September 1947 Radio News
Wax nostalgic about and learn from the history of early
electronics. See articles from
Radio & Television News, published 1919-1959. All copyrights hereby
Until maybe 20 to 30 years ago, there was still a certain amount
of awe associated with new applications of technology. It seems
anymore people are so accustomed to new and amazing things -
usually at affordable prices - that the wonder is gone. Advancements
are expected. The world is moving so fast that it is difficult
to absorb and fully appreciate all the work being done. In 1947,
both airplanes and electronics were still relatively new to
a lot of people, especially in more rural areas, so a whiz-bang
scheme like broadcasting messages from an airplane was a big
deal to many. It was an area of science that had not yet been
explored to a large degree.
Sound Broadcasting from Airplanes
The installation and servicing of sound equipment in planes
can provide an added source of income for the radio serviceman.
By Saul J. White
Chief Engineer, University Loudspeakers, Inc.
During the war high-powered sound systems were used in military
aircraft for sound broadcasting to the ground during invasion
operations, and for propaganda purposes. The Coast Guard has
equipped planes with sound systems for its Air-Sea Rescue Division.
Now, with the peace, there is a growing interest in "sky broadcasting"
for commercial advertising. Former military pilots and radiomen
have turned to this novel field of advertising as a lucrative
and exciting occupation. One enterprising flying corporation,
organized by ex-G.I.'s, has equipped five airplanes for air-to-ground
advertising. A Morristown, New Jersey, flying service has equipped
an auto-gyro similarly.
Temporary Installation of 100 watt loudspeaker
and amplifier in a Piper Cub. Speaker projects out of open door.
For effective airplane broadcasting of sound, audio power
of 100 watts or more is required. Energy below this is unreliable
because of the effect of wind, noises on the ground, and the
necessity for flying at considerable heights, especially over
congested areas. It is preferable that this full power be handled
by a single loudspeaker. If greater sound coverage is desired,
additional speakers each capable of handling 100 watts should
be installed, and corresponding increases of amplifier output
made available. Speakers with narrow projection angles are most
effective because of the concentration of sound intensity on
the ground. To obtain maximum intelligibility and acoustic output,
both the loudspeaker and the amplifier should be so designed
as to cut off all frequencies below 200 or 300 cycles. Low frequency
or bass notes do not add to the clarity, but only serve to overload
the equipment. Most plane engine noises are in the low frequency
range, and hence this type of pickup through the microphone
is minimized where the bass is cut off in the amplifier and
speaker. Where high intensity and wide ground coverage are required,
the practice is to use either a greater number of speakers or
a single speaker with wide dispersion angle capable of handling
greater audio power, 200-300 watts.
The dispersion angle of the loudspeaker is of some importance
in the case of high speed planes from which an announcement
or message of any length is to be broadcast. It can be realized
that if the beam of the speaker is narrow or sharp and the plane
is traveling at 100 m.p.h. or more, that the projection area
on the ground would pass over a stationary listener in a very
short space of time, usually a matter of seconds. Therefore,
it becomes necessary that all announcements be made short and
that they do not exceed the estimated "ground time" which is
determined by the dispersion angle, the rate of speed of the
plane, and its height. Consider the case shown in the following
illustration. Here a plane flies at 1500 feet, at a speed of
100 m.p.h. The speaker has a dispersion angle of 450, this representing
the width at which maximum intelligibility and intensity is
obtained. It is, therefore, a matter of very simple computation
to realize that a message from the plane covers a ground diameter
of 1250 feet and could be heard by a listener for only nine
Determination of dispersion angle of loudspeaker
is necessary in order to gear an advertising message for maximum
intelligibility with relation to the listener.
Reduction of Acoustic Feedback
Because of the high power required and the fact that the
loudspeaker is within a few feet of the microphone, acoustic
feedback may occur before the necessary sound intensity is obtained.
The following recommendations are made to permit larger volume
to be built up before feedback occurs.
First, the loudspeaker should be so mounted that it points
slightly to the rear of the plane. In other words, in addition
to pointing downward, it must also point to the rear. This allows
the sound to flow partially with the slip stream. The microphone
should be located "upwind" from the loudspeaker.
A great improvement will result if the loudspeaker mouth
projects about a foot into the slip stream or beyond the fuselage
of the plane. The best possible results are obtained where the
entire loudspeaker is hung outside of the fuselage and mounted
on a wing strut or under the nose of the plane.
Schematic diagram of a 150 watt audio amplifier.
The mike input is for a single-button carbon lip mike, while
the low level input may be used for the input of a wire recorder.
If the reproduction is to be obtained from a microphone,
it is imperative that only a close talking microphone be employed.
The recent lip type microphones and especially the differential
type are excellent for reducing feedback to a minimum as well
as eliminating a lot of the engine noises which would otherwise
be picked up by the microphone and reproduced over the speaker.
Reproduction from a phonograph would be extremely difficult
from an airplane because of the vibration and the banking. Unless
an extremely complicated design were worked out, it would be
impossible for the pickup to ride in the record groove. Reproduction
from a magnetic wire or tape recorder, or film sound track,
however would be excellent since these are not affected by vibrations
and other normal maneuvers of the airplane. Magnetic recorders,
however, are unquestionably the best source for the message.
Announcements are recorded in advance on the ground.
Wind is an element which frequently ruins what would otherwise
have been a good performance, but unfortunately the results
can not be made uniform because of the variation in wind velocity
and direction of the airplane. The effect of variable wind in
any direction would be to cause considerable fading and this
effect will increase as the distance between the listener and
the loudspeaker increases. If the wind is gusty and the plane
engages in changes in heading, the sound as the listener hears
it will vary in intensity.
Most sound installations in aircraft operate from a rotary
converter, using 24 or 32 volts d.c. storage battery input and
having 110 volt a.c. output, which is fed to the amplifier.
This naturally entails considerable amount of weight and, of
course, the plane must be capable of carrying this load in addition
to the weight of the amplifier and loudspeaker. The rotary converter
method is the simplest because it permits the use of available
commercial amplifiers. However, where weight is at a premium,
as in a light plane of the Piper Cub class, and it is imperative
that equipment weight be reduced, the amplifier should be custom-built
to operate from 12 volts d.c., utilizing a genemotor delivering
the required d.c. plate voltage. This eliminates the weight
of the high voltage and filament transformers. The circuit should
eliminate all non-essential features, contain only the necessary
input and output channels. Where a low frequency cut-off of
300 cycles is specified, the weight of the output transformer
can be reduced. The circuit should be reduced to the simplest
A Consolidated Vultee AT-6. purchased by
an ex-AAF pilot·from Army surplus, is shown being equipped with
a University Model B-6, 150 watt loudspeaker unit.
In certain planes it may be possible to couple a 110 a.c.
generator to some part of the power plant so that at cruising
r.p.m. the generator driven by the engine will deliver the correct
voltage. However, this is difficult in most engines below 200
h.p. because of inaccessibility to any portion of the crankshaft
for power transmission.
In large planes it is possible to install a complete gasoline
driven generating plant. This requires careful precautions against
Wind driven generators, if procurable, are an excellent solution.
Propeller-driven d.c. generators can be used to keep the batteries
up to full charge where a storage battery converter has been
installed. If the owner can obtain a 60-cycle wind-driven generator
with a 110 volt a.c. output, any conventional amplifier could
be used. Wind-driven generators of 400 cycles have been built
for aircraft electrical requirements. These are lighter in weight
than the 60 cycle models, and would be most suitable, especially
where the amplifier is designed for 400 cycle supply. For 100
audio watts to operate a 100-watt loudspeaker, the generator
should have a capacity of 300-500 watts. Where 200 to 300 audio
watts are required the generator should have a capacity of 750
As pointed out elsewhere, weight can be saved where the amplifier
is custom-built to the precise requirements of the installation.
Assuming a carbon microphone of the close talking type is to
be employed, the amplifier need have no more than 70-80 db.
gain. This is easily obtained in a 3-stage amplifier. It is
recommended that the output stage be designed around a pair
of 811 tubes operating in push-pull "Class B." This will afford
power output in the neighborhood of 150-200 audio watts when
using a plate supply of 1500 volts. The generator delivering
the high voltage for the plate supply should, of course, be
capable of meeting the peak current requirements.
Of course, high-powered amplifiers can also be designed to
operate around a group of 6L6 tubes with a plate voltage of
400-450 volts. This lower plate voltage may be found more desirable
because of the availability of genemotors delivering this output.
The output stage should consist of six 6L6's arranged in push-pull
multi-parallel. This should be designed for "Class AB" conditions
and will deliver close to 100 watts.
Since loudspeakers of the order of 100 watts or more are
designed to utilize a group of individual driver units mounted
on a common mixing chamber and air column, a great deal of flexibility
in wiring is available. This makes it possible to utilize a
group of medium-powered amplifiers. For instance, the University
Model 4A4, 100-watt loudspeaker contains four driver units.
each rated at 25 watts. They can thus be wired to four amplifiers
each with 25 watts output power, or connected so as to be fed
from two amplifiers of 50 watts output each.
The University Model B-6 loudspeaker rated at 150 watts contains
six individual driver units inside the housing. The Model B-12
loudspeaker rated at 300 watts contains 12 driver units, each
of 25 watts capacity. Thus, a number of output stages or boosters
of, say, 50 watts each can be used and these are commercially
available from a number of amplifier manufacturers. This type
of installation, namely the utilization of several power output
stages, affords a great deal of safety factor since if one of
the power stages should fail, there remains sufficient power
to keep the equipment operating without complete interruption
of the service.
Installation in the Piper Cub
When the installation is to be temporary, the door of the
plane should be removed and the loudspeaker mounted at such
an angle that the sound can be projected downward and somewhat
to the rear. The mouth of the horn should project six inches
to 1 foot outside of the cabin.
For a more permanent installation in a Cub, the rear seat
and the rear floorboard should be removed and a hole corresponding
to the diameter of the loudspeaker should be cut into the fabric
at this point on the bottom of the fuselage, just back of the
rear control stick. The loudspeaker should be securely braced
inside the cabin, standing directly over the opening of the
fuselage. This type of installation offers no parasitic drag
to the aircraft but may cause acoustic feedback. The edges of
the hole in the fabric must be properly protected against possible
wind damage and must conform with CAA requirements. A wire mesh
screen should be used across the hole, held down with two plywood
escutcheons or large washers, one inside and one outside which
would reinforce the fabric edges.
In other models of ships it is possible to locate the loudspeaker
under the fuselage between the wheel struts. This is an excellent
location. In all cases the airplane must have adequate pay load
for the weight of the sound equipment and this equipment must
be so located in the plane that it does not upset its center
of gravity, especially laterally. For a longitudinal displacement
of the center of gravity, the trim tab or stabilizer can, of
course, be adjusted for correction.
The removal of the door, the opening of the fabric, and the
installation of heavy equipment may involve special permission
from the Civil Aeronautics Authority. Whenever changes in the
plane structure or serious redistribution of weight results,
the CAA should be notified. Generally such permission, with
special restrictions, is given but in many cases the NC license
is replaced by a NR or "restricted" one.
A complete high-powered sound system for reliable high altitude
operation would weigh almost 300 pounds. This is proportioned
as follows: Loudspeaker (100 watts) - 60 pounds; Amplifier -
75 pounds; Storage Batteries and Converter - 150 pounds; a total
of 285 pounds.
Posted May 23, 2015