Lessons in Sound – Lesson 1: Critical Distance

One of the most important concepts for recording sound is the idea of ‘Critical Distance’.

Wikipedia defines ‘Critical Distance‘ as:

“the distance at which the sound pressure level of the direct and the reverberant sound fields are equal when dealing with a directional source.”

What this essentially means is at the threshold of ‘Critical Distance’ there is an equal amount of direct sound (ie, the sound you hear when you are close to the source) and reverb sound (ie, the sound reflected by the walls of the room).

The amplitude of sound, over a distance, is governed by the ‘Inverse Square Law‘ which states that the decrease in power of sound is inversely proportional to the square of the distance.

Relative Sound Pressure Level (aka, volume) is measured in decibels (dB), a logarithmic ratio which simplifies much of the calculations required for the measurement of the intensity of sound. Without explaining the mathematics behind this concept it is easy to just remember that a drop in 10dB means the volume is cut in half (also, 10dB+ = 2x volume).

Back to the ‘Inverse Square Law’. What this amounts to is that with every doubling of distance there is a drop in 6dB (not quite half). This becomes very noticeable when one considers a singer next to a microphone where a movement of a few cm can nearly double (or half) the volume.

But once you hit the ‘Critical Distance’ threshold, the volume of the sound source will no longer drop and a constant dB level with be maintained.

After the ‘Critical Distance’, there will be no direct sound, and before the ‘Critical Distance’ the volume of the reverb will be much less (although still present, unless very close to the source).

This threshold will be different for every room and microphone.

Thus, when recording audio, it is important to find the ‘Critical Distance’ and decide on a distance which works well.

Here is a video which demonstrates this idea:

Lesson 1: Critical Distance

Posted in Sound | Tagged as: , , , , ,

Comments are closed.