بحث بعنوان Ultrasonic Transducers for Use in Air

بحث بعنوان
Ultrasonic Transducers for Use in Air
FRANK MASSA
MASSA DIVISION, DYNAMICS CORPORATION OF AMERICA
HINGHAM, MASS.
Abstract—Fewer applications were found for the use of ultrasonics in air than for ultrasonics in liquids or solids. This was primarily because of the inherent limitations of generating high-intensity sound levels in a gaseous medium, and also because of the extremely high attenuation that accompanies in the propagation of ultrasonic energy through air. Several types of transducers are described
for the generation and reception of ultrasonic sound in air. Data are
presented in the form of engineering design charts which include
several quantitative relationships between piston displacements,
acoustic power, sound pressure level, beam patterns, attenuation,
and additional fundamental information for use in the design of ultrasonic equipment to achieve the transmission of high-frequency
sound over a specified range. Due to the natural limitations in the
transmission of ultrasonics in air over large distances, the major applications have been in remote control systems, proximity indicators,
automatic counting, burglar- and fire-protection systems, and shortrange carrier telephony.
Ultrasonic sound may be used for short-range proximity indicators, such as, for example, the determination
of the last several feet of height of a plane from the landing strip for the possibility of automatic guidance of the
plane in the final landing operation. Such a proximity
system has also been successfully employed in maintaining a predetermined altitude of a helicopter hovering a
short distance above water or land.
A very interesting application of an ultrasonic proximity indicator has been in an experimental blind aid
which uses the transducer shown in Fig. 15 in a handheld case resembling a flashlight. An ultrasonic pulse is
emitted at about one-second intervals from the transducer when the operator turns on the equipment. If the
ultrasonic beam finds an object in its path, the reflected
sound from the object will return to the transducer to
automatically trigger off another pulse to establish a
steady repetition rate for successive reflections. The
repetition-rate frequency is fed to a tiny earphone which
permits the blind person to know the relative distances
of objects that he intercepts with the ultrasonic beam.
Future applications of ultrasonics in air will probably
see an expanded use of control devices and proximity
indicators. One interesting application of ultrasonic
control has been considered for use as a burglar-alarm
system in which an ultrasonic transducer radiates sound
in a vacant room and sets up a diffuse standing-wave
pattern within the enclosure. A receiving microphone,
also placed within the room, picks up the sound, and the
frequency of the received signal is compared with the
oscillator frequency which drives the transducer. Both
frequencies will be identical if everything within the
room remains stationary. If a person enters the room,
the sound-wave pattern is disturbed and the frequency
of the received signal will be changed by the Doppler
effect caused by the person’s acting as a moving target
for the sound reflected from his body. The same system
would also act as a fire alarm, since a fire in the room
would cause large convection currents of air due to the
heat, which, in turn, would shift the frequency because
of the Doppler effect due to the motion of the medium
transmitting the ultrasonic sound.

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