Digital-Computer Method of Measuring Concert-Hall Acoustics

Abstract

A digital-computer method for measuring sound transmission and diffusion in concert halls is described. Specially shaped tone bursts are generated on the computer and recorded on magnetic tape. These tone bursts are radiated in the hall and recorded at various locations, using omnidirectional and directional microphones. The recorded sound-pressure responses are first “cleaned” by the computer, using a matched-filter technique, and then squared and integrated over various time intervals to obtain (1) the total energy for each frequency band, location in the hall, and microphone condition; (2) the “early” energy (direct sound and that arriving within the following 50 msec); (3) the energy of the direct sound; (4) the energies of various early reflections (ceiling, walls, etc.); and (5) the ratio of early-to-total energy. These energies and ratios are converted to decibels and plotted as functions of frequency by a special microfilm plotter attached to the computer. Frequency averages of these data as functions of location in the hall and spatial averages over all or certain selected locations are also computed and plotted. For more-detailed analyses, “echograms” are plotted on both linear and logarithmic scales. The computer also time-averages the sound-pressure responses and plots smoothed decay curves on a logarithmic scale, together with one or more straight lines fitted to the decay curves. Measures of time-varying sound diffusion are computed from the outputs of directional microphones. Results obtained by this method in Philharmonic Hall (New York) are discussed in a companion paper. Parallel studies are aimed at assessing the subjective significance of the measurements.