# Acoustic Glossary

## Sound Energy Terms, Definitions, Units, Measurements ...

Sound Energy is the energy present in a sound field, the unit is the joule (J) = watt·seconds and is transmitted from the sound source by the sound waves.

Sound Energy causes mechanical vibration in any media a sound wave passes through, including our ears, which convert the sound energy into the sounds we hear.

Sound Energy Definition ASA 2.67, total energy in a given part of a medium minus the energy that would exist at that same part of the medium with no sound waves present. Unit, joule (J).

Sound Energy is related to the sound pressure, particle velocity and the density of the media.

We can hear sound energy levels as low as 0.000000000001 joules, so when discussing sound energy the joule is not a useful descriptor. The solution, used in other branches of acoustics, is the decibel (dB) which reduces the wide range of sound energy levels into manageable numbers, as discussed in the following section.

Sound Energy Level is the logarithmic ratio of the sound energy to the reference sound energy.

Sound Energy Level Lw = 10 lg (w/wo) dB, where w is the sound energy and wo is the reference sound energy level of 10-12 joule ≡ 0 dB

3 dB is a factor of 2, twice or half the energy.
10 dB is a factor of 10 increase or decrease in the sound energy.
20 dB is a factor of 100
40 dB is a factor of 10000 etc.,

### Related Terms - listed alphabetically

Instantaneous Kinetic Sound Energy Density Definition IEC 801-21-40, half the product of the density of the medium and the square of the velocity of a particle in the said medium.

Instantaneous Potential Sound Energy Density Definition IEC 801-21-39, half the quotient of the square of the instantaneous sound pressure by the modulus of elasticity of the medium.

Reference Sound Energy (Wo) = 10-12 joule ≡ 0 dB
Reference Sound Energy Density (wo) = 1 pJ/m3 = 10-12 J/m3 ≡ 0 dB

Sound Energy Density (w) is the time-averaged sound energy per unit volume, the SI unit is J/m3 (joule per metre-cubed).

Sound Energy Density Definition IEC 801-21-41, sum of the instantaneous potential and kinetic energy densities and is also known as the total energy density

Sound Energy Density Level is the logarithmic ratio of the sound energy density to the reference sound energy density

Sound Energy Density Level (LE) = 10 lg (E/Eo) dB, where E is the sound energy density
and Eo is the reference sound energy density level = 10-12 J/m3 ≡ 0 dB

20 dB is a factor of 100
40 dB is a factor of 10000 etc.,

Sound Energy Flux (J) the rate of flow of sound energy through a specified area. SI Unit is the watt (W).

Sound Energy Flux Definition IEC 801-21-37, time average product of the in-phase components of the instantaneous sound pressure and the volume velocity through the surface element considered, and is also known as the sound power through a surface element

Sound Energy Flux Density (I) is the average sound energy flowing through a specified point and the SI units are watts per square metre (W/m²).

Sound Energy Flux Density Definition IEC 801-21-38, sound energy flux in a specified direction and sense through an area perpendicular to that direction, divided by the area, and is also known as sound intensity and sound power density

Sound Energy Flux Density Level Definition IEC 801-22-06, logarithm of the ratio of a given intensity of sound in a stated direction to the reference sound intensity. Such intensity level in decibels is ten times the logarithm to the base ten of the ratio, and is also known as the sound intensity level.
Note : unless otherwise specified, the reference sound intensity is 1 pW/m2