The generation and transmission of crank rumble noise in vehicles equipped with
four-cylinder engines have been studied from the viewpoint of a source-path-receiver
model using a series of in-situ vehicle experiments. The proposed setups comprised
of both vehicle level operating and stationary experiments applied to two identical
vehicles with opposing rumble quality. The results revealed new information that
Fig. 14. Comparison of specific loudness reductions (phon) for actual operating and simulated air-borne
noise cases. (a) bark 5; (b) bark 6; (c) bark 7; (d) bark 8; (e) bark 10.
T.C. Lim, A.J. Witer / Applied Acoustics 60 (2000) 45±62 61
has not been documented previously. Specifically, our findings show a direct correlation between the modulation frequency of rumble and one-half order of the fundamental engine rotational speed signifying probable cause by one specific cylinder
or main journal bearing. However, the frequency ranges carrying rumble signatures
are time-invariant, and do not seem to be aected by the specific operating conditions and vehicle speed, which implies direct participation from certain structural
modes in the engine. Also, we discovered that the primary difference between the
quiet and noisy vehicles of identical design is due to the level of parametric excitation coming from the engine components. This is because there is no significant
measurable difference in the characteristics of transmission paths. Furthermore, our
experimental results indicated that the primary transmission path for crank rumble
is of structure-borne nature, except at higher frequency or Bark range where airborne noise contributions become more dominant. Although this experimental
study provided new information about the characteristics of crank rumble signatures and transmission, more research is still needed to fully understand the generation of rumble excitation possibly by using a controlled engine dynamometer
setup. This is left for future research. Finally, it may be noted that the proposed
experimental approach is in fact quite general and applicable to other time-varying
vehicle noise problems.