Specifications

9
The illustrations below demonstrate how the DFR11EQ works in a sound system. The system on
the left shows a feedback loop, where the microphone picks up sound from the loudspeaker and
sends it back into the sound system. The system on the right shows how an installed DFR11EQ
reduces the gain on the worst case frequencies.
DFR11EQ
MIXER
AMPLIFIER
MICROPHONE
MIXER
AMPLIFIER
MICROPHONE
After the DFR11EQ stops the feedback at the dominant frequency, the sound system may start
feeding back at the next dominant frequency. In this case, the DFR11EQ inserts another notch filter
into the audio path at the new frequency. The DFR11EQ can insert a total of 10 notch filters to reduce
feedback.
DFR11EQ Limitations
The DFR11EQ (or any other notch filter system) cannot entirely eliminate feedback in a sound
system, it can only help to reduce it. In a typical system, a point of diminishing returns is reached after
4 to 8 notch filters are set. This is because generally there are only a few dominant frequency
response peaks above the response of the entire system (figure 4A). The DFR11EQ works very well
controlling these peaks. The user can expect a 6 to 9 dB improvement of gain-before-feedback in a
typical system. However, if the system has too much overall gain, then all of the frequencies have too
much gain; instead of trying to notch out all of the frequencies, better results will be obtained by
lowering the gain of the system (Figure 4B). If the system still has insufficient gain, then other
changes must be made to the sound system such as different microphone or loudspeaker placement.
FREQUENCY RESPONSE OF AN UNEQUALIZED SOUND SYSTEM
Feedback Threshold
Figure 4A
Use notch filters to lower peaks
Figure 4B
Lower overall gain of system
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