Page 733 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

back-up a hunch as to where the noise is coming
Table 22.1-4 Suggested target noise levels for achieving type
from).
approval under 9297/EEC
Sound intensity mapping – where the machine is run
Target levels at 7.5 m, continuously, and an intensity probe is passed over an
acceleration test (dBA) notional surface set around the machine. Regions of
high intensity give an indication of the position and
Passenger car Light truck Heavy truck
level of noise sources.
Engine 69 72 77 Spatial Transformation of Sound Field (STSF)
method. This is a relatively new technique that
Exhaust 69 70 70
uses an array of microphones in a plane (or for sta-
Intake 63 63 65 tionary sound fields, a single scanning microphone)
and the principle of near-field acoustical holography
Tyres 68 69 75
along with the Helmholtz Integral Equation to cal-
Transmission 60 63 66 culate the three dimensional sound field. Taylor and
Bridgewater (1998) give an example of the technique
Other 60 72 65
in use. Kim and Lee (1990) give a more detailed
Combined level 74.2 77.3 80.1 description of the technical background.
Noise from vibration – where the machine is run and
measurements of vibration velocity are made on the
relative contributions made to the pass-by level by the surfaces of potential noise sources. Radiation effi-
various sources (intake, exhaust, tyres, etc.) so that the ciencies are calculated or assumed, and the noise
most significant sources can be controlled first. This is contribution from each potential noise source is
achieved using noise source ranking. estimated.
First, a list of potential noise sources should be com- Modelling – where the noise contribution from each
piled, and then the next task should be to rank all of the potential source is calculated using empirical or
sources in order of significance. The term ‘significance’ mathematical models.
can have several different meanings according to the case
in hand. Significance could mean:
overall linear sound power level or sound pressure 22.1.2.1 Noise source ranking using
level at a particular point in space; shielding techniques
overall ‘A’-weighted sound power level or sound
pressure level at a particular point in space; The most commonly used noise source ranking pro-
sound pressure or power level in a certain frequency cedure in the automotive industry is still (arguably) the
band; rather laborious shielding or encapsulation method. This
long-distance propagation; usually consists of first eliminating the various noise
subjective rating. sources, using acoustic treatments, and then removing
the treatments systematically and measuring the noise
The noise source ranking procedure may be undertaken contribution from each source as it is uncovered. The
in a variety of ways. The more popular methods include: shielding technique is used for both vehicle and engine
Isolation – where each component of the machine is noise. The vehicle noise case shall be discussed further
run in isolation, where possible, and its noise con- here.
tribution is measured directly. The interior noise contribution from each source is
Shielding – where the machine is run several times, tested using microphones placed at each seat in the ve-
and each time a different noise source is encapsu- hicle, and the exterior noise contribution is assessed
lated in a sound-retaining structure. The reduction in using the relevant standard drive-pass test. A typical
noise level achieved yields that particular noise noise-source ranking programme might take the follow-
source’s contribution to the total noise level. A ing form:
practical example of this technique is given by Perform baseline noise tests on the programme vehi-
Balcombe and Crowther (1993). cle. Interior noise levels are recorded at each seat
Close microphone techniques – where the machine position with the vehicle accelerating uniformly in
is run in an anechoic chamber and a microphone second or third gear, first with wide-open throttle
is passed in the near field over the surfaces of the and then part throttle. Exterior noise for the baseline
machine to locate regions of high pressure level (an vehicle is assessed using the relevant drive-pass
approximate indicator only usually used to quickly standard.

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