Apparatus and Technique for Reverberation Measurements
Frederick Vinton Hunt · The Journal of the Acoustical Society of America · 1936
New apparatus has been constructed for obtaining accurate reverberation data. The conventional method of timing the decay to an adjustable threshold is made automatic by a system of relays which control the following sequence: when a predetermined sound level is attained in the sound chamber the source is turned off and an electric timer is started; when the sound level falls to the adjustable threshold the timer is stopped and the source is turned on. If this recurrent cycle is interrupted after ten repetitions the average time for a single decay can be read directly from the timer. The apparatus is entirely a.c.-operated and 80 db of decay can be recorded in the middle audiofrequency range. Using this apparatus we have investigated the merits of employing a low pass electric wave filter to remove the rapid fluctuations in the rectified output of the microphone amplifier. This is approximately equivalent to the use of a recorder which is just capable of following the average decay rate. Determination of the average decay rate is not affected by the use of the filter but the average deviation from linearity in the decay curves can be reduced by a factor of at least two.