SECTION I - GENERAL

• Document Type: CREST
• Collection: General CIA Records
• Document Number (FOIA) /ESDN (CREST): CIA-RDP78-03642A001700030001-9
• Release Decision: RIFPUB
• Original Classification: C
• Document Page Count: 32
• Document Creation Date: December 22, 2016
• Document Release Date: February 22, 2012
• Sequence Number: 1
• Content Type: MISC

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Section
General
CONFIDENTIAL'
1. SCOPE - This technical manual contains information pertaining to
the description, assembly, operation and maintenance of High Powered
Voice Amplifier, Type DE-1492-A, manufactured by Applied Electro
Mechanics, Inc., Alexandria, Virginia.
-/
2. PURPOSE AND CHARACTERISTICS - High Powered Voice Amplifier
AEM Type DE-1492-A provides for a portable, lightweight, compact, high
gain tactical sound system, illustrated in Figure 1. It is designed for
point-to-point dissemination of sound over land, water or from the air.
The equipment described herein is capable of satisfactory operation over
the temperature range of minus 30 degrees Fahrenheit (-35C) to plus 130
degrees Fahrenheit (55C), relative humidity up to 97 per cent, and alti-
tudes from 0 to 12, 000 feet above mean sea level. The maximum useable
range is greater than one mile when operated from a favorable vantage
point. The maximum rated audio power output is 250 watts feeding a suit-
able horn array. Amplifiers may be operated in combinations of two, four
or more. Information concerning operation in combinations or for other
special requirements will be furnished upon request.
3. OPERATING COMPONENTS - Components and cables normally supplied
with the amplifier are identified in Figure 2, which includes unit dimensions
and weight. Figure 3 is a functional block diagram showing the intercon-
nection of operating components.
ORIGINAL CL BY
1:1 DECL yREVW ON
EXT BYND 6 YIJ
REASON
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4. DESCRIPTION OF AMPLIFIER- Amplifier DE-1492-A is pictured in
Figure 5. It contains the amplifying circuits necessary to furnish 250
watts of audio power into a suitable speaker load with an input from a low
level dynamic microphone. It also contains the proper operating controls
and suitable monitoring devices to check for proper operation. Its reli-
ability is enhanced by conservative design and a unique built-in protective
circuit which shuts off the amplifier whenever excessive input or loading
conditions threaten to impair vital circuit elements.
a. SPECIFICATION
(1) Supply Voltage- The amplifier will normally be powered from
a 24 volt storage battery with a terminal voltage, when charged, of 28 V DC
Lower voltage supplies may be used with lower resultant audio power being
generated. Supplies having higher than 32 volt steady or peak values will
cause the amplifier protective circuit to trip. No supply used should have
an internal impedance greater than 0.15 ohms.
(2) Input Signal - The amplifier will give full power out with an
input voltage of 0. 01 volts rms.
(3) Frequency Response - The amplifier response is flat within 3 db
over the frequency range of 500 to 5000 cps.
(4) Harmonic Distortion - Because amplifier DE-1492-A operates
largely in a saturated state, its wave form at full power approaches a square
wave. Operating at 0. 4 rated power, its amplitude distortion is 12% or less,
measured at a frequency of 2000 cps with sine wave input.
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(5) Load Impedance- The specified resistive load for the DE-1492-A
amplifier is 1. 75 ohms. This is the nearest equivalent to a nominal 2 ohm
speaker load. Usual load is furnished by two 4 ohm speakers connected in
parallel. However, higher directivity may be gained by paralleling four
8 ohm speakers to load one amplifier.
(6) Output Power - Specified output power is 250 watts into a 1. 75
ohm resistor with 0. 01 volt rms., 2000 cps. sine wave input. Tests for
equivalent power delivered to a speaker load are determined by watt meter
measurement.
(7) Temperature Range - Amplifier DE-1492-A is capable of oper-
ation over the temperature range of -30 ? F to + 130 ? F (-35 to +55 ? C).
Operation at the high extreme, however, will necessitate a lower than normal
duty cycle to prevent the temperature sensitive protective circuits of the
amplifier from interrupting the operation.
(8) Altitude - The amplifier is capable of operating at any altitude
up to 12, 000 feet.
(9) Input Impedance - The low level MIC input impedance is 60 ohms.
The high level Recorder input is approximately 1200 ohms.
b. EXTERNAL FEATURES
(1) Construction- Amplifier DE-1492-A is built into an extruded and
welded aluminum chassis which provides both a rugged case and an excellent
heat sink. All heavy duty power dissipating components are mounted to the
chassis to minimize their internal temperature rise. An overall anodize
processing gives both a durable finish and excellent heat radiating capability.
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Along the sides and the rear, removable panels protect the power components
mounted between the cooling fins from accidental contact with metallic objects.
These panels may be easily removed to gain access to the many test points
provided.
(2) Jacks and Connectors - A five pin receptacle, type MS 3102A-18-
11P provides the power connection for input DC supply voltage and for AC
power to the speaker cable. A three pin connector, type MS 3102A-10SL-3P
connects the amplifier to microphone and recorder or telephone signal sources.
The MIC jack on the front panel allows a microphone to be plugged in directly
for local operation or testing purposes. A MONITOR jack allows the use of
headphones to monitor the audio signal from the amplifier.
(3) Controls - A power switch, located near the power receptacle,
connects or disconnects the input DC line to the amplifier protective cir-
cuits. These in turn connect the DC to the amplifying circuits when suitable
conditions are met, i. e., correct polarity, voltage, temperature, etc. In-
dividual gain controls attenuate the signals coming from the recorder and the
microphone inputs to desired operating levels.
(4) Lights - The application of DC power is indicated by a red "Push-
to-Test" pilot lamp. Normally, this lamp indicates the application of power to
the amplifying circuits (following the protective circuit). When pressed, the
lamp indicates that power is available at the input power receptacle.
Three lamps are used to monitor the output audio power level. The
lamp labeled. "MIC" should flash with very low output levels; "HALF" and
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"FULL" require correspondingly higher levels of power. Because these
lamps are voltage sensitive, they will indicate erroneously if the speaker
load is disconnected.
5. DESCRIPTION OF AUXILIARY COMPONENTS - Amplifier DE-1492-A
is designed to operate with specified commercial components in order to form
an effective sound system.
a. MICROPHONE- The microphone normally used in the sound system
is a noise cancelling, dynamic type AEM-MIC-1. This microphone has an out-
put impedance of 50 ohms. It comes equipped with cord and a type MS 3106 A-
10SL-3S cable connector.
b. SPEAKERS - Amplifier DE-1492-A commonly powers two Atlas
type CG-44 speakers equipped with SA-67 drivers and 4 ohm voice coils. They
are electrically connected in parallel to furnish a nominal 2 ohm load. Other
combinations and types are permissible provided the correct or somewhat
higher load impedance is obtained and the combined power handling capacity of
the speakers is sufficient.
c. POWER SUPPLY - Storage batteries of the lead acid or nickel
cadmium type with a terminal voltage of 24-28 V DC are suitable power
sources for the sound system. Rectifier type power supplies which operate
from a 50-60 cycle power line can be supplied for shop or permanent installa-
tions.
6. ACCESSORIES- Certain accessories are available for testing and com-
plementing the sound system.
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a. DUMMY LOAD - AEM-TE-1422A is a two (2) ohm load with
three foot cable and type MS 3106A-16-11P connector for ready substitution
of the speaker circuit or connecting to the Power Meter Test Unit described
below. The use of resistive dummy loads in place of the normal speaker
load facilitates testing and power measurement.
b. POWER METER TEST UNIT - The AEM-TE-1421 Power Meter
Test Unit is a convenient test unit with connectors and cables for inserting
into the power circuits of an amplifier and measuring both the DC current
drawn and AC voltage generated across the load. The load may be either
speakers or the 2 ohm dummy load described above.
c. PROTECTIVE CIRCUIT TEST UNIT- A special unit is available
to test the protective circuit so that its proper adjustment is assured. This
unit is used with the 2 ohm dummy load described above.
d. RECTIFIER POWER SUPPLY- The rectifier type power supply
permits the amplifier to be powered from a 50-60 cycle power line.
e. PARALLELING TRANSFORMERS - Paralleling transformers are
available for driving 2, 4, or 6 amplifiers from the same audio signal source.
f. REMOTE CONTROLS - Units may be obtained to control, or to
control and monitor, any number from one to several amplifiers in parallel.
All remote units provide audio gain controls and on-off power switching. A
solenoid actuated contactor and cabling are also obtainable for remote operation
with above.
g. SPEAKER RACKS AND STANDS - Lightweight racks and stands for
mounting various combinations of speakers on vehicles or at stationary sites
can be obtained.
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Section II
System Assembly
1. GROUND SITE SELECTION - Sound waves, which are transmitted by
as well as through the air, are strongly affected by wind, weather, time of
day, and intervening objects. Satisfactory sound projection over great dis-
tances is possible only from a well situated operating site which takes maxi-
mum advantage of these natural elements.
? Optimum location of the speaker stand is the prime controlling factor
in site selection. This is especially important when maximum voice pro-
jection is required.
An operating location with a line-of-sight observation of the target
objective is desirable. For example, a hillside location with the horns
directed down or across the valley would be ideal. Sound carries well across
water, consequently, advantage can be taken of a river or a lake, to improve
sound transmission considerably. When prevailing winds are objectionable
in the operating area, it is preferrable to locate upwind of the target in order
that the carrying power of the wind can be used to advantage. If the above
conditions cannot be entirely met for various tactical reasons, a site devoid
of foliage or other obstructions in front of the speaker stand, for at least
several hundred yards, will be adequate.
Echoes constitute wasted power; therefore, the speakers should be
aimed so that the "sound beam" clears any intervening mass, such as build-
ings or hills. However, it is sometimes possible to utilize the echo effect to
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reach an obsured objective by reflecting the sound beam from a distant
cliff or steep rise. When it becomes necessary to transmit cross wind,
the horns should be directed slightly upwind of the target, the amount depend-
ing on the distance and the wind velocity. The approximate amount of cor-
rection is 25 yards per mile of transmission for each 10 mph. of cross wind.
2. AIRBORNE INSTALLATION - Experience shows that projection of
sound from fixed wing or helicopter aircraft is feasible up to two (2) miles.
If a circling course is planned, the horns should point horizontally abeam from
the aircraft frame. This permits the bank of the aircraft or the downward
thrust of the rotor blades to direct the sound beam toward the ground target.
The speakers may be mounted in an open doorway to reduce air resistance or
fitted into a streamlined pod which is 'mounted under wing or fuselage. In the
case of helicopter use, a bracket on the side or front of cabin will suffice.
3. ASSEMBLY - The sound system assembly is straightforward and con-
siderably simplified by the use of factory prepared cables. Information given
by the block diagram of Figure 3 and the instructions given here shluld suf-
fice to assemble a properly operating system for any purpose.
Power cable AEM-CBL-1 connects to the 5 pin power receptacle (P-3)
on the amplifier, connector type MS 3102A-18-11P. At the other end of CBL-
1, connector type MS 3101A-16-1IS attaches to either extension speaker cable
(CBL-10 to 100) or directly to "Y" connection, AEM-CBL-2.
One half of the CBL-1 cable terminates in dressed red and brown leads.
These wires must be clamped to a 28 volt DC power junction board or battery
with the RED lead to the POSITIVE terminal. CAUTION - Failure of the POWER
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pilot lamp to operate when the power switch is turned to UN may inaicate
that the input polarity of the DC power is reversed.
The microphone, AEM-MIC-1, fitted with connector type MS 3106 A-
10SL-3S may be used alone by coupling to the MIC receptacle (P-1) on the
amplifier chassis.
If an input audio line from microphone, radio, recorder, etc. is
used, precautions are necessary. The high gain of the amplifier requires
care in prevention of feedback either electric or acoustic. To prevent electric
feedback, shielded lines should be used. ALSO, the shield of the shielded
line (s) should not be connected or allowed to touch any electrical ground or
potential circuit. This may mean that an isolation transformer will be neces-
ary if not otherwised provided by the signal source. Both shields should be
securely tied to pin C at the type MS 3106A-10SL-3S connector.
Acoustic feedback is prevented by shielding the microphone from direct
sound waves from the speakers. This may be effected by an enclosure or oper-
ating at a considerable distance behind the speaker pattern.
A further precaution relates to the relative phasing of the speakers. It
is important that identical connections be maintained to both speaker driving
coils, otherwise sound cancellation will greatly reduce the projected power.
The proper phasing is provided by the cables supplied. Wiring may be checked
against the diagram of Figure 3 for corr ectness if repairs or recabling is re-
quired.
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System Operation
1. OPERATING PROCEDURES - Operating the voice Amplifier system
is relatively simple. However, several techniques, some of which will be
learned by experience, are important for the most favorable results.
a. Techniques of Voice Transmission- The system possesses the
capability of high quality voice transmission over long-distances and for
long periods of time. However, effective utilization of these features depends,
to a great extent, upon proper operating procedures.
To successfully project the voice over long-distances, it is necessary
to develop a microphone technique different from the normal conversational
manner. Talk clearly and distinctly, separating each word with noticeable
pauses, keeping the voice volume up, and the lips touching the mouth piece.
It is important not to obstruct the vent located at the top rear of the microphone
when depressing the "MIC" switch. A normal (medium pitched) voice is more
intelligible and carries farther than a bass voice.
For proper voice operation the "Gain" control is adjusted until the
Audio Power "Full" Indicator illuminates intermittently while a "Test-count"
is being made.
To minimize feedback (which if allowed to persist will overdrive and
possibly damage the components) it may be necessary to adjust the input "Gain"
control or improve the sound shielding of the microphone position.
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b. Operation With Recorder - When a recorder, or other sound source,
is to be operated with the amplifier, connection is made through Pins B & C
of the audio input connector and the "Radio Recorder Gain" control is ro-
tated to the proper gain position. Because of the very high gain of the
amplifier, care must be exercised when coupling to the input circuit to pre-
vent undersirable feedback. Battery operated recorders can be coupled dir-
ectly without adverse effects, however, it is necessary to keep the recorder
case above ground. An alternate solution is to employ an isolation transform-
er between the radio /recorder output and the amplifier input pins.
If the recorded sound is voice the correct input level is adjusted in
the same manner as for microphone operation. If the recorded sound is music,
it is advisable to operate at a reduced "Gain" control setting. Failure to do so
will cause excessive average audio power to be dissipated in the speaker
units resulting in probable failure of the voice coils.
2. RESETTING PROTECTIVE CIRCUIT - Amplifier DE-1492-A is design-
ed with self protecting circuitry to interrupt the operation whenever unusual
conditions exist which might endanger the amplifier components. Such a con-
dition might be an excessive voltage, voltage spike, current surge or temper-
ature rise. Once the protective circuit is activated, the flow of power is
stopped and no further operation is possible without corrective action. If the
trouble was due to a temporary or transitory cause, operation may be resumed
by simply turning the power switch to OFF and back to ON. If the protective
circuit again shuts the amplifier off, the power swticla should be immediately
turned to OFF and the cause of trouble determined and corrected.
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3. CARE AND PRECAUTIONS- The transistors of this system are subject to
. ?
damage when exposed to excessive heat, voltage, and current for even short
periods of time. The design incorporates a protective circuit which disables
the amplifier when safe operating currents, voltages or temperatures are ex-
ceeded. Operation of the protective circuit is indicated by the extinguishing of
the amplifier panel light. These protective circuits are adequate in most in-
stances; however, they themselves are subject to damage when extreme or
sustained stresses are allowed to exist. Careful monitoring is therefore
necessary when operating under unusual environmental extremes. When oper-
ating under temperature and solar conditions more extreme than those des-
cribed in Section I paragraph 4a, precautions should be taken to maintain temp-
erature within recommended limits. Other stress conditions causing activation
of the protective circuits are excessive battery charging voltages; short circuits
within the system; transmission of compressed, high level music and reversed
polarity from the battery supply.
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HORN ARRAY
AEM -CBL -2
Y CABLE
AEM-DE- 1492A
AUDIO AMPLIFIER
kiNwaltexz.:
AEM-CBL -10
SPEAKER CABLE
FIGURE 1
AEM-CBL - 1
POWER CABLE
NO ISE CANCELLING
Mi CROFHONE
HIGH POWERED VOICE AMPLIFIER SYSTEM TYPE DE-1492-A
A-2
?-?1".
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QUANTITY
PER
SYSTEM NAME OF UNIT DESIGNATION DIMENSIONS
WT /UNIT
System Components
1 Audio Amplifer
1 Microphone and Cable
2 Speaker with 3 foot
Cable and Connector
AEM-DE-1492-A
AEM -MIC -1
Atlas Horn CG-44
Atlas Driver SA-67
8 5/8 x 9 3/4 x 3 3/8 in.
Hand Size
22 x 12 x 20 in.
10.0
1.0
13.5
Cables
1 Power Cable
1 Extension Cable
1 "Y" Cable
AEM -CBL -1
AEM-CBL-10 to 100
AEM -CBL -2
10 feet
10 feet to 100 feet
18 in.
2.5
3.51
.5
Accessories or Optional Equipment
Dummy Load (2 ohms) AEM -TE -1422A
Power Meter Test Unit AEM -TE -1421
Protective Circuit Test Unit AEM -TE -1424
Rectifier Type Power Supply AEM-DE-1522
Paralleling Transformer AEM -JBT -2A
(for use with 2 amplifiers )
6 x 3 1/2 in. with
3 feet Cable and
connector
8 1/2 x 31/2 x 3 in.
3 x 4 x 6 in.
13 x 7 x 19 in.
4 1/4 x 2 3/4 x 1 3/4 in.
with 3 feet Cables and
Connectors
1.5
6.0
1.5
95.0
1.0
Figure 2 Table of Components and Accessories
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7.5
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(Cont'd) DESIGNATION DIMENSIONS
WT /UNIT
Accessories or Optional Equipment
Paralleling Transformer
AEM -JBT -4A
(For use with 4 amplifiers)
4 1/4 x 2 3/4 x 1 3/4 in.
with 3 feet Cables and
Connectors
1.0 lbs.
Remote Control
AEM-CON-1
5 x 2 3/4 x 2 1/4 in.
1.0
(For use with 1 or more
amplifiers with no monitoring)
Remote Control
AEM-CON-2
8 1/2 x 3 1/2 x 3 in.
1.5
(For use with 2 amplifiers
Remote Control
AEM -CON -4
8 1/2 x 3 1/2 x 3 in.
1.6
(For use with 4 amplifiers
Remote Control
AEM-CON-6
3 1/2 x 4 x 9 1/2 in.
1 . 8
(For use with 6 amplifiers
Speaker Rack
AEM -RAC-5
9 x 52 x 10 1/2 in.
8.2
Speaker Stand
AEM-RAC-6
96 x 96 in.
43.7
(For 4 amplifier systems)
Speaker Stand
AEM-RAC-8
72 x 72 in.
39.0
(For 2 amplifier systems)
Tripod
TP-1
4 to 7 feet high
8.75
(For 1 amplifier)
Figure 2 Table of Components and Accessories
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,
041,1')
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0.4.!)-0.1.9:1-? 4)C} N:..)
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atflaqi I
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(2).1
k
Signal Input
Recorder & Mic
Signal Input
Mic Only
On-Off Power Switch Power Receptacle
Figure 5 Audio Amplifier Type AEM-DE-1492-A
A-7
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D I APHRAM
Figure 6 Speaker Driver Unit Showing Internal View of Diaphram,
Voice Coil and Magnet with Air Gap
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MAGNETIC
GAP
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ComspliiinCuraz) o?605TH AIR COM MAN DOS
Communicat ions Section
COMMUNICATIONS TECHN ICA AID
DOC ---
GRID COMP
CMG CLASS
JUST
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I.
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"THE VOICE FROM THE SKY"
COMMANDO AIRBORNE LOUDSPEAKER SYSTEM
GENERAL
a. Purpose
b. Theory of operation
c. Equipment used
d. Characteristics
1. Physical
2. Drawbacks
II. Theory of OPERATION
a. Principle of sound
b. Mike techniques
c. Aim of speaker
d. Bank of aircraft
e. Amplifier checks (Maint)
DI. AIRCRAFT SPEAKER CAPABILITY (Alt & Radius)
a. Cessan 185/U-10/Light Aircraft
b. C -46/C -47.Similar Cargo Aircraft
c. Cessna; Chopper
IV. EQUIPMENT USED AND COST
a
a. Cessna 185/U-10 Small Aircraft
b. C-46/C-47/SA-16
c. Speaker Mounts
d. Speaker hookup
V. FURTHER INFORMATION
a. AEM
b. 605th,Commandos
GENERAL
1. The purpose of this paper is to present theoretical and operational infor-
mation on the Commando Airborne Loudspeaker System (The Voice from the Sky) so
that this information can be used by interested personnel. This information is
the most up to date on the system and has been prepared by the 605th Air Commando
Squadron's (Composite) Communications Section, Howard AFB, Canal Zone.
2. The primary aim of the Airborne Loudspeaker is to provide aural air to ground
communication where none other exist. The system can be used in almost any type
of aircraft and under varying conditions.
3. The adaption of a public address system to an aircraft is a relatively new
form of communication which offers unlimited possibilities both in civilian and
military operations. The systems is ideal for psychological warfare, disaster
Jr
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control, ground control from the air, rescue operations, riot control, hih.way
patrol, for emergency traffic instructions and other applications requiring a
"Voice from the Sky" with an overseeing eye.
If. The 605th ACS initiated and supervised the construction of an air field in
the ranama Jungle from the air by the use of the Airborrie Loudspeaker System in
a b-10 Aircraft. It also has been used successfully to notify the local popu-
lace, in remote and difficult areas to reach, of the arrival time of supplies,
personnel and help.
5. The whole concept of an aerial loudspeaker system has peen well received by
C.
all .1.d ixoved worth its cost. f.
C. The Coast Guard has adopted the system and use it quite successfully in
rescue operations. Companies are being formed using this system as an advertis-
ing media. Cities can disseminate information to their populace in a matter of
minutes by employing this aerial public address system.
7. The operation of the system is relatively simple and can be set up in a mat-
ter of rlinutes with the availability of proper equipment. Thd system is basic-
ally the same as public address systems found in churches, schools, offices, etc.
except that it is airborne.
C. Below is a block diagram of the system operations:
MIKE
ANPLIFIER
SPEAKER
<
9. A single 12V or 24V battery provides all the necessary power for the whole
system. The mil:e changes the sound pressure of intelligence to electrical i?-?
pulses. Tile tr:J1sistorized amplifier, which is high powered, lighweight, and
rutjc,ed, ;.tmplifies these electrical impulses and feeds it to speakers protruding
from an opening in the aircraft. The speakers convert the electrical impulses to
intelligence and beams this sound earthwards. This is basically the same pro-
cess a basic public address system uses. The equipment used is as follows:
a. AEM-DE-1492 Amplifier
b. SA-C7 6peaker driver
c. CJ-44 Speaker horn
d.. AM-MIC-1 Microphone
e. Associated AEM electrical cables.
The system is sold by: Applied Electro Mechanics, Inc., 2350 Duke St, Alexandria,
Virginia
10. The amplifier used is the most compact, reliable, rugged and has the great-
est power output. Technical data on the amplifier (the heart of the system) is
as follows:
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at.
j thc
Li
T1
is
;tc.
T1
Lkle L
,rceI
nit 1
_,-----
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a. Power Requirements - 15 amps (peak) at 28 volts D.C.
Standly less than 1.5 amp. Operates on battery voltage between 12 VDC -
30 VDC.
L. Power Output - 250 watts of audio power at .2 duty cycle.
c. Gain - 901)13
d. 1eight - 9 3/4 pounds.
e. Size - 3;i2 x 9 3/4" x 34"
f. Controls - Microphone and recorder input levels, on-off switch.
g. Monitors - Power on, microphone alive half and full power indicuting
lights.
h. Construction - Extremely rugged design to withstand mobile use in all
types of applications. Aluminum chasis design provides heavy frame for compon-
ent support as well as heut sink for the high power transistors.
i. Modular construction..
j. Protective Circuits - Reversed voltage, high voltage, high current, short-
ed output, temperature.
k. Operation - Iith microphone, tap recorder or radio receiver, input
perature - 300 F .(35?C) = 130? F (55 C) Altitude 0 to 12,000 feet.
THEORY _OF OPERAIlOgS
1. Sound by definition is an alteration in pressure propagated through the air
or other eleastic material. Sound is also the sensation produced through the ear
by the above alterations. Sounds travels in air at approximately 1100 feet/se-
cond. The sensation produced through the ear by sound does not vary directly as
the sound (stimulus) but approximately as the logarithm of the unit measuring thc
sound.
2. Ssation = constand x log (sound)... The amplitude of sound is measured in
terms of pressure, intensity or power. Intensit,z is the ,Alergy transmitted by
the sound wave per unit areas, in watts/CM2. Pressure is the force per unit
areas perpendicular to the path of the wave, exerted by the sound wave on the me:J.-
ium. Power denote 's the energy transmitted per second. It is equal to the in-
tensity times the perpendicular areas through which the sound is radiated.
3. Tho range of sound intensities and pressures, is very great. A decibel or
DB is a unit used to express the ratio between two poser, pressures, intensities,
etc.
4. The sJnsation law expressed above applies to the perception of sound ampli-
tude and so the DB unit of amplitude ratio is more accurate in relation to human
perception. When we say that the intensity of a sound has increased 3DB, it means
that the intensity has approximately doubled; then we can conclude from this, that
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Declassified and Approved For Release 2012/02/22 : CIA-RDP78-03642A001700030001-9
the intensity of sound on a person = DB = 10 log P,/P2 where P, is the output
power and P2 is the input. Thus increasing our DB we increase sound intensity by
more amplifications in an amplifier system. Intensity values for various sounds
range from 0 DB to the threshold of hearing and to 120 DB threshold of pain.
5. A sound wave travels in a straight line along the direction in which it was
radiated unless it is abosrbed, reflected, refracted or diffracted.
Absorption is the dissipation of sound.
Reflection is the simple bouncing of sound off of objects, etc.
Refraction of sound is the slowing down or speeding up of it when it encounters
new mediums. The behavior is the same as that exhibited by light waves.
Deffraction is the change of direction of a sound wave shich is caused by travel
around an obstacle.
Interference is the reception of sound from more than one scurce; either adding
of subtracting to the intensity of sound.
6. The change of pitch in sound is called the Doppler Effect. This happens when
the source of sound and point of reception are moving with respect to one another,
the frequency detected by the listener will not be that of the vibrating source.
Suppose that the listener is approaching the source and since he is moving to-
wards each wave impulses at the same time that it is moving towards him, he meets
it a little sooner than if he had merely waited for it, and the perceived per-
iod between impulses is shortened, which is to say the perceived frequency is
higher. When source and listener are moving apart, the effect is the opposite
that of lowering pitch. Motion which does not change the distance between source
and listener has no effect on pitch.
7. Distortion of sound results when the instantaneous response is not directly
proportional the instantaneous stimulus (sound) at all times.
8. The discussion above on the theory of sound effects the Airborne Loudspeaker
system. As we go on the different properties and shortcomings of sound will be
discussed as applied to the Airborne Loudspeaker system.
U / the
9. Proper mike techniques, speaker aim and bank of the aircraft will determine
e success or failure of this system. If the proper techniques are used, the
shortcomings of sound discussed previously can be eliminated.
10. The system possesses the capability of high quality voice transmission over
long distances for several hours. However, effective utilization of these fea-
tures depends, to a great extent, upon proper operating procedures.
1:11 91'3 N?,?,,44, D:c-q -,),cit, WiC.0 c,yoN, 7tc,; cWItanc:es it is necessary to
, develop a microphone technique eifferent from the normal ocr.verseLaI me.=.,;:.
ITalk clearly and distinctly, separating each word with noticeable pauses, keepir_E
the voice volume up and the lips touching the mouthpiece. A medium pitch voice
is more intelligible and carries farther than a bass voice.
; ink. Wto-tt 1-14H}. ; WO 00 POW(' PWAtOh to ON and make a test count into
),,, ,, , " . , ..; ,. -
the mike. Adjust gain control to OD and advanoe oloohwinc to the Oint where
Audio Power meter just swings between the two red marks, to minimize feedback
(a singing sound whenever microphone switch is depressed) the gain control may
4
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oc:
13.
01:o:
roc(
pos:
Jec.
coul
hat.
3L 1a:
arl
14.
for
to (
1c,A
15.
aim
0.1
a:1d
pro.
erec
16.
shol
ed ?
belc
Alt
;ity
iOtindis
1.
; was
Iters
,ravel
Lding
ks when
,nother,
lurce,
to-
1 meets
er-
is
,ite
source
ctly
eaker
I be
mine
the
Direr
rea-
:o
:eping
>ice
; into
re
ay
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,,s! backed off, or preferably, better sound shielding provided betwcan
and microphone. This latter can be accomplished by increasing the sep-
cr enclosing the microphone position.
eration with Recorder - When a recorder or other sound source is to be
with the amplifier system, connection is made through the "Recorder"
cje and the "Gain" control is rotated to the counter-clockwise "REC"
Aion. A choice of high (2000 ohms) or low (500 ohms) impedance is available.
of the very high gain of the amplifier, care must be exercised when
?.11r.g to the input circuits to prevent undesirable (electrical) feedback.
operated recorders will give little trouble if the recorder case is'in-
from electrical ground. An alternate solution, in any case, is to use
? ::slation transformer between the signal source and amplifier.
If the recorded sound is music, the correct input level will be the same as
the microphone operation. If the recorded sound is music, it is advisable
rate at a reduced average power level. Transmission of compressed high
music at full power may cause overheating in the amplifier or speaker units.
The speakers must be aimed like a gun to give best voice projections. The
%!..7. of the speakers is extremely important and this can be easily demonstrated
t,:st flights. The spread of the speaker is 120 degrees in the vertical plane
I...! )0 degrees in the horizontal plane. At an altitude of 1000 ft with direct
ection on the ground, approximately 1.35 million square feet of area is coy-
t:rcd.
Circular patterns are best for air to ground communications. The plane
bank when using the loudspeakers so that the plan of the speakers are point-
ti towards an object or area -which you want to project your sound to. The sketch
mlew shows the pattern described and speaker effects:
120?
speaker
spread
Aircraft in bank flying circular
patterns
30?
speaker spread
ffective audio area
17. Amplifier checks - With the output level indicator, the amplifier chassis is
largely self-indicating as far as its normal operation is concerned. When the
operation is not normal, the following possibilities should be checked.
a. Polarity of input D.C. voltage. If reversed, the panel light will not
indicate until pressed.
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Declassified and Approved For Release 2012/02/22 : CIA-RDP78-03642A001700030001-9
b. A short in speaker or cable. Pilot light will not indicate until pressed
and protective circuit is energized.
c. Excessive D.C. voltage. Battery may not be properly connected. l'anel
light does not iadicated until pressed.
d. Power switch OFF. Panel does not indicate until pressed.
e. No D.C. to chassis. Panel light does not indicate normally or when pres-
sed.
f. Speaker coil or cable open. Output level indicator will illuminate with
little or no signal.
g. An external ground in the audio input circuit through auxiliary device
(recorder, etc.) Indicator lamps illuminate with not input signal. Feedback
caused by "loop" currents.
h. Amplifier may be faulty. This will require shop maintenance.
18. Power supply checks - Battery voltages can vary between 12 VDC to 30 VDC.
19. Speaker and cable checks - Testings of speakers and cables may, to some ex-
tent, be done visually. All connections may be examined for freedom from dirt,
moisture and open or short conditions. All receptacle threads should be through-
ly engaged. Use of an ohmmeter may be made to establish the state and correctness
of wiring.
AIRCRAFT SPEAKER CAPABILITY
1. The Airborne loudspeaker sy3tem can be used from 0 to 3000 feet altitude and
out to 1 mile radius. These values are the results of the testing by the author
of this paper. It should always be kept in mine that when operating these speak-
ers, you have the initiative. When someone hears a voice from the sky you uncon-
sciously have his undivided attention consequently increasing the systems auda-
bility factor. Transmissions in clear areas or better thah hilly or congested
areas. Echoing of signals, increases and decreases sound intensity, consequently
causing the intelligence to be garbled and distorted. When flying in areas where
hills exist, use clear areas where a line of sight exists to the target on the
t;round. Follow the iastructions contained above and the system will work amazing-
ly well.
2. The following is recommended altitudes and amplifier's power setting when
used in an area of less than 5 knots ground wind relatively clear area, and noise
level normal or relatively low and using an airspeed between 80 - 120 knots:
a. Light Aircraft Operation - Cessna 185/U-10 Helio
Altitude 500 feet
6
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ed
s-
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Radius of turn in miles Amplifier setting
-4- 1
2
2
3/4 3
1 4
Altitude 1000 - 1200 feet
Radius of turn in miles Amplifier setting
A 2
4
12
lz-
tJa 3/4 3
1 4
gh-
ness
Lk-
)n-
;re
Lng-
Lse
Altitude - 1500 feet
Radius of turn in miles Amplifier setting
* 3
i 3
3/4 4
1 4
Altitude - 2000 feet
Radius of turn in miles Amplifier setting
4- 3
i' 4
3/4 4
1 4
b. Dual Engine Aircraft - C-46/C-47
Altitude - 500 feet
Radius of circle in miles
Amplifier setting
_1. 2
I 2
3/4 3
1 4
Altitude - 1000 - 1200 feet
Raduis of circle in miles Amplifier setting
I" 2
i 3
3/4 4
1 4
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Declassified and Approved For Release 2012/02/22 : CIA-RDP78-03642A001700030001-9
Altitude - 1500 feet _
.,.._
r,10 ft
Radius of circle in miles Amplifier setting
ff-: f!:,
4
/ ft
-2
3/4
-, 4 'J73 1.;
t \
,..):C ft
Altltudo - 2000 - 3000 font
CO
Radius of circle in miles Amplifier settig t_ ft
7CC. ft
'0ft
4 .166 ft
a00 ft
2 4
3/4 4 (in .ric: 1
1 4 LIL a:id a
cad be u
SPEnR SYSTEM ON CESSNA CH-1C HELICOPTER
1. The following- equipment is required to install the airborne speaks: system on
the CESSNA CH-1C SkYHOOK Helicopter.
1. AEI: DE1942 AMPLri-1GA
1. MICROPHONE, M-34 with SHIGLD
1. POWER CAjLE
2. 6J-47 SPEAO,RS & JRIVERS
12. FT OF DEXION STEEL
4. 10/32 2" INCH BOLTS, STEL.
1. ALUM BRACE 12 INCHS LONG
2. The 2 6J-47 speakers are mounted on the left side of the chopper. Dexion is
run from the front landing strut to the rear landing strut and secured with the
4 steel bolts mentioned above. The 2 SJ-47 speakers are mounted to this dexion.
A short lenght of dexion is then used to bolt the 2 speakers together and the
12 inch brace is then added from the rims of the speakers to the landing runner..
To make the 2 speakers rigid itis required to bolt the rims of the 2 speakers to-
gether.
3. This epeaker system was tested at alt of 50 to 850 ft. There is no feed bac4
at all wiLn this system and max volume could 1:4; used all the time with no bad side
effects. The distance from the objective point was varied from directly overhead
to 1 mile away and clairity was good at all times.
4. The CESSAN CHOPPER is a 4 place a/c and has ample room to place the ampl any-
where in the cabin so all passengers are able to use the system without changing
positions in the a/c. The CESSNA CHOPPER has a 24 VC battery that is easy to
string the ampl power cable to and has a small vent window that the speaker cable
can be run through to the speakers.
5. The last test-run of the speakers gave the following results:
8
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1 JO
a.
?
?
P
3 1 -
-285.
In on
n is
the
men.
rs to-
back
d side
rhead
any-
zing
to
cable
Declassified and Approved For Release 2012/02/22: CIA-RDP78-03642A001700030001-9
50 ft
30 ft
150 ft
300 ft
500 ft
700 ft
350 ft
850 ft
700 ft
COO ft
500 ft
400 ft
300 ft
On the
at and
can be
DISTANCE TO OBJECT
Overhead
Mile
J- Mile
2
-1- Mile
2
1 Mile
2
I Mile
? Mile
2
1 Mile
1 Mile
1 Mile
1 Mile
1 Mile
1 Mile
CLhRITY
Very Good
II
1.1 It
It It
II It
It 0
It It
It It
II II
It It
It II
II 0
11 11
landing run the CHOPPER was approximately Zi miles from the object point
altitude 1000 ft and could still beeard but not with ideal clarity. This
used on other types of helicopters just as well.
C. Equipment used and cost.
The following is a list of equipment used in each aircraft and its cost:
a. Light Aircraft - Cessna 165/U-10 Helio employing two speakers and one
amplifier.
F6N
5831-ND-000230D
5831-ND-000231L
Pox:C.4." F.E.AJ
ITEM 2TY Cosa.
:Amplifier AM-DE-1492 1 ea ec,75.00 .
Speaker Driver - 75 watts with 9 ft. /
cable and connector 2 ea .0
5831-ND-000232L Speaker Horn CJ-44 Standard J-t, 2 ea 42\50
, o /5.00
5965-285-0641--Micr oilio.le .)lic d-RX.-1334-A/C--_____ /15.00
5831-ND-000233L Cable amplifier, power 10 ft, AEMCBL-1 1 ea , 17.50
1--44-1-0.0235L "Y" paralleling cable AEM-C3L-2 1 ea 15.Q0
TOTAr7UST77:7:7777.7.:4.$1229.50
b. Dual engine
sjstem.
FJN
aircraft - C-46/C-47/SA-16 - Four speakers, two amplifiers,
ITEM 005'2
5831-ND-000230L
DE-1492 Amplifiers
2 ea
.e975.00
5331-ND-000231L
SA-67 75 Watt driver
4 ea
53.50
5831-ND-000232L
CJ-44 Horns
4 ea
42.50
5631-ND-000233L
AEM-BL-1 Power cables
2 ea
17.50
5831-ND-000234L
AEM-CBL-20 Extension cables (50 ft)
2 ea
13.00
5631-ND-000235L
AEMBL-2 "Y" Paralleling cables
2 ea
15.00
5965-254-9226
Microphone M-34 lAC
1 ea
15.00
5965-265-0641
Microphone shield MX-1334 AC
1 ea
5.00
Paralleling transformer AEM-JBT-2
1 ea
41.5(
TOTAL COST
r2505.56
7. Speakers can be mounted in the aircraft numerous ways. The best way, nat',Irallj
is to manufacture a cloor around the speAcer. A more simple and less expensive is
to use dexion steel as shown in the pictures. A more simple way is to physically
tie the speakers down in the aircraft doorway.
9
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Declassified and Approved For Release 2012/02/22: CIA-RDP78-03642A001700030001-9
8. Construction of speaker doors will be determined by material available but
initative and strong imagination will greatly expedite matters. All structures
should be safely secured to the aircraft to prevent loss of equipment.
Cessna 185 / Light Aircraft Hookup
Wke & Cable
1 AEM-CBL-10
SpeakeTable
AEM-1492
I>4i Audio
[ Amplifier
SIVA e r y
AEM-a1-1
M-34 with Power Cable
Shield recommended
to prevent feedback
C-47 / C-46 / SA-16 Transport Aircraft
IF
Dynamic
Mike
M-34
Recommended
10
1 AEM-1492
Audio
Amplifier
Paralleling
I kTransformer
AEM,JBT-2A
AEM-CBL-10
or 50
Speaker Cable
AEM-CBL-2
"Y" cable
lakers
AEM-CBL-:
"Y" Cabl.
Speake:
AM-1492
Audio
Amplifier
foam
- -Battery
Lam--AEM-CBL-1
Power Cable
Spaa
Declassified and Approved For Release 2012/02/22: CIA-RDP78-03642A0017nnnmnn1_a
BL-2
able
:aker&
eakers,
Declassified and Approved For Release 2012/02/22 : CIA-RDP78-03642A001700030001-9
Allr'71;
Fii I 11
" 101
,
,
,
.k
1117--111Z _
DL
ila-j.XCYJ ,J C11
Mucosa,