Improvement of acoustic-feedback stability by bandwidth compression

Shifting the input signal´s frequencies by a fixed frequency Δf has been proven to be effective in improving the stability margin of public address systems operating in reverberant spaces. This paper describes the effect of an alternative approach of improving the acoustic feedback stability by compressing the bandwidth of the speech signal and yet maintaining full speech intelligibility. This technique requires small portions of the input signal to be discarded periodically since the operation is performed in real time. Criteria are developed for the bandwidth compression technique to effectively improve the stability margin of public address systems. A speech-processing system has been developed which incorporates these principles, and a series of tests has been performed both in a room environment and in an acoustic chamber to show how much the stability margin can be improved. The test results have shown that up to 10 dB improvement can be obtained in the stability margin by employing this technique.