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c)
Improvement
of sound
quality
(i.e.,
greater clarity
of
music
and words).
The 562 Feedback Suppressor
is
not
a
cure-all for serious
sound system deficiencies.
It
will not make inferior components
perform
like superior ones, nor will
it
completely correct poor
acoustic conditions or poorly engineered installations.
3.2.2
SUSCEPTIBILITY TO FEEDBACK.
In
public address
and sound
reinforcement systems,
the
maximum
acoustic gain that may be obtained for
a
microphone
in
the
vicinity
of
a
loudspeaker which
is
part
of
that system, will
be
determined
by the
positive feedback loop created when sound from
the
loudspeaker
enters
the
microphone, reinforcing
the signal level
until the
system goes into oscillation.
This positive feedback problem
is
aggravated
if
some frequencies
in the
audio range are reproduced
at
a
higher level than
the rest of the
frequency band.
If
the level
of
these frequencies which are being reproduced at an exaggerated
level can be reduced, then the sound system gain
may
be increased
to
some degree
without feedback.
An improvement of
a
system's tendency
to
feedback should
be
attempted after
the
general equalization
of the
frequency response
is
performed.
The result will
be
a
higher amplifier gain setting
than was possible before feedback suppression.
Remember:
an
improvement
of
3
dB
is
equal to twice
the
previously available
power.
To
prevent ringing,
it
is
best
to
adjust
the
gain
at
least
3
dB
below the threshold
of
feedback.
3.2.3
HOW MUCH ADJUSTMENT?
Theory says that for
a
given change
in
amplitude response,
using
a
minimum-phase filter network, there will
be
a
corresponding
phase change.
While you may not hear
the
amplitude response
change, you may hear the effects of the phase angle rotation.
Since
the
minimum amplitude response change generally causes
the
minimum change
in
phase response, the shallower
the
notches
for
feedback suppression, the less likely that they will
be audible.
When adjusting
the Model 562
in the
field,
it is
often possible
to
ignore the above considerations.
Program material
and
deficien-
cies
in
the
speaker system
and
acoustic environment
are usually
several orders of magnitude larger than
the
effects introduced
by
the notch filters,
even
at
extreme settings.
This
is
especially
true
in
vocal reinforcement; music reinforcement systems
are
typically less tolerant.
When using
the Model 562 in
a
system
intended
to
reproduce music,
care should
be
taken not
to
notch out
too much
program material.
Once
the
feedback modes have been found
and the filters are
adjusted according
to the
procedure
in
Section
3.1, the
depth controls may
be
backed off
to
determine
if
a
notch
less
deep will still suffice.
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