The FD23 provides a quick inexpensive means of detecting which frequency is causing the feedback. Generally, feedback (ringing) is a self-sustaining and erroneous oscillation that occurs when sound arriving at a microphone is greater than the sound produced by the person or instrument using that microphone. Feedback is controlled through the use of a graphic or parametric equalizer, and these units are most effective when the sound engineer can quickly recognize the frequency at which feedback is occurring.

The FD23 is a rugged, inexpensive, hand-held instrument that can be used with either alkaline or nicad batteries. It can also be powered with a wall-wart providing 12Vdc @ 200mA. Nicad batteries can be recharged through the external power jack.

The FD23 operates using the same filter design that is found in Gold Line's spectrum analyzers. Ease of operation is through a single on/off/sensitivity control. If a frequency is ringing and the sensitivity is set accurately, the display of the FD23, which has LEDs for standard ISO third-octave frequencies from 80Hz to 12.5kHz, will light an LED representing the frequency that is ringing (within that frequency's tolerance). Most equalizers have adjustments (sliders) at the same frequencies and by moving the slider that matches the frequency indicator on the FD23 you will reduce the feedback.