Projection engineering (Jan 1932-Mar 1933)

JANUARY, 1932 Page 17 Sound absorption balance in the acoustics of auditoriums+ By V. A. SCHLENKER^ In this paper is presented new studies of the relation between reverberation and articulation, as employed in sound transmission in auditoriums A REGRET expressed by engineers now delving into architectural acoustics is that the father of modern acoustics is no longer living. Professor Wallace Clement Sabine was never content to consider any phase of acoustics a closed book. He was always willing to reopen and continue with any investigation where there was hope of extending experimetal data in a practical way. Unfortunately, many of his successors have shown a tendency to assume that his contributions were sufficient for the solution of practically all acoustical problems which the engineer encounters. Without detracting in the least from the monumental pioneer work of Professor Sabine, it can truthfully be said that his contributions are not entirely adequate to arm the present engineer to successfully cope with the acute conditions caused by sound pictures, radio, and television. Had Sabine lived he would no doubt still be the leader in experimental investigation with our present electro-acoustical instruments. That he was able to make such accurate and intricate measurements with the crude equipment available in his time is beyond the comprehension of most of us. The most common assumption made by the young acoustician is that his job is done when he adjusts the reverberation of an auditorium to the socalled "optimum." Just what or why there is or should be an optimum period of reverberation has always been somewhat obscured with a veil of mystery. That music is enhanced with some reverberation is well known and it follows that a definite period, not too long and not too short, can be deter mined by the sensing of artists who react positively to the variation of the reverberation as it is adjusted. Professor Sabine reported that "a difference of five per cent in reverberation is a matter for approval or disapproval on the part of musicians of critical taste." (P. 80) Collected Papers on Acoustics.) In the case of speech, however, it is not so simple. In a general way, the clarity should increase as the reberberation is reduced. At the same time the intensity at the ear of the listener is reduced by the introduction of acoustic absorption which is employed to reduce the reverberation. In small auditoriums the attenuation is not especially objectionable so long as the intensity is well within the limits o f audibility. Therefore, if there are no other factors in control no optimum period of reverberation can be determined. In the case of the large auditorium we have two essentially opposite effects on the quality of speech — one tending to increase, the other tending to reduce '",; ' 1\ 5Peccn ,„ Titf*TKC 'A' /if ren r#r*rrtrnr *mm Fig. 2. Effect of reverberation on speech. the reverberation. If there were no other important factors, naturally, a compromise or optimum would result. It is quite apparent, in the small inclosure, at least, that some other important factor must be in control to account for the optimum period of reverberation. The reverberation of a room is usually measured by timing the decay of sound. Present methods involve a microphone to pick up the sound and a chronographic means of recording the time required for the sound intensity to drop a certain number of decibels. The number of seconds required for a 60 decibel reduction in level is universally designated as the period of reverberation regardless of frequency. An oscillographic trace of such decay of sound can be seen in Fig. 1. The two traces are recorded simultaneously — the lower one is monitored on the upper channel electrically but set at an adjustable number of decibels above it in gain. In this particular case, the lower trace was recorded at 24 db. above the other. The distance between the points of equal amplitude on the two traces is a measure of the time for the sound to decay 24 db. By multiplying this figure by 2l/2 the period of reverberation is obtained. The difference of level chosen is determined by the noise level and the maximum inten Fig. 1. Oscillographic trace of decay of sound. f Presented before the Radio Club of America, November 11, 1931. * Consulting Acoustical Engineer, Chanin Bldg., New York City.