Some musicians may relish the howling, shrieking feedback generated
when amplification systems are turned up to high power, but for most it
is a pain in the neck. Now an electronic gadget from the US, called Exterminator,
will automatically hunt out and kill feedback, allowing the engineer in
charge of the sound to double the amplifier power. Musicians such as Rod
Stewart and the group Simply Red have already started using the Exterminator
at their concerts.
Public address systems and concert amplifiers start howling when someone
on stage puts their microphone or electric guitar too close to a loudspeaker.
The sound from the loudspeaker is taken up by the microphone or guitar pick-up,
re-amplified, and fed through the speaker to the microphone or pick-up again.
The sound goes round and round the electronic loop, getting louder all the
time, and peaking at any resonant frequency in the hall.
Sound engineers guard against feedback by keeping the overall level
of sound on stage low. They also use an electronic filter, like a hi-fi
tone control, which reduces the sound level at frequencies likely to produce
resonance. But existing filters cover quite a broad band of frequencies:
over a third of an octave. There may also be several other resonant frequencies
which need filtering, so the overall sound can end up badly distorted. It
is also sometimes difficult to identify all the resonant frequencies.
The Exterminator, made by Sabine Musical Manufacturing of Gainesville
in Florida, converts the sound from the microphone into digital code. A
digital sound signal can be filtered in a narrower band of frequencies:
just one-tenth of an octave. The filter can also change frequencies to target
the resonance exactly. Because the filter removes a narrower slice of frequencies,
up to nine can be targeted at the same time without noticeable distortion.
The filters pick their frequencies automatically. Before a concert,
the sound engineer turns all the amplifier controls up to full volume so
that the system starts howling. A sensor circuit inside the Exterminator
analyses the energy content across the range of frequencies and registers
the peaks caused by the feedback. The filters then align with the peaks.
When the concert begins, the engineer turns the sound down to normal
working level, where there is normally no feedback, and switches off the
filters. If anyone on stage brings their microphone close to a loudspeaker,
the Exterminator registers a sudden increase in energy and activates the
matching filter to block feedback.
Training the system before the concert stops the Exterminator mistaking
sustained musical notes for feedback and filtering them out.