Anti-howling software is designed to address acoustic feedback or howling that occurs in a closed acoustic loop (CAL) in which there is a gain. Feedback systems are stable when the gain of the system is less than one for the entire frequency spectrum. Figure 1 illustrates a scenario for closed acoustic loop.
The frequency response of a CAL has many peaks and valleys. Feedback howling typically occurs at peaks of the frequency response, but any frequency in which there is a gain, howling can occur. The original signal energy will be looped back continuously growing in intensity. This will result in ear-piercing feedback and/or annoying echoes.
The obvious way to design a system that does not produce howling is to make a system with a CAL in which the entire frequency response spectrum is less then one. But sometimes being able to provide that extra amplification to the microphone signal would be of great benefit to the listener. As mentioned previously, typical frequency responses have peaks and valleys, thus if a frequency shift is applied to the microphone signal, within a couple cycles, the signal can be shifted into a valley where the gain is less than one. In most systems the difference between peaks and valleys is about 10dB and in some systems over 30dB. By applying a frequency shift, an audio system can amplify the loudspeaker signal by an additional 10dB without creating ear-piercing howling.