Introduction to Acoustics

high enough that there is little sound power reflected
back down the outlet duct, and the transmission loss of
the silencer (the ratio of the sound power at the outlet
and the sound power at the inlet) is a good measure
of silencer performance. This may not be true at low
frequencies.
Duct silencers are manufactured in a wide variety of
sizes, with linings, with splitters, and with aerodynamic
shapes on the inlet and outlet. Standards are available
for the measurement of silencer performance and guidance
on their use [23.135–139]. The insertion loss of the
silencer is usually measured, and its value may depend
on the direction of air flow relative to the direction of
propagation of sound. When the two are in the same
direction (outlet duct), a sound wave is bent toward
the lining whereas when the two are in opposite directions
(inlet duct), sound is bent toward the center of
the duct [23.140].
23.4 Noise and the Receiver
Section 23.2 was devoted to the characterization of
sources in terms of their noise emission. The third part of
the source–path–receiver model involves immission of
sound at the receiver. The sound may not be unwanted,
and is therefore not technically noise, but this section
is generally devoted to the effects of noise on people.
Section 23.4.5 is devoted to sound quality, a subject of
increasing importance in the design of products.
23.4.1 Soundscapes
As discussed in Sect. 23.0.2, a sound field may be described
in terms of a sound pressure, p(r, t), that varies
both in space and time. In practice, it is the RMS pressure
that is measured since the time average of the pressure
itself is zero, and several quantities measured by modern
sound level meters are discussed in Sect. 23.1. The sound
field can be described in the time domain or the frequency
domain, or as a short-time spectrum that varies
with time. Other descriptions are also possible.
The sound field p(r, t) may, for the purposes of this
section, be called a soundscape, an overall acoustical
environment – both indoors and outdoors – that includes
all sound, both wanted and unwanted. The interaction
between an observer and this physical soundscape can
then be described in terms of immission of sound. In
some cases, it has been found that this interaction depends
not only on the properties of the soundscape itself,
Noise 23.4 Noise and the Receiver 999
Calculation of Duct Transmission Loss
Computer programs for the determination of the transmission
loss of ducts are available [23.134]. The
duct lining may consist of air gaps, one or more
layers of porous material, resistive sheets and perforated
facings. As one example, Fig. 23.27 shows
the transmission loss for a duct 2.5 m long having
a cross section of 0.6m × 0.6 m. The sound-absorptive
material has a steel perforated facing 1.6mm thick
with perforations 3 mm in diameter and a 30% open
area. The duct liner is 200 mm thick in one case,
and 100 mm thick in the second. The transmission
loss shown in the figure is from 50 Hz to 500 Hz
because the calculation is for plane-wave transmission
only. It can be seen that the peak attenuation
is about the same for both cases, but the thicker
liner provides a much higher transmission loss at low
frequencies.
but on the visual environment of the observer (the landscape).
A discussion of this effect is beyond the scope
of this chapter.
The soundscape includes indoor environments such
as living space and industrial plants, urban and suburban
areas, parks, and wilderness areas. The effects
of the sound field on observers in different portions
of the soundscape are varied, and in many cases difficult
to quantify. These effects include hearing damage,
annoyance in various forms – which can range from
mild dissatisfaction to frustration and anger – as well as
interference with speech communication in many settings,
including meeting rooms and classrooms. Other
effects include interference with sleep, and loss of productivity.
Since the soundscape includes both wanted
and unwanted sound, the soundscape, when properly
managed can induce a sense of well-being in the observer
which can have a positive effect on the quality
of life. Examples include listing to natural sounds in remote
areas where unwanted sound is either nonexistent,
or has been reduced to an acceptable level. It is a fact that
“acceptable” means different things to different persons.
A further complication is that using conventional
measures of noise immission, the acceptability of
a sound may depend on the source. For example, it
has been found that the same level of noise from aircraft,
road traffic, and rail traffic produces different
human reactions. In recent years, the quality of the
Part G 23.4