Northern Airborne Technology PA700 - Manual (Page 12)

PA700 Series High Power Voice Amplifier
SM44 Installation and Operation Manual
Section 2 Rev: 1.01 Issue 4 Page 2-3
ENG-FORM: 805-0117.DOT
CONFIDENTIAL AND PROPRIETARY TO NORTHERN AIRBORNE TECHNOLOGY LTD.
2.4.3 Ground Site Selection
Sound waves transmitted through air are strongly affected by wind, weather, time of day and intervening
objects. Satisfactory sound projection over great distances is possible from a well-situated operating site
that can take maximum advantage of these natural elements.
Optimum location of the loudspeakers is the prime controlling factor in site selection. This is especially
important when maximum projection is required.
An operating location with a line-of-sight observation of the target objective is desirable. For example, a
hillside location with the loudspeakers directed down or across the valley would be ideal. Sound carries
well across water and consequently advantage can be taken of a river or a lake to improve sound
transmission. It is preferable to locate upwind of the target in order to benefit from the carrying power of
the wind. Whenever possible, the site should be free of foliage or other obstructions for several hundred
yards in front of the loudspeakers.
Directional sound bends away from warm surfaces. For example, transmissions over warm pavements
attenuate rapidly with distance, as the sound ‘curls upward’ into the cooler upper air.
2.4.4 Airborne Installation
Experience shows that sound projection from fixed wing or rotor aircraft is possible up to 2 miles slant
range. If a circling course is planned, the loudspeakers should point horizontally alongside or at right
angles to the airframe. This permits the bank of the aircraft or the downward thrust of rotor blades to
direct the sound beam toward the ground target. To reduce air resistance, the speakers may be mounted
in an open doorway or fitted into a streamlined pod. From a hovering helicopter, intelligibility is improved
by focusing the directional sound to an area outside the rotor blast.
When an input audio line from an intercom, radio, recorder, etc. to REC input is used, precautions are
necessary. The high gain of the amplifier requires care in prevention of feedback. Complete elimination of
ground loop and stray currents in the input line is mandatory. This may mean an isolation transformer is
necessary, if not otherwise provided by the signal source. Refer to Dwg No. AA22\492\403-1 for details of
an aviation-specific interconnect.
Shielding the microphone from direct sound waves from the speakers prevents acoustic feedback. This
may be achieved using an enclosure, or by operating at a considerable distance behind the speaker
pattern.
Multiple speakers require correct phasing, otherwise sound cancellation will greatly reduce the projected
power. The proper phasing is provided by the cables supplied. If any repairs or re-cabling are required,
the wiring should be checked from Dwg No. PA700\407-0.
2.4.5 Remote Installation
For installations where the Audio Power Amplifier will be remote mounted, it is recommended that the
knob be removed from the GAIN control and be replaced with the compression fitting (NAT Part No.
25-01-002, Smith p/n 181) supplied in the PAREC-IKS and PAMIC-IKS install kits. Do not remove the nut
that secures the GAIN potentiometer to the control panel. Once the compression fitting is installed on the
bushing of the potentiometer, set the GAIN control to the desired setting then carefully tighten the
compression fitting on to the shaft of the GAIN control potentiometer.