2.4 Loudspeakers
2.41 Consistency of phase between drivers of the same type, and operating over
the same frequency band.
Connect the source to the drive unit, either directly (if ambient noise is low), or via a power
amplifier and a uni-directional microphone to the detector. Set the source frequency range
to suit the particular driver, and hold the microphone up to the driver, on axis, and observe
the detector's phase LED. It will be necessary to select the required frequency by
experimentation, and adjusting via the recessed pre-set adjusters. Any frequency will do
that gives a good strong indication, and which is generally uncritical of microphone distance
or placement. Correct frequency choice is necessary due to the considerable phase
movement that occurs within loudspeakers and their cabinets. A hesitating detector
indication will generally indicate an incorrect choice of frequency. Avoid picking up reflected
sound from the sides of the cabinet horn flares, or room reflections, as these will be of
reverse phase as a consequence of their reflection, and will be added to the direct signal by
the microphone.
Having obtained a phase LED colour for this driver or cabinet, other units of the same type
can then be checked quickly for their consistency. Changing to other types of drive units will
probably necessitate a change in tone frequency. It will be observed that for a given drive
unit, the phase relationship between the electrical input signal, and the acoustical output, as
measured by the microphone, will change the frequency. This is quite normal, and indicates
the quite considerable phase movement within a drive unit and its cabinet.
In general the colour of the phase LED is unimportant, as long as all drive units and cabinets
of the same type, give the same result, when tested in the same manner. On cabinets with
more than one drive unit fitted, the detector will pick out reverse wired, or non working units.
Note that a reverse phase indication for one unit, which is mounted within the same
acoustical chamber as the rest, can mean that it is not working rather than the reverse
wired, as the cone of this unit will be driven by the back pressure generated within the rear
chamber by the other working unit.
2.42 Correct phase alignment at the crossover points
It is of prime importance in multiple speaker systems, where different drive units handle
different parts of the frequency spectrum, that at the crossover region, both drive units are
working acoustically together. This can be checked by setting the source tone frequency to
this crossover point, and then driving each unit, in turn, to give the same phase LED colour.
During this type of test, only the drive unit being measured should be switched on, otherwise
interaction can occur between the two units. It is also important that any crossover or phase
correction units normally used to drive the system should be incorporated within the test
system, as these are sometimes trimmed to suit the speaker at its critical crossover region.
BSS Audio
Linkside House,
Summit Road,
Potters Bar,
Herts EN6 3JB
England
Tel +44(0)1707 660667
Fax +44(0)1707 660755
WWW http://www.bss.co.uk
