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3.2.4
INTELLIGIBILITY AND FREQUENCY MASKING
Most
of the
discussion
on the use of the Model 562 has focused
on the
reduction
of
feedback.
However,
in some
instances
this
may
not be its most important function.
Sometimes
a
system may already
provide adequate gain before feedback,
and
other times
a
system may
have
so
many modes that notching out
four or five of them with
a
Model 562 will not provide
a
signficant increase
in
useable gain.
In
either
of
these cases, however, there may be masking
of
important
mid and high frequency sounds by peaks
in the
bass range.
Bass
peaks tend to occur due
to
typically longer reverberation times
at low
frequencies.
Here, the Model 562 may be used to reduce the
bass peaks
and
hence improve speech intelligibility and reduce
frequency masking
of
music.
3.2.5
USE OF ENDCUT FILTERS
The endcut filters of the Model 562 are used to restrict the
bandwidth
of the
program material
for one
of
several reasons:
1)
It
may
be
necessary
to
remove feedback modes
at
very
high and very low frequencies that are beyond the range
of
the
notch filters.
This might occur
in
a
situation where
the
microphone
and
loudspeaker
are fairly close to each
other.
For example, consider
a
stage
monitor system
where
a
speaker
is
located close
to the
microphone
and
aimed directly at
it.
In
this instance, feedback at
frequencies (above
6
kHz)
may occur due
to an
unusually
high level
of
high frequency gain available for feedback.
(Very high
frequencies do not generally reach
the
micro-
phone due
to
their greater absorption
in
air, and due to
greater speaker directivity at high frequencies.)
2)
In
small rooms, feedback at very low frequencies may be
a
problem.
Bear
in
mind that
the
acoustical treatment
of
many smaller rooms that have
not
been designed for music
utilizes acoustic materials
and
methods that
are only
effective
at
middle
and
upper frequencies.
For instance,
it
would not
be unusual to find
a
conference room designed
for NC-30* which actually has 90% absorption
of
signals
in
the
mid
to
high frequencies, but less than
10%
absorption
for low frequencies.
Additionally, low frequency reflec-
tions can be greater because
of the
proximity
of the
walls.
*
NC-30
is
a
standard Noise Criteria curve
of
frequency
vs.
level
showing an average
of 30 dB
SPL permissible background noise.
3)
It is
usually desirable
to
restrict
the
upper
and lower
bandwidths
to
only that necessary for
the
program material.
In the
case of
voice
this may be
a
considerable reduction.
This avoids
the
amplification
of
such
extraneous sounds
as
rustling paper, power line
hum, air
conditioning
noise,
mic stand rumble, and leakage from on-stage bass instrument
amplifiers
.
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