Near field.

Far field

As the listener moves away from the source the ratio of direct sound to reflected sound changes. At a certain distance the amplitude of the reflected sound may become equal, or greater than the amplitude of the source. This area is called the Far field.

Critical distance

At a certain distance away from the source the direct and reflected sound are equal in amplitude. This point is called the critical distance.

Reverb fields

What happens to sound when it hits an object or surface

When a soundwave hits a surface or object 4 things can happen to it ...

It is reflected away again
It is absorbed
It can travels (or be transmitted) through or bend around it
A combination of 1,2 & 3

What determines what happens to a sound when it hits an object or surface

Whether a sound is reflected, absorbed, transmitted or bends round is entirely dependent on ...

The characteristics of the surface
The size of an object
The thickness of a surface (such as a wall)
The wavelength of the sound

The characteristics of the surface

The type of surface on a wall or object will have an effect on what happens to a soundwave when it hits it. For example, a tiled wall will reflect sound better than one covered in wallpaper. A curtain hung in front of a wall will absorb high frequency sound but allow mid and low frequencies through to be reflected (see the effect of wavelength later).

The size of an object

Sound can bend around objects but the bigger an object is the harder it will be for sound to do this.

The thickness of a surface (such as a wall) and a sounds wavelength

When an object is large in relation to a sound waves wavelength it will be partially reflected and partially absorbed. So for example, high frequency soundwaves at 20kHz (with a wavelength of 1.7cm) will be easily reflected or absorbed by soft furnishings because they will be thicker than 1.7cm.

In short, high frequency sound is more easily absorbed than mid and low frequency sound.

The same wall would not be able to entirely absorb soundwaves at 20Hz (with a wavelength of 17 metres) so they would be partially transmitted through. This explains why low frequency sound is more obtrusive and carries further.