Dolby Laboratories CP65 - Manual (Page 77)

A.1
APPENDIX A
CHECKING PHASING OF SPEAKERS
Loudspeakers are two-wire devices that can be connected incorrectly as well as correctly.
Incorrectly wired loudspeakers in a multiple-speaker installation cause degraded performance.
For example, woofers in an array must work together to generate a solid wavefront of sound
pressure. If some woofers are moving in one direction at the same time that other woofers
are moving in the opposite direction, the result is partial cancellation and hence loss of bass.
(The lower the frequencies being handled, the greater is the cancellation.) Wiring can be
reversed inside a connector, a transformer can be mislabeled, and the internal wiring
polarities of mixers, crossovers, voice coils, and other inaccessible equipment may be
unknown.
The Cat. No. 85C pink noise generator can be used to determine correct speaker phasing.
Position a microphone as shown on page 5.7 and connect it to the RTA. Switch on pink noise
to the center channel in phase (C switch down) and observe the RTA display. Switch on pink
noise to the left channel in phase (L switch down). The level in each band displayed on the
RTA should rise by 2 to 4 dB. If the level in any band decreases then check phasing of the
speaker connections. Repeat this procedure for center and right channel speakers.
A number of hand-held devices are available to assist the installer to determine if the speakers
in a theatre installation are correctly wired. Virtually all such devices consist of two units —
a pulse or tone generator which is connected to the speaker being tested — and a polarity
detector which is driven either by an integral or external microphone. The detector contains
LEDs which indicate the direction of motion of the speaker. It is necessary only to connect
the generator to the speaker being tested and turn on the test signal. The detector LEDs
automatically indicate the direction of motion. It is sometimes difficult to interpret the indication
given on these devices, but they are useful in determining whether several loudspeakers
behave identically. (If a special adapter cable is made, the generator can be connected to the
CP65 aux input or the L and R output test points on the Cat. No. 240 module so the phase
check will include all of the system after the optical preamplifier.)
It is strongly recommended that the phasing of all of the speakers be checked before any of
the alignment procedures are started.
Typical phasing checkers are as follows:
SCV Audio Model PC80-439 available from QMI, 15 Strathmore Road, Natick, MA 01760
Cricket Polarity Test Set available from Galaxy Audio, 625 E. Pawne, Wichita, KS 67211
Check with your dealer for complete information on these and other systems.
IN PHASE
OFF
OUT OF PHASE