# Acoustic Glossary

## Sound Pressure Terms and Definitions ...

### Sound Pressure (P)

is the change in the local air pressure when a sound wave passes through. The air particles 'assist' the transmission of the sound wave and then return to their original state i.e. no net movement of any media particles after the sound transmission. Sound level meters measure changes in the atmospheric pressure of the 'local' air due to sound waves passing through.

Sound Pressure is a field quantity also known as a root-power quantity, not a power quantity.

Sound pressure unit is the pascal (Pa), however the range of our hearing, 0.00002 pascals up to 200 pascals means the pascal is not practical for everyday use. The sound pressure level in decibels, solves this problem.

Sound Pressure Definition (IEC 801-21-20) root mean square of the instantaneous sound pressures over a given time interval, unless specified otherwise.

Sound Pressure Relationships:
Sound Pressure = sound intensity ÷ particle velocity.
Sound Pressure = (sound intensity × acoustic impedance).

Sound Pressure = particle velocity × acoustic impedance.

### Sound Pressure Level (SPL)

is the sound pressure measured in decibels, to compress the immense range of pascals we can hear into manageable numbers. See our sound pressure levels table below for some examples.

Sound Pressure Level Formula = 20 log (p/po) where p is the sound pressure and po is the reference sound pressure of 0.00002 pascals = 0 dB the threshold of hearing, in air at 1 kHz.

Sound Pressure Level Unit is the decibel (dB).

Sound Pressure Level Definition (IEC 801-22-07) logarithm of the ratio of a given sound pressure to the reference sound pressure in decibels is 20 times the logarithm to the base ten of the ratio.
Note 1 : unless otherwise specified, the reference sound pressure is 20 μP for airborne sound and 1 μPa for sound in media other than air.
Note 2 : unless otherwise specified, the sound pressures are understood to be expressed in root-mean-square values.

Sound Pressure Level is a sound field quantity and uses the 20 log factor so, as a rule of thumb:
6 dB = a factor of 2 in sound pressure levels (double or half)
10 dB = a factor of 3 in sound pressure levels
20 dB = a factor of 10 in sound pressure levels.

The following table demonstrates the huge range of pascals we hear and the more practicable sound pressure levels in decibels.

Sound Pressure Level Table
 Sources at 1 m Sound Pressure SPL re 20 μPa Rifle 200 Pa 140 dB Threshold of pain * 20 Pa 120 dB 2 power mowers 2 Pa 100 dB 1 power mower 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 Reference sound pressure 0.00002 Pa 0 dB

Sound Pressure Related Terms - listed alphabetically.

### Band Sound Pressure Level

is the sound pressure level in a standard, frequency band, usually of one octave or 1/3 octave bandwidth.

Band Sound Pressure Level Definition (IEC 801-22-12) level of the sound pressure produced within a specified frequency band.

Note : the band may be specified by its lower and upper cut-off frequencies, or by its geometric centre frequency and bandwidth. The width of the band may be indicated by a modifier such as one octave band sound pressure level, one-half octave band level, one-third octave band level.

### Effective Sound Pressure

is the root-mean-square of the instantaneous sound pressure, also known as the average sound pressure.

### Equivalent Continuous Sound Pressure Level

is the time-averaged sound pressure level over a stated measurement period in decibels.

Equivalent Continuous Sound Pressure Level Definition (IEC 801-22-11) logarithm of the ratio of a given root-mean-square sound pressure during a stated time interval, to the reference sound pressure. Average sound pressure level in decibels is 20 times the logarithm to the base ten of that ratio.
Note : unless otherwise specified, the reference sound pressure for airborne sound is 20 μPa.

Not to be confused with the Equivalent Continuous Sound Level (IEC 801-22-16).

### Instantaneous Sound Pressure

is the sound pressure at a particular instant in time and specific point in the sound field.

Instantaneous Sound Pressure Definition (IEC 801-21-19) at a point in a medium, is the difference between the pressure existing at the instant considered and the static pressure.

### Peak Frequency-weighted Sound Pressure Level

is the maximum instantaneous sound pressure level during a measurement, also known as the peak sound level.

Peak Frequency-weighted Sound Pressure Level Definition (IEC 801-22-15) greatest instantaneous value of a standard frequency-weighted sound pressure level, within a stated time interval.
Note : if frequency weighting is not specified, the A-frequency weighting is understood.

### Peak Sound Pressure

is the maximum instantaneous sound pressure during a measurement period or noise event. Also known as Lpeak but should not be confused with the Lmax.

Peak Sound Pressure Definition (IEC 801-21-21) greatest absolute instantaneous sound pressure during a given time interval.

### Reference Sound Pressure

(Po) = 20 μPa ≡ 0 dB, the threshold of hearing at 1 kHz in air.

Reference Sound Pressure Definition (IEC 801-21-22) the sound pressure conventionally chosen, equal to 20 μPa for gases and to 1 μPa for liquids and solids.

Sound Pressure Inverse Square Law

### Sound Pressure Reference Coefficient

, when a plane sound wave reaches the boundary between two media, part is reflected and part is transmitted, the portions are frequency dependent.

Sound Pressure Reflection Coefficient Definition (IEC 801-31-05) at a given frequency and for a given angle of incidence, in plane waves, ratio between the sound pressure amplitude of the reflected sound wave and that of the incident sound wave.

Time-averaged Sound Pressure Level

### Weighted Sound Pressure Level

is the frequency and time weighted sound pressure in decibels and is also known as the sound level.

Weighted Sound Pressure Level Definition (IEC 801-22-14) logarithm of the ratio of a given sound pressure to the reference sound pressure of 20 μPa, the sound pressure being obtained with a standard frequency weighting and with a standard exponential time weighting. Sound level in decibels is twenty times the logarithm to the base ten of that ratio.

Note 1 : standard frequency weightings  A, B and C and standard exponential time weightings fast (F), slow (S) and impulse (I) are given in IEC 651 sound level meters.

Note 2 : time and frequency weightings employed should be specified, but if not stated explicitly, Fast (F) exponential time weighting and A-frequency weighting are understood.

See also Sound LevelsLA • LAF • LAFmax • LAFmin • LCpeak etc., and the IEC Definition of Level 