From Wikipedia, the free encyclopedia
Audio feedback (also known as the Larsen effect
after the Danish scientist, Sψren Larsen, who first discovered
its principles) is a special kind of
feedback which occurs when a sound loop exists between an
audio input (for example, a
microphone or
guitar pickup) and an audio output (for example, a
loudspeaker). In this example, a signal received by the
microphone is
amplified and passed out of the loudspeaker. The sound from
the loudspeaker can then be received by the microphone again,
amplified further, and then passed out through the loudspeaker
again. This is a good example of
positive feedback. The
frequency of the resulting sound is determined by resonant
frequencies in the microphone, amplifier, and loudspeaker, the
acoustics of the room, the directional pick-up and emission
patterns of the microphone and loudspeaker, and the distance
between them.
More specifically, the conditions for feedback follow the
Barkhausen criterion, namely that an
oscillation occurs in a feedback loop whose delay is an
integer multiple of 360
degrees and the
gain
is equal to or greater than 1 (both at the given feedback
frequency). If the gain is greater than 1, then the system can
start to oscillate out of noise, that is to say: sound without
anyone actually playing.
Most audio feedback results in a high-pitched squealing noise
familiar to those who have listened to bands at house parties,
and other locations where the sound setup is less than ideal
this usually occurs when live microphones are placed in the
general direction of the output speakers. Professional setups
circumvent feedback by placing the main speakers a far distance
from the band or artist, and then having several smaller
speakers known as
monitors pointing back at each band member, but in the
opposite direction of the microphones.
Audio feedback is usually undesirable. However, it has
entered into musical history as a desired effect beginning in
the early 1960s. Although it is now well associated with the
history of rock music where
electric guitar players such as
Pete Townshend and
Jimi Hendrix have used it extensively, it was the
contemporary American composer
Robert Ashley who first used feedback as sound material in
his infamous work Wolfman (1964). The Beatles inclusion
of feedback, the same year, in the opening of "I
Feel Fine" is rather tame compared to the twenty minutes of
vocal feedback in Ashley's composition. However, Beatles'
single, released in the UK in November 26, 1964, is widely
considered the first example of feedback included in a
commercial recording. It was used extensively after 1965 by the
The Monks and
The Velvet Underground, first played onstage by
The Who. Used in this fashion, the artist has some control
over the feedback's frequency and amplitude as the guitar
strings (or other stringed instrument) form a
filter within the feedback path and the artist can easily
and rapidly "tune" this filter, producing wide ranging effects.
Artists can even manipulate feedback by shaking their
instruments (in the style of Pete Townshend) in front of the
amplifier, creating a throbbing noise. More recently,
Audioslave guitarist
Tom Morello (formerly of
Rage Against the Machine) has employed feedback in
conjunction with a "killswitch" on his guitar and heavy use of
the
whammy bar to create inventive
hip hop-influenced solos. The most extensive use of feedback
in a commercially released recording is
Metal Machine Music by
Lou Reed.
Also note that desirable feedback can be created by an
effects unit by using a simple delay of about 50 ms fed back
into the
mixing console. This can be controlled by using the fader to
determine a volume level.
To avoid feedback, automatic anti-feedback filters can be
used. (In the marketplace these go by the name "feedback
destroyer" or "feedback eliminator".) These electronic devices
are a multi-band
parametric equalizer combined with a
spectrum analyzer which applies a
notch filter to frequencies with a very high peak compared
to the rest of the audio spectrum. This can also be accomplished
by an audio engineer using either a parametric equalizer
(normally found on a mixer's input channels) or a
graphic equalizer (commonly connected between the mixer's
outputs and the amplifier inputs to correct acoustical problems
in a room or to address feedback issues).
See also
