Dissection of Automobile Interior Noise Spectrum with Emphasis on the Infrasound Region

*† ‡ This paper documents noise spectra measured in the interior of a large number of automobiles. The original goal was to measure the vehicle interior acoustic amplitudes in the infrasound region, but since the noise was measured in the audible region also, spectra have been analyzed over a frequency range of 0-20 kHz. These experiments were carried out on public roads around Atlanta, Georgia, USA. The tests were performed with various side window openings, from fully closed to fully open. Typical automobile noise spectra analyzed for ∆f of 0.078 Hz and dissected in low, mid, and high frequency regions are discussed. Contributions to the measured noise by tire rotation, engine firing, and apparent source of infrasound are identified. Past examinations of vehicle interior noise are also reviewed. Changes in vehicle interior noise of various automobiles over the last decade are then discussed. The cars tested include Ford Explorer, Dodge Ram Van, Toyota Corolla, Toyota Camry, and Honda Civic. At frequencies higher than 2 kHz, interior noise is found to be around 20 dB for ∆f of 0.078 Hz. In the mid-frequency region of 500-2000 Hz, the spectrum is made up of the harmonics of the lower frequency region superimposed over a broadband spectrum. Comparing the harmonics in this lower frequency region of different cars, old and new, it is found that over the last decade, on the average, the noise from tire has been reduced by 15 dB and that from engine firing has been reduced by 20 dB. Two infrasound humps were noticeable, one in 0-10 Hz region and another in the 10-20 Hz region of the spectrum. The first hump appears to be related to the vehicle wakes and the second hump is related to Helmholtz resonance of the vehicle interior. The amplitude of sound in this region increases by 12-15 dB at the center of the car on opening the driver's side window by as little as 2 inches. Unlike the audible range, it is found that this infrasound hump has remained unchanged during the last decade, in spite of large improvements in the audible region over the same period.