Sound Pressure : Definitions, Terms, Units and Measurement

Sound Pressure Level : SPL is the RMS value of the Instantaneous Sound Pressures measured over a specified period of time, measured in decibels (dB).

Sound Pressure : is the difference between the pressure produced by a sound wave and the barometric (ambient) pressure at the same point in space, symbol p or p.

Sound Pressure is a measure of Force per unit
Area.

The immense range of human hearing, see the table below, means that absolute sound pressure values are impractical for everyday use, a neat solution is the LogarithmicSound Pressure LeveldB descriptor.

Sound Level : Lp is a logarithmic measure of the RMS sound pressure of a sound relative to a reference value, the Threshold of Hearing. It is measured in decibels (dB).

Lp = 20 lg (p/po) dB re 20 µPa for air ... water, steel, etc., are different.

Some typical sound levels

Sources at 1 m

Sound Pressure

Lp re 20 µPa *

Rifle

200 Pa

140 dB

Threshold of pain

20 Pa

120 dB

Pneumatic hammer

2 Pa

100 dB

6 dB = double the Pa

1 Pa

94 dB

Street traffic

0.2 Pa

80 dB

Talking

0.02 Pa

60 dB

Library

0.002 Pa

40 dB

TV Studio

0.0002 Pa

20 dB

Threshold of hearing

0.00002 Pa

0 dB

* Reference Sound Pressure po in air = 2 x 10^{-5} Pa = 20 µPa = 0 dB

The Reference Sound Pressure was chosen conventionally to correspond to the quietest sound at 1000 Hz that the human ear can detect.

Lp uses the 20 lg equation so, as a rule of thumb:

6 dB = a factor of 2 in sound pressure
10 dB = a factor of 3 in sound pressure
20 dB = a factor of 10 in sound pressure

Time Averaged Sound Pressure Level : LAT : the equivalent steady level over a given period of time that contains the same amount of noise energy as the actual fluctuating level.