Recording and Voice Processing, Volume 1

Appendix 1Sound Unit

A1.1. Introduction

The decibel (dB) is the main unit of measurement in the field of audio, but it is not the only one. It is easily associated with the pascal, the volt and others, without counting its variations the dB SPL, the dBu, the dBv, the dBA and so on.

In the following, you will find some details about all these units and the possible shortcuts to switch from one to another.

A1.2. Bel and decibel

When the first physicists and inventors became interested in audio phenomena, which gave rise to acoustics and then years later to electro-acoustics, they very quickly needed a way to quantify and measure the perception of a sound signal.

Alexander Graham Bell (1847–1922), an American engineer and scientist, to whom we owe in part¹ the invention of the telephone, was also the person who established the first unit of measurement of an acoustic phenomenon. His mother’s deafness and his father’s work to teach deaf-mutes (as they were called at the time) to read lips and articulate led him to become fully involved in acoustics.

Bell’s approach was based on the ear and its threshold of audibility. After having chosen a reference frequency, 1,000 Hz, he evaluated this threshold for a human being on a rather large population sample.

His work focused on the minimum and maximum perception levels of a sound signal at this frequency.

On average, for the minimum level perceptible by the ear, the result obtained was close to 10^-12 W/m² or 10^-12 W.m^-2 (watt per square ...