Page 679 - Automotive Engineering Powertrain Chassis System and Vehicle Body

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

The listening room method is different. Here an ar- supplementary attributes for rating noise from particular
tificial head (and torso) is placed in a passenger seat of sources such as diesel engines (called dimensions in the
the vehicle and binaural recordings of the interior noise reference):
are made. Thereafter, ranking, paired ranking, magnitude
overall level;
estimation or semantic differential questioning can be
low-frequency content and ‘boom’;
undertaken for a statistically significant number of panel impulsiveness;
members in a room (see Johnson (1995) for example).
Each panel member can be tested separately, or juries harmonic content;
often persons can be assembled. Otto et al. (1999) de- strong tones;
scribe the process well. irregularity;
Apart from safety and cost, one significant advantage high-frequency content.
that the listening room method has over the driving
method is that the recorded sounds can be manipulated
digitally and the altered sounds used to establish cus- 21.1.2 Noise path analysis
tomer preferences without the cost of constructing pro-
totype vehicles. Russell et al. (1988, 1992) describe an 21.1.2.1 Background
early system with more sophisticated versions being
demonstrated by Maunder (1996) and Naylor and Willats Interior noise levels can be controlled under certain
(2000). Such advanced digital techniques can be used by circumstances by adding sound-absorbing material to
panels of refinement specialists to diagnose the causes of the passenger compartment. The advantage in control-
poor sound quality. Otherwise, magnitude estimations for ling sound by absorption in the vehicle interior is that it
the driving method are commonly used when the panel will take effect on noise in a certain frequency range
has the opportunity to drive a candidate vehicle. Most regardless of origin or noise path providing that the
involve a panel of people driving and riding in the car(s) receiver is at some distance from the source. The dis-
along a predetermined test route on public roads and advantage is that its effect is usually rather small unless
rating the following noise (and vibration) attributes: the vehicle had little sound-absorbing trim in the first
place.
wind noise
As an alternative, a noise path analysis may be used to
road noise determine the contributions to interior noise levels made
engine noise by noise using different paths between the source(s) and
idle refinement the vehicle interior. Depending on the dominant noise
cruising refinement path(s) identified, the following noise control options are
transmission noise available:
general shakes and vibrations Structure-borne noise from the engine
squeaks, rattles and tizzes Improve the vibration isolation provided by engine
ride quality mounts.
driveability Reduce the vibration produced by the engine.
noise that is a ‘feature’ (sporty exhaust notes, etc.). Add damping treatments to resonant portions of the
firewall and floor.
The ratings are made to a common scale from 1 to 10 as
shown in Table 21.1-1. Airborne noise from the engine
Improve the TL of the firewall and/or floor by adding
A rating of less than 4 is unacceptable for any
attribute. a barrier layer (Wentzel and Saha, 1995), commonly
a mat of EVA, PVC or natural rubber (surface density
A rating of 5 or 6 is borderline. 1–7 kg m ) glued to a decoupling layer of chip foam
2
A rating of 7 or more on any attribute is acceptable.
or something similar (volumetric density in the range
3
Most new cars are launched with a subjective rating of of 20–60 kg m ) or a fibrous matting (density in the
3
7 or 8 on most attributes. Russell et al. (1992) supply range of 60–80 kg m ).

Table 21.1-1 Common subjective rating scheme

1 2 3 4 5 6 7 8 9 10
Not acceptable Objectionable Requires improvement Medium Light Very light Trace No trace

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