Spectrum of Infrasound and Low-frequency Noise in Passenger Cars

The interior sound pressure spectrum of modern passenger cars running at various speeds on various road conditions was analyzed in terms of a 1/3 octave band over a frequency range of 1 to 250 Hz. The car speed varied from standing idle to a motorway speed of 110 km/h on a flat asphalt road and a cement road and in a tunnel. The 1/3 octave spectrum consisted of a nearly flat sound pressure level in the infrasound region up to 10 Hz, followed by a gradual decrease with frequency. The spectrum contained several humps, which corresponded to the resonance frequencies of engine firing and rambling noise. The spectrum profile suggests that much of the noise energy is in the infrasound region and increases with car speed. The levels of infrasound and audible sound pressures in modern Hyundai cars were shown to be significantly lower than those in the equivalent cars of 1970 models due to the incorporation of new materials with improved structural design.