Page 672 - Automotive Engineering Powertrain Chassis System and Vehicle Body

Vehicle refinement: Purpose and targets C HAPTER 20.1

Table 20.1-5 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

(BS 3383 – 1988, ISO 226 – 1987). Each contour is The ratings are made to a common scale from 1 to 10 as
labelled with X phons, X being the sound level at 1000 per Table 20.1-5
Hz for that particular contour. A rating of less than 4 is unacceptable for any
Articulation index (AI) in the range of 0.5–0.6. The attribute.
calculation method is given in ANSI S3.5 – 1969 A rating of 5 or 6 is borderline.
and the long-term rms. speech level in 1/3 octave A rating of 7 or more on any attribute is acceptable.
bands is compared against the long-term rms.
masking noise level, and the 1/3 octave results so Most new passenger cars are launched with a subjective
generated are aggregated using separate weighting rating of 7 or 8 on most attributes.
functions for each 1/3 octave band.
Speech transmission index (STI) and the more rapid 20.1.6.4 Targets for ride quality
RASTI (rapid speech transmission index) (BS 6840: (including tactile vibration)
part 16 – 1989) are commonly used alternatives to
articulation index. Rust and co-workers suggest Ride quality is taken here to be the subjective response to
a RASTI of 0.8 at 50 km hr 1 reducing linearly to 0.4 a low-frequency vibration phenomenon. There are sev-
1
at 125 km hr . eral different strategies for the assessment of in-vehicle
vibration levels. A summary is measured vibration levels
are rated according to objective criteria, and the most
20.1.6.3.3 Interior noise: subjective targets commonly used criteria are offered in:
There are several different strategies for the subjective ISO 2631 Part 1 (1985);
assessment of interior noise. These are discussed, along BS 6841 (1987);
with target levels in Section 21.1.1.4. An engineering NASA discomfort level index (1984) (Leatherwood
method for subjective appraisal involves a panel of people and Barker, 1984).
driving and riding in the vehicle(s) along a pre-determined Experience of using all three has led to the conclusion that
test route on public roads and rating the following noise a family class vehicle for either the European or the US
(and vibration) attributes:
Federal markets will be ready for sale when the appropri-
wind noise; ately frequency-weighted seat rail vibration levels mea-
road noise; sured at 80 km hr 1 on a straight road with 5–10-year-old
engine noise; tarmac and a few spot repairs (in other words, a typical
idle refinement; B-class inter-urban road) are:
cruising refinement; Close to the four-hour reduced comfort boundary in
transmission noise; the vertical direction as defined in ISO 2631 Part 1
general shakes and vibrations; (1985).
squeaks, rattles and tizzes; Have an rms. level less than 0.63 m s 2 (classed as
ride quality; better than ‘a little uncomfortable’ according to BS
driveability; 6841 (1987)).
noise that is a ‘feature’ (sporty exhaust notes, etc.). Have a NASA discomfort rating below 4.0 (Bosworth
et al., 1995).

References

Ashley S November 1997 Keys to development of low noise engines SAE Bosworth R, Trinick J, Smith T, and
Chrysler’s comeback, Mechanical Paper No. 1999-01-1745. Horswill S 1995 Rover’s system
Engineering. Bies DA, and Hansen CH 1996 Engineering approach to achieving first class ride
Beidl CV, Rust A, and Rasser M 1999 Key noise control – theory and practice, comfort for the new Rover 400, IMEchE
steps and methods in the design and Second edition. E&FN Spon, London. Paper No. C498/25/111/95.

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