Behringer DSP1100P user manual Graphic equalizers, Typical feedback loop

	Speaker Box
Microphone
Mixing Console	Power Amp

Fig. 4.3: Typical feedback loop

Basically, any microphone signal passing through an amplifier is liable to generate feedback. Unfortunately, the feedback frequencies of P.A. systems differ, and even one single system can have varying feedback frequencies, as these depend largely on the room acoustics. Feedback can be caused by the following condi- tions:

sThe microphones are too close to the speaker boxes or the speakers are badly positioned (monitor sys- tem).

sThe microphone channels on the mixing console are not set up correctly.

sThe microphones used are not operated in accordance with their directivity (e.g. cardioid/super-cardioid).

sThe acoustic properties of the room are unfavorable. Tiled walls and floors heavily reflect the sound.

All rooms feature a number of natural resonances sometimes with “high” quality factor. At such frequencies the potential risk of feedback is increased. In addition to the acoustic properties of the room, the relative position of the microphone with reference to the loudspeaker plays a decisive role for the generation of feedback. In practice, this phenomenon can be observed with microphone held directly in front of a speaker, thus producing feedback. Of course, the first thing to do in such a case is to move the microphone away from the speaker. When doing so, you can hear the feedback frequency change, because a variation in the distance between microphone and loudspeaker results in a variation of feedback frequencies. It is therefore very difficult to anticipate feedback frequencies and avoid their occurrence by means of equalizers with fixed settings.

4.2 Graphic equalizers

Graphic equalizers are part of the audio engineer’s standard equipment for live applications. In this context, graphic EQ’s usually perform three main tasks:

sFine tuning the mix to your taste and the style of music.

sFine tuning the mix to the room and speaker acoustics.

sTo some extent, audio engineers can use graphic EQ’s to manually suppress annoying feedback.

The 1/3-octave design with 31 faders per channel has become the standard among graphic equalizers. Here, the spacing between individual filter frequencies is 1/3 octave. The quality factor (Q) of the filters (usually 1/3 octave) is fixed as are the frequencies controlled with the 31 filters.

1/3-octave equalizers are very popular (e.g. our ULTRA-CURVE PRO DSP8024) because they are so easy to operate. The fader positions clearly show how the signal is being processed, especially since graphic EQ’s have fixed frequencies based on the so-called ISO standard. So, all graphic equalizers designed to meet the ISO standard feature the same fixed frequencies. Once you’ve got used to work with a 1/3-octave equalizer, you will find the FEEDBACK DESTROYER PRO a highly convenient tool, as it splits up the audio spectrum into the ISO frequencies (see table 1.2), which enables you to access the most important frequencies quickly.

22 4. TECHNICAL BACKGROUND