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 30 to 40 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 when this "Sound Broadcasting from Airplanes" article appeared
in Radio News magazine, 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. BTW, the spell checker
flagged a new word (for me, anyway):
genemotor which, as it turns out, is the generic
name for the line of dynamos, generators, engines, and motors manufactured by
Pioneer Gen-E-Motor Corporation of Chicago, Illinois.
Sound Broadcasting from Airplanes
Temporary Installation of 100 watt loudspeaker and amplifier
in a Piper Cub. Speaker projects out of open door.
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
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.
Determination of dispersion angle of loudspeaker is necessary
in order to gear an advertising message for maximum intelligibility with relation
to the listener.
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.
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 seconds.
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.
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.
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
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
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 fire hazards.
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 watts.
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
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 August 12, 2022
(updated from original post