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Thomann's Cool Online Guides: Acoustic Guitar Amps

3. Feedback Elimination

Notch
FilterA circuit used to remove or accentuate frequencies in a sound source. Common types are found on mixing consoles equalisation, synthesizers and dedicated processors such as graphic equalisers.
Filters
& Dynamic
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
Feedback
Rejection

One of the inherent dangers in amplifying acoustic instruments is that of
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
. While some manufacturers have attempted physical remedies - thinner bodies and sound-hole covers for instance - these all have a detrimental effect on the sound of the instrument, and the main means of tackling the problem today are electronic.

Most
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
occurs at a specific frequency, typically a ‘howl’ at an identifiable pitch. Most electronic
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
solutions therefore involve some form of EQ to remedy the situation. The standard frequency bands of most equalisers, however, are far too wide to reduce
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
without seriously affecting basic sound quality – that’s what they’re for, after all.

Seek and Destroy

The device required is a notch
FilterA circuit used to remove or accentuate frequencies in a sound source. Common types are found on mixing consoles equalisation, synthesizers and dedicated processors such as graphic equalisers.
filter
: a very narrow, high rejection EQ band.

While fixed or manually variable notch
FilterA circuit used to remove or accentuate frequencies in a sound source. Common types are found on mixing consoles equalisation, synthesizers and dedicated processors such as graphic equalisers.
filters
have their uses, most
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
is unpredictable, occurring at different frequencies depending on the instrument, venue and
TrimThe initial gain control on a mixer or other audio device such as a mic preamp, often labelled 'gain'. Trim/gain adjustment is crucial to maintaining a clean gain structure and good signal-to-noise ratio.
gain
structure. Dedicated
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
elimination works by continually analysing the incoming signal, and then varying the frequency, width and
TrimThe initial gain control on a mixer or other audio device such as a mic preamp, often labelled 'gain'. Trim/gain adjustment is crucial to maintaining a clean gain structure and good signal-to-noise ratio.
gain
reduction of the notch
FilterA circuit used to remove or accentuate frequencies in a sound source. Common types are found on mixing consoles equalisation, synthesizers and dedicated processors such as graphic equalisers.
filter
in order to eliminate any
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
detected. By this means, the
FilterA circuit used to remove or accentuate frequencies in a sound source. Common types are found on mixing consoles equalisation, synthesizers and dedicated processors such as graphic equalisers.
filter
frequency can usually be extremely narrow, with little other audible effect on the signal.

Some amps also feature a manually activated notch
FilterA circuit used to remove or accentuate frequencies in a sound source. Common types are found on mixing consoles equalisation, synthesizers and dedicated processors such as graphic equalisers.
filter
which can be set to reduce the known resonant frequency of your instrument, usually in the
RegionA region is usually a section of MIDI or audio first defined by recording or imported length, which can be copied, edited or deleted non-destructively as part of the editing process in a DAW.
region
of 120Hz.

PhaseIn audio and signal processing in general, phase describes the relationship between one or more signals and time. If two signals are identical but one is delayed by less than one complete wave cycle, the difference is described as a phase difference. This is normally expressed in degrees, with 360 representing a full wavelength.
Phase
Reversal

Another technique for dealing with
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
involves simply inverting the
PhaseIn audio and signal processing in general, phase describes the relationship between one or more signals and time. If two signals are identical but one is delayed by less than one complete wave cycle, the difference is described as a phase difference. This is normally expressed in degrees, with 360 representing a full wavelength.
phase
of the amplified signal. The reason this works is due to the very nature of
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
: the guitar resonates in response to sound from the speakers - if a certain frequency is highly resonant, it will be heavily present in the amplified signal, which will cause the guitar to resonate even more at that frequency, and so on in a vicious circle. When this happens, it happens very quickly – the effect increases with every cycle until limited by the maximum
TrimThe initial gain control on a mixer or other audio device such as a mic preamp, often labelled 'gain'. Trim/gain adjustment is crucial to maintaining a clean gain structure and good signal-to-noise ratio.
gain
of the
AmplifierA device for increasing the power of a signal by taking power from a supply and shaping the output to match the input signal.
amplifier
. Inverting the
PhaseIn audio and signal processing in general, phase describes the relationship between one or more signals and time. If two signals are identical but one is delayed by less than one complete wave cycle, the difference is described as a phase difference. This is normally expressed in degrees, with 360 representing a full wavelength.
phase
of the amplified signal means that any vibration of the guitar body at that frequency is ‘discouraged’ by the amplified signal, because at any given time it is pushing or pulling in the other direction. In the real world, the efficacy of this method depends on many factors including the distance and angle between the guitar and
AmpereUnit of current; the amount of electricity flowing through a conductor. One Ampere is equal to the flow of 6.25x10^18 electrons through a conductor in one second.
amp
, but it remains an interesting technique and is featured on some acoustic amps.

High-Pass Filter

Very low frequency
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
can be suppressed with a simple high-pass
FilterA circuit used to remove or accentuate frequencies in a sound source. Common types are found on mixing consoles equalisation, synthesizers and dedicated processors such as graphic equalisers.
filter
. This is usually set with a cut-off frequency of around 80Hz to eliminate any low frequency
FeedbackThe phenomenon whereby audio picked up by a microphone or guitar pickup that is then played from a speaker close or loud enough for it to be captured again by the same source. If left the signal will continuously loop, with any resonant frequency causing the undesirable 'howling' sound often heard at concerts.
feedback
, mains
HumUnwanted sound of a specific frequency constantly present in an audio signal, often caused by interference from AC mains power and therefore occurring at 50Hz or mathematically related frequencies.
hum
and guitar body
DistortionIn most cases distortion is an undesirable alteration to a signal which occurs when a piece of equipment is driven with a input level that is too high for its operating level. Sometimes, as in the case of guitar distortion, this can be an intentional and desirable effect.
noise
below the guitar’s musical range (the guitar’s open low E string has a frequency of 82.4Hz).

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