transducers.
Physical
driver positioning and relative
electrical
polarity will affect frequency response in
the crossover region, and improper phase or
polarity relationships between channels can
cause
a
loss of
bass
response and/or degradation of the
stereo
image.
NOTE:
Where this
discussion
involves the exchange of
signal-carrying
conductors,
POLARITY
is the topic, not
phase.
Polarity
reversal
is not a 180°
PHASE
SHIFT,
although
some
people loosely
describe
it as
such.
Physical
driver positioning affects the relative
phase
of signals, but, for the purposes of this dis-
cussion,
it is assumed that the loudspeaker system
is
already constructed and therefore
nothing
can be
done to alter the phase relationship between trans-
ducers.
It is true that in a multi-way system
with
separately
packaged mid and high frequency horns
and
bass
bins, it is advisable to experiment
with
relative phasing by moving drivers to align the
acoustic
plane of all the transducer diaphragms.
These
techniques are beyond the scope of this
manual,
however. We have
limited
this discussion
to the method of establishing the correct signal
polarity for the components in any one channel
multi-way system, as well as the correct polarity
relationship between channels.
Two
suitable methods for establishing polarity
of components in a two-way system are described
in the
following
paragraphs. Either one may be
used,
depending on the availability of test equipment. A
three-way or four-way system would be treated in a
similar manner by first establishing the proper
polarity for the transducers of the low frequency
transducers,
and then progressing to the midrange
and high or ultra-high frequency transducers, as
applicable.
Objective
Method
of Polarity Determination
The
following
test equipment is
necessary:
a.
A real-time 1/3 octave or continuous
spec-
trum
frequency response analyzer.
b. A microphone of known, calibrated frequency
response.
c.
A pink noise source.
NOTE:
To optimize the
results
obtained with this procedure,
the loudspeaker
systems
should be placed in their normal
operating locations.
NOTE:
If different loudspeaker
systems
are
used,
or in a
stereo
system,
first establish common polarity among the
low frequency drivers as follows:
1.
Connect the pink noise source to two channels,
set
the two channels for the same comfortable level,
and observe low frequency response on the
real-
time analyzer
(RTA).
2.
Now intentionally reverse the polarity of one
low frequency transducer and see whether the low
frequency response diminishes or
increases.
If it
diminishes,
the system had been correctly wired
initially; return the transducer wiring to the original
polarity. If low frequency response
increases,
the
two channels' low frequency transducers were
reversed
in polarity relative to one another; leave
the wiring in the "improved" configuration.
NOTE:
In
stereo
or multi-channel
systems,
folio w
steps
3
and
4 below for one channel at a time.
3.
Using pink noise as program material, adjust
the system volume to a comfortable listening level.
Then
set the level of individual transducers to dis-
play the flattest overall frequency response on the
real-time analyzer.
4.
Reverse
the polarity of the high frequency
driver and observe the effect on the frequency
response
through
the crossover region. Proper
transducer polarity is indicated by the flattest
frequency response
through
the crossover region
(as
shown by the RTA).
5.
Once proper polarity among the transducers
of a loudspeaker system has been determined,
other loudspeakers systems in the installation
(assuming
they are the same model) may be con-
nected accordingly and double-checked
with
the
RTA.
Subjective
Method
of Polarity Determination
The
following
method is recommended to
establish
proper polarity of the loudspeaker system
when no test equipment is available. To optimize
the results obtained
with
this procedure, the loud-
speaker
systems should be placed in their normal
operating locations.
NOTE:
If different loudspeaker
systems
are
used,
or in a
stereo
system,
first establish common polarity among the
low frequency drivers as follows:
1.
Connect a program source known to contain
substantial
bass
to two channels, set the two chan-
nels
for the same comfortable level, and listen to
the intensity of the low frequencies.
2.
Now intentionally reverse the polarity of one
low frequency transducer and see whether the low
frequency response diminishes or
increases.
If it
diminishes,
the system had been correctly wired
initially; return the transducer wiring to the original
polarity. If low frequency response improves, the
two low channels' low frequency transducers were
reversed
in polarity relative to one another; leave
the wiring in the "improved" configuration.
NOTE:
In
stereo
or multi-channel
systems,
follow
steps
3
through 5 for one channel at a time.
3.
Adjust the volume for a comfortable listening
level
and set the approximate system balance using
the noise heard between stations of an un-muted
FM
tuner.
4.
Listen to a recording of a male voice long
enough to be accustomed to the performance of
the loudspeaker system.
5.
Reverse
the polarity of the high frequency
loudspeaker, which will produce a change in voice
character.
When the transducers are in correct
polarity, a recorded male voice should sound
natural and exhibit presence (an "up
front"
quality), in contrast to the undesirable "hollow"
sound heard when the transducers are out of phase.
NOTE:
If the
crossover
point
lies
above 2 kHz, reversing
the polarity of the high frequency driver
will
create
little
(if any) perceptible difference in
system
performance. The
15
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