The tendency of public‐address systems to become unstable, when operated in reverberant spaces, can be reduced by shifting all frequency components of the audio signal by about 5 cps. Frequency shifts of this magnitude will raise the gain at which a public‐address system remains stable by about 10 dB. However, heating the direct‐sound signal and the frequency‐shifted signals limits the usable additional gain to about 6 dB. Electronic devices for effecting frequency shifts of the desired magnitude and stability have been described previously and are now available commercially. In this paper, a theory of the stability of public‐address systems with and without frequency shifting is presented and compared with experimental results obtained in rooms of various dimensions. Good agreement is found between theory and experiment. In addition, frequency responses of rooms are simulated by a Monte Carlo method on a digital computer and the permissible gains for stable operations of public‐address systems are evaluated on the computer. The Monte Carlo results fall within 0.6 dB of the theory.

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