With increasing use of the constant speed pass-by conditions to capture the noise generated by this portion of the vehicle operating cycle, knowledge of the contributing sources of noise was become increasingly important. For frequencies above 400 Hz, the noise is dominated by tire/pavement noise as can be demonstrated by comparing on-board sound intensity (OBSI) measurements to constant speed pass-by noise levels. At lower frequencies, direct on-board measurements become more difficult as the tire/pavement noise source strength decreases with decreasing frequency and microphone induced wind noise increases. To investigate the contribution of sources at these lower frequencies, cruise and coast pass-by measurements were made for a number of different pavement types and two different tire designs at test speeds of 56, 72, and 97 km/h over a frequency range from 50 to 10,000 Hz. OBSI measurements were also conducted for these same conditions. From the pass-by measurements, powertrain noise was separated from “rolling” noise by comparing cruise and coast operation. For coast conditions, the noise levels below 400 Hz were found not to depend on pavement type and texture or on tire design unlike the higher frequencies. Further, the speed dependencies of the levels below 400 Hz were more consistent with aerodynamic noise than with those of the tire/pavement noise. Contribution of aerodynamic noise was also indicated with wind tunnel measurements of vehicle exterior noise. Using these results, the relationship between the various noise sources at these lower frequencies was investigated.
