TOA bs-1030 Mode D'Emploi

TOA Electronics Speaker Guide

17

The Bel, named in honor of Alexander Graham Bell, was originally defined as the loss of signal 
level over one mile of telephone cable. A decibel is 1/10th of a Bel. Neither the Bel nor decibel 
have an explicit level, but are specified as a logarithmic ratio. 

Sound Pressure Level (SPL) is the acoustic pressure reference for the dB. The minimum 
threshold of undamaged human hearing is considered to be 0 dB SPL. The threshold of pain 
for undamaged human hearing is 120 dB SPL. 

Since the decibel is an expression of relative level change, it can be used to describe volume 
levels in both the acoustical and electrical domains. 80 dB SPL refers to an acoustic volume 
(loudness) level relative to the standard 0 dB reference. Changes in electrical power and voltage 
can also be described in terms of the dB (see Sound System Engineering by Don and Carolyn 
Davis for an in-depth discussion on the use of the decibel in sound system design). The following 
rules of thumb will help properly utilize the decibel in speaker system design:

•

A change of 2 dB SPL in overall volume is the smallest change perceptible to the 
average listener.

•

Increasing the volume by 3 dB requires doubling the amplifier power. 

•

Multiplying amplifier power by a factor of 10 increases SPL by 10 dB.

•

Increasing the level by 10 dB SPL is perceived by a typical listener as doubling the volume.

•

Voltage is not the same as power. Doubling voltage increases volume by 6 dB and mul-
tiplying voltage by 10 increases volume by 20 dB. 

For the mathematically minded: The following equation converts power differences to volume 
changes: level change in dB = 10 * log (P1/P2), where P1 and P2 are the power figures being 
compared in Watts.