Resonance in Rooms

The observed characteristic frequencies (or eigentones) in a small room are shown to agree with the theoretical frequencies n for the vibrating volume of air in a six-sided rectangular room, namely n = c2p2l12 + q2l22 + r2l32 where c is the velocity of sound, l1, l2, and l3 the dimensions of the room, and p, q, and r are integers. One or more of these integers vanish when one or more surfaces in the room are covered with absorptive material. When a tone, which is nearly of the same frequency as the frequency of one of the eigentones, is sounded and then stopped in the room, the decay, especially during the latter stages, consists of the free rather than the impressed vibration. Thus, in a room in which the fundamental frequency was calculated to be 70 cycles, oscillograms revealed that impressed frequencies of 65, 68, 70, 72, or 75 cycles all decayed at the fundamental frequency of 70 cycles. The reverberation was about 50 percent longer for the resonant frequency (70 cycles) than it was for frequencies only 5 cycles lower or higher. Similar effects were observed in the vicinity of several of the overtones for the room. The results show conclusively that volume resonance must be considered in determining the absorptivity of materials by the reverberation method, and that resonance is an important factor in determining the acoustics of small rooms.