Acoustic response variability in automotive vehicles

A statistical analysis of a series of measurements of the audio-frequency response of a large set of automotive vehicles is presented: a small hatchback model with both a three-door (411 vehicles) and five-door (403 vehicles) derivative and a mid-sized family five-door car (316 vehicles). The sets included vehicles of various specifications, engines, gearboxes, interior trim, wheels and tyres. The tests were performed in a hemianechoic chamber with the temperature and humidity recorded. Two tests were performed on each vehicle and the interior cabin noise measured. In the first, the excitation was acoustically induced by sets of external loudspeakers. In the second test, predominantly structure-borne noise was induced by running the vehicle at a steady speed on a rough roller. th types of excitation, it is seen that the effects of temperature are small, indicating that manufacturing variability is larger than that due to temperature for the tests conducted. It is also observed that there are no significant outlying vehicles, i.e. there are at most only a few vehicles that consistently have the lowest or highest noise levels over the whole spectrum. For the acoustically excited tests, measured 1/3-octave noise reduction levels typically have a spread of 5 dB or so and the normalised standard deviation of the linear data is typically 0.1 or higher. Regarding the statistical distribution of the linear data, a lognormal distribution is a somewhat better fit than a Gaussian distribution for lower 1/3-octave bands, while the reverse is true at higher frequencies. For the distribution of the overall linear levels, a Gaussian distribution is generally the most representative. As a simple description of the response variability, it is sufficient for this series of measurements to assume that the acoustically induced airborne cabin noise is best described by a Gaussian distribution with a normalised standard deviation between 0.09 and 0.145. is generally considerable variability in the roller-induced noise, with individual 1/3-octave levels varying by typically 15 dB or so and with the normalised standard deviation being in the range 0.2–0.35 or more. These levels are strongly affected by wheel rim and tyre constructions. For vehicles with nominally identical wheel rims and tyres, the normalised standard deviation for 1/3-octave levels in the frequency range 40–600 Hz is 0.2 or so. The distribution of the linear roller-induced noise level in each 1/3-octave frequency band is well described by a lognormal distribution as is the overall level. As a simple description of the response variability, it is sufficient for this series of measurements to assume that the roller-induced road noise is best described by a lognormal distribution with a normalised standard deviation of 0.2 or so, but that this can be significantly affected by the tyre and rim type, especially at lower frequencies.