Page 697 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

10 0
rigidly mounted subframe
compliantly mounted subframe

10 –1
Force (arbitrary units) 10 –2

10 –3

10 –4
0 20 40 60 80 100 120 140 160 180 200
Frequency (Hz)
Fig. 21.1-7 The beneficial effects of the compliant subframe: data taken from Hardy (1997).

Airflow over the underside of the vehicle causing glasswork and door seals. The intensity probe can also be
transmission of airborne sound to the interior, par- used outside the vehicle to map the contours of aero-
ticularly in the wheel-arch areas. dynamic noise from bodywork protrusions and from the
Noise transmission through door and glass seals due greenhouse surfaces. A form of noise path analysis is also
to aspiration (leakage) or due to aerodynamic exci- possible by making measurements of structural vibration
tation of doors and glass caused by disturbed airflow on the greenhouse surfaces and then combining these
over the seals. with NTFs measured between positions inside the in-
Vortex shedding over protrusions in the bodywork terior and the various surfaces (see Section 21.1.2.3 for
(such as aerials, roof bars, etc.) causing tonal air- a discussion of the NTF). The character of the aero-
borne noise. dynamic excitation can be determined using a combina-
Cavity flows through partially open windows and tion of static pressure tappings and flush mounted
sunroofs causing intense low-frequency (below 25 Hz microphones embedded into thin rubber mats that are
commonly) noise and buffeting. simply laid on the greenhouse surfaces.
Aerodynamic noise is best investigated at full scale in
a windtunnel although some promising results have been
obtained at model scale (Kim, 2003). The use of 21.1.7 A note on brake noise
a windtunnel allows the car to be stationary, thus re-
moving noise contributions from the powertrain and Brake noise has been an issue of concern to vehicle
from the tyres. Frequently, aerodynamic noise de- manufacturers for decades. There are several distinct
velopment is undertaken on a subjective basis. Windows categories of brake noise (Betella et al., 2002):
and doors are taped up, lead sheeting is added to wheel
Brake squeal (occurring at higher speeds and having a
arches and to the underfloor, bodywork protrusions are
tonal character with components well above 1000 Hz).
removed and, at every stage, binaural recordings are
made in the car using a dummy head system. The Brake moan (occurring at moderate speeds and char-
resulting recordings can be played to juries, or Zwicker acterised by frequency components around 100 Hz).
loudness or Articulation Index are frequently used as Brake creep-groan (occurring at speeds less than
a measure of improvement (see Section 20.1.6.3.2). walking pace and characterised by frequency com-
Publications by Anderson (2002) and by Coney et al. ponents around 100 Hz).
(1999) detail objective methods of assessment. These Brake judder (occurring at speeds less than walking
include the use of an intensity probe inside the vehicle to pace and characterised by frequency components
check for aspiration noise. An inexpensive and traditional around 10 Hz).
alternative is to use a length of plastic tubing, to place one The source for all four classes of brake noise is the pairing
end in your ear and to pass the other end around the of friction surfaces at the pad and rotor disk. Brake squeal

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