A series of measurements was undertaken in the summer and fall of 1977 to determine the noise radiated by a wide variety of cars and trucks, and the effect of an acoustic wall, or barrier, on suppressing the transmission of that noise. The assumption of the "correct" effective radiating height was found to be the most critical parameter in making the calculations agree with the measured results. The "best height" value varied greatly from vehicle to vehicle, and with speed, load, and throttle setting for a given vehicle. The variability of this parameter made it impossible (solely from these tests) to assess such considerations as whether Fresnel's or Maekawa's curves should be used to calculate the attenuation produced by the wall.
The most noteworthy observation from this study is that newer trucks not only are quieter (to meet noise radiation standards) but have an effective radiating height as low as 2 feet compared to as much as 11 feet for older trucks. Thus, as the newer types of trucks become predominant in the highway traffic mix, the effectiveness of walls and barriers in shielding the community from noise will increase considerably. This is good news, because wall effectiveness has been marginal at best for pre-1970 trucks because of their radiation heights.