Acoustic Glossary

Sound Energy : Definitions, Terms, Units and Measurements

Sound Energy is the amount of energy available in a Sound Field and is related to the Sound Intensity, Particle Velocity and Sound Pressure as explained below.

The SI units of Sound Energy are joules = watt seconds.

Energy : J :   = Force x Distance = Work, a quantity equivalent to the capacity of a physical system to do work; the SI unit of energy is the same as the unit of work - the Joule J and is a Scalar quantity.

1 J = 1 N·m = 1 kg·m2/s2 = 1 W·s = 107 ergs

Energy Flux Density : sometimes known as Intensity

Energy Spectral Density : ESD : describes how the energy of a signal or a time series is distributed with frequency.

Sound Energy : W : is the energy present in a sound field.

W = Wo · 10LW/10 J  ....  the units are Joules = Watt seconds

Standard Reference Sound Energy Wo = 1 pJ = 10-12 J = 0 dB

Sound Energy Level : Lw = 10 lg W/Wo dB re 1 pJ or 10-12 Joule.

Sound Energy Density : E : is the Time-Averaged Sound Energy per unit volume, the units are J/m3 or W·s/m3

E = I/c : where I is Sound Intensity in W/m2 and c is the Speed of Sound in m/s.
E also equals Sound Particle Velocity squared x Sound Pressure.
E is the sum of the instantaneous Kinetic Energy Density and Potential Energy Density.

Once again because of the wide range of the human ear the absolute values of Sound Energy Density are enormous. The decibel ratio to a threshold level produces manageable values - hence the

Sound Energy Density Level : LE = 10 lg (E/Eo) dB re 10-12 J/m3 or W·s/m3

Standard Reference Sound Energy Density Level = 1 x 10-12 J/m3
  0 dB = 1 x 10-12 J/m3
20 dB = 1 x 10-10 J/m3
40 dB = 1 x 10-8 J/m3
20 dB is a factor of 100
40 dB is a factor of 10000

Kinetic Sound Energy Density : half the product of the Density of the medium and the square of the Particle Velocity in the said medium.

Potential Sound Energy Density : half the quotient of the square of the Instantaneous Sound Pressure by the Modulus of Elasticity of the medium.

Total Sound Energy Density : sum of instantaneous potential and kinetic energy densities.

Sound Energy Flux : q : SI units : W/m2 = J s/m2
is the time average product of the in-phase components of the Instantaneous Sound Pressure and the Volume Velocity through the surface element considered. Also known as sound power through a surface element.

Sound Energy Flux Density : is the Sound Energy Flux in a specified direction and sense through an area perpendicular to that direction, divided by the area.

Also known as Sound Intensity and Sound Power Density.

Sound Energy Flux Density Level : is the logarithm of the ratio of a given Sound Intensity in a stated direction to the reference sound intensity.

Also known as Sound Intensity Level.

Specific Energy : is the energy per unit mass : joule per kilogram - J/kg

See also : Noise Dose l Sound Exposure
Related Terms : Leq l Sound Intensity l Sound Power l Sound Pressure