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Feedback
Feedback is often a problem with amplified acoustic instruments. Much more so than for solid bodied instruments.
There are several ways to tackle feedback. In many situations the player will need to employ a combination of these measures. The physical approaches are:
* Positioning
* String/body damping
* Soundhole blocking
There are also electronic measures:
* Phase switching
* EQ
Positioning
Simply positioning the player and his instrument away from speakers can do a lot to eliminate feedback. If the player is playing into an external mic then it is wise to ensure that speakers and foldback monitors do not point into those directions that the mic is designed to monitor. Most specifically, you don't want the amplified sound coming from behind the player and going into the mic.
String/body damping
In some marginal situations the feedback occurs when the instrument is idle. In this case it is often enough to simply place a hand across the strings, or across the strings and soundhole. This stops sympathetic vibration and reduces the tendency to feed back.
Soundhole blocking
You've probably seen guitarists with black rubber or plastic plugs in the soundholes of their guitars. These "feedback busters" can greatly lessen feedback, sometimes eliminating it altogether. They are available in several sizes, although they can be hard to find in uke sizes. However, it is easy to improvise plugs for most uke soundholes by using various plastic stoppers and plugs. For example, a mailing tube end plug can be adapted to fit a Makala soprano, and the plastic lid from a gravy granule container will fit a Kala tenor.
These plugs do have a noticeable detrimental effect on the tone of the instrument, so some players try to balance maximum feedback reduction with a minimal effect on tone by having small holes in the soundhole plug.
Electronic measures
Phase switching
A phase switch is usually just a two-position switch on a preamp or amplifier. If feedback occurs with the phase switch in one position, flipping it to the other position often cures or reduces the problem.
The Fishman Pro EQ II preamp and the
Marshall AS series acoustic amplifiers both feature this facitlity.
EQ
The simplest EQ-based anti-feedback measure to use is the notch filter. A notch filter is one of the most effective measures to be found on most acoustic amplifiers. It is simply a knob which, when switched on, suppresses a narrow band of EQ frequencies. When feedback occurs one simply switches on the notch filter and then turns the knob until reaching the point where feedback is eliminated or minimised. In my opinion, this is one of the most useful features to look for on an acoustic amplifier.
The
Marshall AS acoustic amplifier series models feature notch filters
If your preamp or amplifier do not have a notch filter facility, a guitar multi-band EQ can help. It is sometimes possible to locate the frequency band within which the feedback is occurring, and to cut it by reducing the level of the slider/knob controlling that band.
Guitar multi-band EQ pedals are available from
Boss and, more affordably, from
Behringer. [...]