Transmission loss (duct acoustics)

Sound measurements
Characteristic	Symbols
Sound pressure	p, SPL
Particle velocity	v, SVL
Particle displacement	δ
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 duct acoustics, together with insertion loss (IL), describes the acoustic performances of a muffler like system. It is frequently used in the industry areas such as muffler manufacturers and NVH department of automobile manufacturers. Generally the higher transmission loss of a system it has, the better it will perform in terms of noise cancellation.

Transmission loss (TL) (more specifically in duct acoustics) is defined as the difference between the power incident on a duct acoustic device (muffler) and that transmitted downstream into an anechoic termination. Transmission loss is independent of the source and presumes (or requires) an anechoic termination at the downstream end.

Transmission loss does not involve the source impedance and the radiation impedance inasmuch as it represents the difference between incident acoustic energy and that transmitted into an anechoic environment. Being made independent of the terminations, TL finds favor with researchers who are sometimes interested in finding the acoustic transmission behavior of an element or a set of elements in isolation of the terminations. But measurement of the incident wave in a standing wave acoustic field requires uses of impedance tube technology, may be quite laborious, unless one makes use of the two-microphone method with modern instrumentation.

By definition the TL on an acoustic component, for example a muffler, is described as:

where:

Note that $p_{i+}$ $p_{i+}$ cannot be measured directly in isolation from the reflected wave pressure $p_{i-}$ $p_{i-}$ (in the inlet, away from muffler). One has to resort to impedance tube technology or two-microphone method with modern instrumentation. However at the downstream side of the muffler, $p_{o}=p_{o+}$ $p_{o}=p_{o+}$ in view of the anechoic termination, which ensures $p_{o-}=0$ $p_{o-}=0$ .

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