Acoustic Absorption in Vehicles and the Measurement of Short Reverberation Times

When characterizing absorption in vehicles, a knowledge of the in situ performance of the acoustical absorbers is often desired. To measure this is a relatively easy task in a large diffuse sound field. However, inside a vehicle where the sound field is not diffuse, the measurement is more difficult. Impulse and interrupted noise techniques have been used to measure absorption inside cavities with extremely short reverberation times. Among several difficulties associated with these techniques are the response time of the filters and the influence of the averager in the instrumentation. In many cases, these limitations make these traditional techniques impractical for use in a vehicle. With the advent of modern digital signal processing, however, there are a number of other readily accessible techniques that are more useful. These include the use of broadband noise based decays that are analyzed in reverse, integrated impulses, time-frequency distributions, and maximal length sequences. The difficulties and benefits associated with each of these techniques will be highlighted in this paper. The usefulness of these techniques in a large reverberant environment will be contrasted with their use inside a vehicle.