The utilization of short pulses of sound has made it possible to perform anechoic acoustical measurements in ordinary rooms. It takes time for bothersome reflections from walls and other surfaces to arrive at the measuring position. If the pulse is short enough, the time gate can be opened only for the desired sound pulse, shutting out the interfering pulses. This tone-burst method can be used to measure the sound-absorption coefficient of a material at any desired angle of incidence.

Such an arrangement is illustrated in principle in Fig. 9-5. The source-microphone system is calibrated at distance x as shown in Fig. 9-5A. The geometry of Fig. 9-5B is then arranged so that the total path of the pulse reflected from the material to be tested is equal to this same distance x. The strength of the reflected pulse is then compared to that of the unreflected pulse at distance x to determine the absorption coefficient of the sample.

Figure 9-5: Determining the absorption coefficients of materials by a tone-burst method. The source-microphone system is calibrated at distance X as shown in (A).

A recent surge of interest in the influence of individual reflections on the timbre of sound is a new and promising development in acoustics. In this new field specific normal reflections, called "early sound," are of special interest. Although random-incidence coefficients are still of interest in room reverberation calculations, for these image control problems normal-incident reflection coefficients are generally required.