Transmission loss

Sound measurements
Characteristic	Symbols
Sound pressure	p, SPL
Particle velocity	v, SVL
Particle displacement	ξ
Sound intensity	I, SIL
Sound power	P, SWL
Sound energy	W
Sound energy density	w
Sound exposure	E, SEL
Acoustic impedance	Z
Speed of sound	c
Audio frequency	AF
Transmission loss	TL

Transmission loss (TL) in general describes the accumulated decrease in intensity as a wave propagates outwards from a source, or as it propagates through a certain area or a certain type of structure. It is a terminology frequently used in optics and acoustics. And it is also a very important indicating value used in the industry of acoustic devices such as mufflers and sonars. Normally transmission loss is measured in dB scale and in general it can be defined using the following formula:

TL = 10 \log_{10} \left\vert {W_i \over W_t}\right\vert

where:

$W_i$ is the power of incident wave coming towards a defined area (or structure);
$W_t$ is the power of transmitted wave going away from the defined area (or structure).

Transmission loss may refer to a more specific concept in one of the field below.

Duct Acoustics

Transmission Loss (duct acoustics) in duct acoustics describes the acoustic performances of a muffler like system.

Room Acoustics

Transmission Loss (room acoustics) in room acoustics describes the decrease of sound intensity that is reduced by a wall or other structure at a given frequency.

Underwater Acoustics

Propagation loss (sometimes referred to as transmission loss) is a quantitative measure of the reduction in sound intensity between two points, normally the sound source and a distant receiver.