European Community legislation will decrease the limits of drive-by noise according to ISO 362 from 77 to 74 dB(A) in 1995. In Germany, the government will also enforce a new emission-related car tax, probably in 1993.
These planned restrictions make it necessary to look more closely at the influence of tire/road noise in relation to the drive-by noise level during vehicle type approval. Volkswagen has built four test tracks according to ISO/DIS 10844 and compared the influence of tire/road noise on these and other tracks in relation to drive-by noise.
The main differences of noise level were measured on different road surfaces. Nearly comparable is the effect of different tires. It must be considered that tire/road noise may be much higher under accelerating conditions than under coast-by conditions. These noise levels generally do not correlate. The effect of temperature has a large influence on drive-by noise, especially when measuring in the third gear. Aging of the test track should be recognized the first year after construction. The car itself influences the sound propagation through the design of the wheelhous-ing and the floor of the car body. Tire inflation pressure and vehicle weight may influence the noise level of low-noise cars.
If these effects are not taken into account, drive-by noise may spread in the range of 3 dB(A) even on an “ISO” test track.