That ultrasonic communications has not proved to be a reasonable
means of transmitting information from one location to another -
even over fairly short distances - is borne out by the obvious lack
of such systems today. With all the technology available in the
form of electronics, mechanics, and software, if it were possible
to efficiently and effectively implement systems of ultrasonic communications,
such devices would be as common as the current plethora of wireless
systems. Some early research efforts at ultrasonic cmmunications
were published in a 1945 edition of Radio News. Regardless
of the era, the electromagnetic frequency bands are always deemed
to be too crowded so researchers constantly look for other transmission
media. There is one revolutionary new potential form of remote communications
on the horizon:
quantum entanglement. Still largely an enigma, entanglement
communications exploits an observed property of some subatomic particles
to be inextricably linked to each other with no discernable medium
or known mechanism. Albert Einstein referred to it as 'spooky action
at a distance.' Don't look for quantum entanglement Internet routers
anytime soon, but once the technology comes to fruition, not only
will it mitigate the need for distribution coaxial and optical cable,
but also local routers; ultimately, only a QE modem will be required
in each device connected to the ubiquitous
Internet of Things (IoT).
An analysis of the principal factors and equipment involved
in conveying intelligence at ultrasonic frequencies through mediums
such as liquids, gases, or solids.
By Robert G. Rowe
Fig. 1. Two types of reproducers (loudspeakers) that
can be used to communicate via a 20 kc. ultrasonic sound
beam. (A) The magnetostriction reproducer and (B) the piezoelectric
reproducer. Photographs of these units are shown in Figs.
2 and 4 respectively.
In order to convey intelligence or to "communicate" through a
medium, be it a liquid, gas, or solid, it is expedient first to
establish in the medium, as a "carrier", some form of periodic wave
motion which can be propagated to and detected at a remote point
in the medium. Secondly, it is necessary to modify the existence
or character of this wave motion in some pre-arranged, decipherable
manner, or "modulate" the wave motion. Obviously, modifying the
existence of the wave may be accomplished by alternately starting
and stopping its generation; whereas, modifying its character may
be accomplished by changing its amplitude, frequency, phase, or
velocity. However, since the velocity of a wave in a homogeneous
medium is fixed, the other aforementioned forms of modulation are
In the art of radio communication, such "carrier" waves are electromagnetic
in character, transmitted through the ether at a uniform velocity
and modulated to carry intelligence by code or telephone.
Table 1. The distance sound travels before its intensity
is reduced to one-half.
Fig. 2. The magnetostriction reproducer.
In the art of sonic communication, such carrier waves are "compressional"
in character, wherein the propagating medium suffers a sort of rectilinear
deformation, which may be transmitted through various liquids, gases,
and solids at velocities determined by the characteristics of the
medium, and modulated to carry intelligence. While electromagnetic
and sound waves may be essentially different in character, they
are alike in that they may each be propagated in suitable media.
Quite obviously, the most universal form of sonic communication
may be exemplified by a conversation between two or more people.
The vocal cords, as well as the configurations and appurtenances
of the oral cavity, cooperate to represent the compressional wave
generator, which is both amplitude and frequency modulated. The
air represents the gaseous medium through which such modified compressional
waves are transmitted at a velocity of some 1100 feet-per-second.
The human ear represents the receiver, which is capable of detecting
and, perhaps, demodulating these compressional waves.
In the art of ultrasonic communication, so designated by virtue
of the fact that the human ear will not respond to the higher frequencies,
such waves still are compressional in character and may be transmitted
through liquids, gases, and solids. It will be appreciated that
ultrasonic communication, while not popularized, has been known
and used for many years. The early work of Langevin, Florisson,
and others describes and discloses means and apparatus to communicate
through a water medium via ultrasonic waves. Part of this work was
directed toward submarine detection and signaling, as well as echo
depth sounding, in which the principles involving the use of compressional
water waves are truly the forerunners of the present-day principles
involving the use of electromagnetic "ether" waves for radar. Early
forms of "absolute altimeters" for avigation also employed sound
waves, but their velocity was too low to render them practical for
the vehicular velocities encountered in practice.
Compressional waves, in passing through any medium, are absorbed
to an extent dependent upon the frequency of the waves, the nature
of the medium, and the distance travelled. All mediums have a certain
"compressibility" and "viscosity" whereby all of the compressional
energy imparted to displace the medium is not returned to the wave
but is partially transformed into heat. From Rayleigh1,
the amplitude A of plane waves at a distance x from the generator,
Ax = A0e-KN²X
where Ao is the original amplitude, e is the base
of the natural logarithms (2.718), K is a coefficient depending
upon the density, compressibility, and viscosity of the propagating
medium, and N is the frequency of the wave motion. By inspection,
the amplitude of the wave at a distance is inversely proportional
to the square of the frequency, so that high frequency compressional
waves fade out in much shorter distances than low frequency waves.
The factor K for a water medium has such a relatively low value
that appreciable distances may be covered by compressional water
waves at frequencies of from 20 to 50 kilocycles. The factor K for
an air medium has a much higher value so that air distances must
be measured in feet rather than in miles. The relative efficiency
of the two mediums, as well as the effect of frequency, may be seen
by examination of Table 1, which shows the approximate range at
which the original sound intensity has been reduced to one half2.
Fig. 3. (A) Mechanical arrangement of the mechanically
resonant microphone. (B) Cross section view of the dynamic
reproducer. Photographs shown in Figs. 6 and 5 respectively.
A redeeming feature of ultrasonic sound waves, like high frequency
electromagnetic waves, is that they are easily directed and formed
into a cone or beam by conveniently small radiators. Provided that
the vibrating surface producing the wave is moving like a piston
with all surface elements in phase, the directivity of the reproducer
may be roughly calculated from
where θ is the half-apex angle of the cone, λ is
the wavelength and r is the piston radius3.
With the foregoing considerations in mind, various apparatus for
communicating via a 20 kilocycle ultrasonic sound beam has been
constructed and employed. The frequency generator to produce the
carrier wave consists of a commercial audio oscillator and ten-watt
audio amplifier. In order to amplitude modulate the carrier wave,
provision is made for plate modulating the output tubes of the ten-watt
amplifier, in accordance with conventional AM radio transmitter
Fig. 4. The piezoelectric reproducer.
Fig. 5. Dynamic reproducer.
Fig. 6. Mechanically resonant microphone.
Fig. 7. When sound-waves along paths D1 and
D2 arrive at the receiver out of phase, as can
occur as shown in diagram, fading and phase distortion occur.
Three general types of reproducers or "loudspeakers," namely
magnetostrictive, piezoelectric, and dynamic, have been used. With
regard to these reproducers, magnetostriction refers to that phenomenon
in which there is a change of length in a bar of ferromagnetic material
attending magnetization. If the magnetic field is alternating (at
20 kilocycles in this case), the amplitude of longitudinal vibration
in the bar will be maximum when the frequency of the applied field
is equal to the fundamental elastic period of the bar. In accordance
with the formula:
where f is the frequency in cycles per second, l is the length
of the bar in centimeters, E equals it's modulus of elasticity in
dynes per-square-centimeter and d its density in grams per-cubic-centimeter,
the length of a bar for resonance at 20 kilocycles is approximately
5 inches. This reproducer is shown in Figs. 1A and 2, clamped for
support at its nodal midpoint and strongly magnetized by an additional
direct current magnetic field. In general this simple type of magnetostriction
reproducer is unsatisfactory because of extreme eddy-current heating.
Further, this particular bar, having an o.d. of one-half an inch,
has a poor match with the air load. Better loading may be obtained
with the addition of a larger diameter thick disc at one end.
The piezoelectric reproducer, illustrated in Figs. 1B and 4,
consists in a 3.5 megacycle X-cut quartz plate cemented between
two identically dimensioned sections of steel rod. In order to determine
the resonant frequency, the required overall length of this steel-quartz-steel
"sandwich" may be approximated from the previously mentioned formula,
because the velocity of sound in quartz and steel is about the same.
This unit has an inherently high impedance requiring a high driving
voltage and necessitating the incorporation of a special matching
transformer as used with piezoelectric instantaneous record cutting
heads. The high voltage appearing across the quartz engenders difficulties
in mounting the reproducer.
The dynamic reproducer, illustrated in Figs. 3B and 5, consists
essentially of a small permanent magnet loudspeaker with the cone
replaced by a resonant brass disc one-quarter inch thick and two
inches in diameter, pinned at its center to the central pole piece
of the permanent magnet, and carrying the voice coil. This unit
has given excellent service in practice with relatively high output.
Various receivers for receiving the 20 kilocycle sound waves
have been employed, all of them requiring some type of microphone,
a carrier amplifier, a demodulator, a carrier filter, and an audio
system, as in conventional radio receivers. Two different microphones
have been successfully used, both being the Rochelle salt crystal
type. The first unit is an unmodified commercial crystal microphone
(Astatic WR-20) having dual crystals and diaphragms, having good
response in the 20 kilocycle region. The second unit, shown in Figs.
3A and 6, employs a stretched foil diaphragm with a small Rochelle
salt crystal cemented directly to the inside face. Provision is
made, as illustrated, for varying the tension on the diaphragm to
permit adjustment for some mode of mechanical resonance at 20 kilocycles.
This mechanically "tunable" microphone has considerably higher output
on equivalent sound intensities than the standard commercial unit,
with the added feature of tending to reject lower, non-resonant
Two types of receivers, one rather unusual in design, have been
employed. The first type consists of several stages of tuned audio
amplification, followed by a diode detector, carrier filter, and
audio amplifier. As an interesting experiment, in an effort to improve
sub-harmonic rejection, improve selectivity, and eliminate the necessity
for a 20 kilocycle carrier filter, another receiver was designed
in which the output from the microphone is successively doubled
to 160 kilocycles, amplified with a two-stage 160 kilocycle r.f.
amplifier, demodulated, and passed to the conventional audio system.
As expected, in this particular arrangement, rather severe audio
distortion is caused by the carrier doublers if a high percentage
of modulation is used. In subsequent experiments it is proposed
to use either push-push doublers or full-wave rectification to elevate
the carrier to the higher frequency. With the frequency raised to
around 160 kilocycles, standard 175 kilocycle i.f. transformers
provide conveniently packaged, high-Q tuned circuits for amplification.
Up to the present time, out-of-door tests with the disclosed
equipment have not been made, but experiments which have been performed
are of great interest. The 20 kilocycle carrier has been modulated
with both tone and phonograph signals and received exceedingly well
over distances of some thirty to fifty feet. Modulation does not
render the carrier audible to the ear. However, with the unmodulated
carrier directed toward the receiver, speaking into the carrier
microphone produces audible signals in the receiver audio output
system. With the carrier removed, the audible signal disappears.
Remarkably pertinent effects may be produced by reflecting a second
carrier wave path, in addition to the original path, from reproducer
to microphone, as shown in Fig. 7. Fading and phase distortion,
such as encountered in standard radio reception, may be produced
realistically in the ultrasonic receiver by varying the position
of the reflector, the motion of the reflector being somewhat analogous
to the motion of the Heaviside layer. The movement of people within
range of the apparatus produces like results and indicates the potential
utility of ultrasonics for intrusion or passage detection in air
mediums, using the beam-of-sound rather than the beam-of-light principle.
In the near future it is proposed to conduct out-of-door tests
in an effort to empirically determine the range of such apparatus.
1. Lord Rayleigh, "Theory of Sound."
2.Dr. L. Bergmann, "Ultrasonics," P. 195.
3. Ibid. (2), P. 194.
Posted January 29, 2015