dB(A)
See under A-weighting.

dB, Decibel
The dB is a logarithmic unit (logarithm with base 10) used to describe a ratio. The ratio may be power, sound pressure, voltage or intensity or several other things. In acoustics the sound power P [in W] and the sound pressure p [in Pa] are expressed in dB values as ratio to a defined reference value:

Sound power level in dB

Decay rate
At a stated frequency, time rate at which sound pressure level decreases in a room after the excitation has stopped. For a room with total diffuse sound field (reverberant room) the decay rate d is constant and is related to reverberation time T by T=60 dB.

Directivity index, directivity factor
The directivity factor, Q, is defined as the ratio of the intensity (W/m2) at some distance and angle from the source to the intensity at the same distance, if the total power from the source were radiated uniformly in all directions.

where Iq=Sound intensity at distance r and angle q from the source I=Average sound intensity over a spherical surface at the distance r And the directivity index (DI) is defined as:

Double construction
In building acoustics a partition consisting of two outer membranes and a cavity between, where the interactions between the outer membranes and the cavity determine the acoustic behaviour for the sound transmission. Normally the sound insulation increases by using absorbing material as Rockwool

inside the cavity. The positive acoustic behaviour of a double construction with absorption is partly reduced by sound bridges (like steel frame in gypsum walls), which mechanically connects the two membranes. Typically used double constructions are lightweight partitions Marine Panels. Mathematically double constructions can be described as mass-spring-mass systems with the two membranes as mass and the cavity (compression of air) as the spring.

Dynamic range
The range between the maximum and minimum sound levels that a sound system can handle. It is usually expressed in decibels.