(DI) The directivity index is the value in dB of ten times the common logarithm of the directivity factor. The directivity factor is the ratio of the sound intensity produced by a test transducer on a specific axis to that of a point source that is emitting the same acoustic power. Since the specific axis is usually one of maximum radiation, the directivity index is usually greater than zero.

See also dB/dt.

Efficiency is defined as the ratio of the acoustic power generated to the total electrical power input. Efficiency varies with frequency and is expressed as a percentage.

Two quantities (x, y) are called proportional if they vary in such a way that one of the quantities is a constant multiple of the other, or equivalently if they have a constant ratio (k = y/x). This constant ratio (k) is called the proportionality constant or constant of proportionality of the proportionality relation.

See also Amplitude, Acoustic Power, and Attenuation Coefficient.

The source level is the acoustic power (sound pressure) in decibels (dB) referenced to 1.0 microPascal measured at 1 meter from the sound source transmitted through a unit area in a specified direction.
The unit of intensity is watt per square meter, but source level is commonly given as a relative intensity in dB.

(SA) The spatial average intensity is the acoustic power within the beam, divided by the beam area.