Consider a basic sound system consisting
of a single loudspeaker, a microphone,
and an amplifier (Figure 5-6). A
performer stands at the microphone,
and a listener sits in the audience.
With the system powered up, by advancing
the gain of the amplifier you
will reach a point where the system
starts to ring. This ringing is called
feedback. The bold arrows in the figure
indicate the feedback path. Some of the
sound from the loudspeaker is picked
up by the microphone and re-injected
into the system, forming a continuous
loop.
Feedback occurs when the gain in
the loop reaches unity (gain of 0 dB).
The feedback locks onto a frequency for
which the loop path is non-inverting (in
phase).