Page 677 - Automotive Engineering Powertrain Chassis System and Vehicle Body

CHAP TER 2 1. 1 Interior noise: Assessment and control

airborne paths, the transmission of sound to the passenger equal to the typical track of a small European car (1.4 m).
compartment will be greatest at low frequencies due to The (0, 2, 0) mode would be expected at 246 Hz.
the action of the mass law for transmission loss (TL) (see The first vertical mode (0, 0, 1) can be expected at
Section 21.1.10.5). In the case of structure-borne paths, 143 Hz if the vertical dimension is assumed to be the
the use of resilient mounts will isolate the passenger typical body height of a small European car (1.2 m). The
compartment at higher frequencies and so transmission (0, 0, 2) mode would be expected at 286 Hz.
will also be greatest at low frequencies. The firing frequency of a four-cylinder, four-stroke
From the above observations, one would expect engine lies in the range of 33–200 Hz for the corre-
1
a strong low-frequency component to interior noise sponding speed range of 1000–6000 rev min . In this
levels. This is found in practice (see for instance Steel range, using the rather over-simplified analysis used
et al. [2000]) in spite of the filtering effect due to the fact above, one might reasonably expect the following body
that steel panels are rather poor radiators of sound at low booms:
frequencies (see Section 21.1.3.7). 1
One of the reasons for this is due to acoustic modes (1, 0, 0) at around 70 Hz/2100 rev min
within the passenger compartment enhancing low- (0, 1, 0) at around 120 Hz/3600 rev min 1
frequency noise levels. The so-called acoustic modes are (2, 0, 0) at around 140 Hz/4200 rev min 1
set up at the natural frequencies of the space, frequencies (0, 0, 1) at around 140 Hz/4200 rev min 1
given by (for a rectangular space): (1, 1, 0) at around 140 Hz/4200 rev min 1
(2, 1, 0) at around 185 Hz/5550 rev min 1
s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1

c ap 2 bp 2 cp 2 The boom around 3600 rev min is often the most
f a;b;c ¼ þ þ (21.1.1) annoying. This is because the lower speed boom is usually
2p x y z
only transient as the vehicle will accelerate quickly
through it. Also the high speed booms are seldom
where
a problem as most drivers will not run their engines for
1
c ¼ speed of sound in air (m s ) long periods at 4000þ rev min 1 and will select a higher
gear when they first hear such booms. However, drivers
a, b, c ¼ integer indices 1, 2, 3, .
x, y, z ¼ acoustic dimensions of the space (m) on high-speed roads may well find themselves having
to cruise around 3600 rev min 1 in top gear with an
For the modern generation of small European car, the annoying boom ever present in the passenger compart-
longest acoustic dimension of the passenger compartment ment. As the occupants tend to sit near to the sides of the
is that between the footwell and the rear screen. This passenger compartment, all find themselves at anti-nodes
dimension is typically a little more than the wheelbase of of the (0, 1, 0) boom and all suffer pressure maxima.
the vehicle, perhaps 2.5 m. The lowest frequency acoustic The passengers in the rear seats of most cars suffer
mode of the passenger compartment (the 1, 0, 0 mode, from pressure maxima at most modes – longitudinal,
a ¼ 1, b ¼ 0, c ¼ 0) therefore has pressure maxima (anti- lateral, vertical (and other non-orthogonal modes). This
nodes) at the footwells and at the rear screen and pressure is not the case for the occupants of the front seats. For
minima at the midpoint of the wheelbase. Assuming this reason, rear seat noise quality is often a greater issue
x ¼ 2.5 m this would occur at 69 Hz. Note that the for concern than the levels and quality of noise at the
driver’s head position is usually just aft of the midpoint of driving position.
the wheelbase and so the driver seldom enjoys the benefits
of being precisely at the nodal position. 21.1.1.2 On the balance between
The 1, 0, 0 mode is commonly expected in the fre- airborne and structure-borne noise
quency range of 65–75 Hz. Our estimate of 69 Hz equates
to the firing frequency of a four-cylinder, four-stroke The vibration isolating effects of resilient components
1
engine operating at 2070 rev min . European four- (engine mounts, subframe mounts, under-carpet treat-
cylinder cars commonly exhibit low-frequency noise ments) tends to limit the significance of structure-borne
peaks (known as interior booms or sometimes as body noise to frequencies below around 500 Hz. At higher
booms) at engine speeds in the 2000–2500 rev min 1 frequencies, noise received via airborne noise paths
range as a result of exciting the 1, 0, 0 mode. generally dominates the interior noise levels.
Thenextlongitudinalmode (2,0, 0) would be expected At frequencies below 500 Hz, airborne noise may
at around 138 Hz. It often occurs at a lower frequency remain a significant contributor to overall interior noise
though in road cars. levels, particularly if the sealing of the passenger com-
The first transverse mode (0, 1, 0) can be expected at partment (door seals, window seals, grommets in the
123 Hz if the transverse acoustic dimension is assumed bulkhead) is not perfect.

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