Sound Broadcasting from Airplanes
September 1947 Radio News Article

September 1947 Radio News [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio & Television News, published 1919-1959. All copyrights hereby acknowledged.

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

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.

Power Requirements

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.

Dispersion Angle

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.

Microphone

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.

Phono

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

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.

Power Supply

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 form.

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.

Amplifier

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.

Loudspeaker Installation

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 on 5/23/2015)