THE JOURNAL OF THE ACOUSTICAL SOCIETY OF JAPAN

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For the field measurement of reverberation time one of the following two methods is usually adopted: (1) field recording of reverberation sound of a room on magnetic tapes, recording of decay curves on recording papers from these tapes by a high-speed level recorder in a laboratory, and reading of these curves to obtain reverberation time of the room; and (2) use of a high-speed level recorder in the room to be measured to record reverberant decay curves. But in case of method (1) we cannot know reverberation time immediately. On the other hand, if method (2) is adopted, mobility is sacrificed. To eliminate such disadvantages, we devised a new reverberation meter small in size and light in weight. The principle of this meter is that of chronometer type, which measures the time interval during which a sound in the room decays from one preset level to the other, and converts them into reverberation time in which the sound decays by 60dB. The mechanism of this meter is briefly as follows: a band noise or warble tone is emitted intermittently in a room; one electric condenser is charged to a voltage corresponding to the sound pressure in the room just before the sound source is stopped; then the source is off; just before the sound is emitted again, another electric condenser is charged to the voltage obtained by amplifying a certain voltage corresponding to the sound pressure at that instant. Comparing these voltage the time interval of two consecutive sound emission is adjusted to equalize both levels. This time interval (t sec) corresponds to the time duration in which the sound in the room decays by a certain amount (L dB) which is equal to the gain of the amplifier. The reverberation time of the room can be obtained by multiplying the time interval t sec by 60/LdB. The reverberation time can be obtained as a mean of the values of repeated measurements. The meter is transistorized so as to be portable except an electrometer tube 5886 for comparison. As a result of practical use, it was found easy to handle.

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