Acoustic Glossary


 

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.

Sound Pressure is a Sound Field quantity and not a Sound Energy or Sound Power quantity.

Sound Pressure is measured in Pascals - symbol Pa, units Newton per square metre.
1 Pa = 1 N/m2 = 1 J/m3 = 10-5 bar = 1 kg/(m·s2)

Sound Pressure Level Measurement


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 Logarithmic Sound Pressure Level dB 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
 
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see other : Standard Reference Levels.

Effective Sound Pressure : more commonly known as the RMS - Root Mean Square of the Instantaneous Sound Pressure over a given period of time.


Instantaneous Sound Pressure : p : is the change in ambient pressure caused by a sound wave at an instant in time.


Peak Sound Pressure : Lpeak or Lpk : greatest absolute instantaneous sound pressure during a given time interval. Not to be confused with Lmax

see also : Time Constants

Sound Pressures measured or 'averaged' over time are known as prms or
Integrating-averaging Sound Levels
Leq, Equivalent Levels
Statistical, L10, L90, etc., Levels


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.

also known as Leq - Equivalent Continuous Sound Level


Related Terms :
Leq
Sound Energy
Sound Intensity
Sound Power