# 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 measured in
Pascals - symbol Pa, units
Newton per square metre.

1 Pa = 1 N/m^{2} = 1 J/m^{3} = 10^{-5} bar = 1 kg/(m·s^{2})

# 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

**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

**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.