Soundproofing (Sound Transmission through Partitions)
Fig. 1 Sound transmission through single wall partion with foam and double wall partition.
Fig. 2 Sound transmission loss of foam-wall partition, predicted versus measured.
Fig. 3 Sound transmission loss of a double wall partition with foam, predicted versus measured.
Fig. 4 Sound transmission loss of a 3-layer constrained partition, predicted versus measured.
Soundproofing is a way of reducing the sound pressure level by introducing damping (absorption) and sound barriers i.e.
partitions. The optimal design and construction of effective sound barriers form an important part of providing high noise
attenuation. The principles governing the design of effective noise reduction partitions are the same whether the sound barrier
is an aircraft fuselage, the wall in a building or the separation in a car between the passenger compartment and the engine
bay.
Fig. 1 illustrates how an incident sound field on a barrier will be partly reflected and partly transmitted. The smaller
the transmission the better the barrier is at reducing the transmitted noise. The effectiveness of the partition in reducing
noise is given as the Transmission Loss (TL) in dB. The transmission loss is calculated as TL = 10 log10 (1/τ) where τ is the transmission coefficient that
has been averaged over all angles of incidence (random incidence).
For most applications it is not only necessary that the partition has a high transmission loss but also that its weight
is low. Single wall constructions are normally not very effective in reducing noise levels, as an increase in the transmission
loss of just 6dB requires a doubling of the mass (i.e. the thickness of the partition). The most common solution is
to use a double wall construction, that is, two walls separated by an gap that is filled with a porous material preferably
with a high flow resistivity. An example of transmission loss for double wall construction is shown in Fig. 3.
The NVC predicted transmission losses in Fig. 2 to Fig. 4 have been calculated using a plane wave propagation model
for layered treatments taking into account non-resonant transmission only. The layered treatments can include solid elastic
materials, thin mass layers, fluid layers and foam layers. In addition to the transmission loss, the model also allows calculation
of acoustic impedance (real and imaginary parts) and absorption.
Apart from providing a means of comparing various soundproofing constructions, the predicted transmission losses can also
be used in Statistical Energy Analysis (SEA) models. An SEA model can take into account all sound transmission paths both air-borne (resonant and non-resonant contributions) as well
as structure-borne (flanking) paths. Such a model can be used to optimise the various soundproofing schemes against cost,
thus preventing that one part is designed with a higher transmission loss than required.
