Acoustic feedback (also known as howling) is inevitable whenever sound reinforcement is necessary. For instance
in large meeting rooms, court rooms, classrooms, auditoriums, theaters, etc. In such large rooms, although audience
(or participants) are seated in the same room, they may be seated far from the presenter or from each other and
may not hear one another clearly.
In sound reinforcement systems, microphones are used to pick up the talker voice, the microphones signals are amplified,
and the amplified signal is played to loudspeakers placed in the same room, so that audience can hear clearly.
The problem, however, is that the microphones not only pick up the presenter's voice. They also pick up the loudspeaker
sound after traveling through the room back to the microphones, causing a closed electroacoustic loop.
If the loop gain at any frequency becomes larger than 1, the loop becomes
unstable at that frequency, and the system starts oscillation, producing the well known howling phenomenon.