Novel sound sources

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Research and Development Report
REPORT 990
17 October 1960

Novel Sound Sources

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THE PROBLEM

Design and develop novel low-frequency sound sources.

RESULTS

Two different types of high-intensity sound sources have
been developed which have both military and commercial
potentialities:

1. The underwater spark sound source is suitable for
explosive echo ranging, particularly from a submarine,
and shows promise as a source for long-range, active
detection and for long-range underwater signaling. The
underwater spark is safer than chemical explosives and,
unlike them, is repetitive. It is simple, reliable, and easily
adapted to working at great depths. Its short-duration pulse
allows discrimination against reverberation and is too short
to "home on,"' particularly if single-pinging or staggered
repetition rates are used.

This source has been tested at a depth of 300 feet
below the surface and generated peak pressures corresponding
to an acoustic level in excess of 3 million watts for a broad-
band nondirectional condition. In the 1000-4000-c/s frequency
band, a corresponding acoustic level in excess of 300, 000
watts was developed for a nondirectional condition.

2. The pneumatic sound source, operating models of
which were built and tested extensively, is probably the
Simplest generator of underwater sound yet discovered (in
its lightest form it weighs less than 1 pound). This source
has generated a peak acoustic level of over 4000 watts for
a nondirectional condition, in the frequency range between
spands 00) c/s.

The pneumatic sound source is suitable for ASW
operation; conversion of coastal defense passive systems
into active systems; explosive echo ranging; mine counter-
measures; anti-limpeteer operations; and coded signaling.
Typical commercial applications could be geophysical
prospecting and explosive metal forming.

WM ONAN MO

RECOMMENDATIONS

1. Conduct operational research to determine the useful-
ness and limitations of both sound sources in the tactical
applications suggested above.

2, Continue the study and experiments described here,
with a view to extending the potentialities of the two sound
sources, and improving the materials and techniques
involved in manufacturing and operating them.

3. Consider the feasibility of resuming study of the
other sound sources partially explored and briefly reported
here.

ADMINISTRATIVE INFORMATION

Work was performed under AS 02101, S-F001 03 04,
Task 8050 (NEL L3-3) by members of the Acoustics
Division. The report covers work from June 1956 to
September 1960 and was approved for publication 17 October
1960.

The author appreciates the assistance of F. D. Parker,
Group Leader, and of M. R. Markland, C. C. Dietrich,

J. Lyons, and P. Olszanecky. The work of John Pflaum in
obtaining the high-speed underwater photographs; of E. Rolle,
who developed many combinations of compounds for trial as
dielectric material; and of R. J. Bolam, who assisted in the
interpretation of the data, is also gratefully acknowledged.

CONTENTS

INTRODUCTION
I. THE UNDERWATER SPARK SOUND SOURCE
Design Considerations
Theoretical Discussion
Tests and Measurements
Experiments with a Magnetic Gap
Experiments with Resonators
Il, THE PNEUMATIC SOUND SOURCE
Background
Early Experiments
Design Considerations
Theoretical Discussion
Tests and Measurements
The Acoustic Bomb
lil. OTHER NOVEL SOUND SOURCES
Conversion of Heat to Sound
The ''Wolf Whistle"
CONCLUSIONS
I. The Underwater Spark Sound Source
ll. The Pneumatic Sound Source
RECOMMENDATIONS
I. The Underwater Spark Sound Source
Il. The Pneumatic Sound Source

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295

26
27-28

ILLUSTRATIONS

Basic design of horn gap

Variety of underwater spark gaps
investigated

Methods used to insulate and encapsulate
spark plugs

High-vacuum switch for energy transfer

Plot of acoustic power vs. sound pressure

Spark gap before and after firing

Laboratory tank and camera installation

Pictures of bubble development produced
in laboratory tank under various
conditions

Deep- water test installation at Lake Pend
Oreille

Assembly for underwater high- speed
photography

Circular spark gap with underwater lights

Inside view of spark capsule

Oscillograph traces and high-speed photo-
graphs of typical underwater spark
signal

High-speed photographs of bubble devel-
opment at NELPOCS

Arrangement for studying effect of magnetic
field upon spark gap

Experimental magnetic spark gap

Typical resonant chamber

Waveforms produced by resonant spark
sound source

Resonating coil for underwater spark

Spark gap in compliant resonant chamber

Waveforms produced by spark source in
compliant resonant chamber

Pneumatic high-power 1f sound source

Molds for rubber spheres

Pneumatic spheres of various wall
thicknesses

Photographs of bubble development from a
2-inch sphere

Experimental models of rubber spheres

Plots of frequency vs. depth for
pneumatic sound sources

Page
53

53
54
54
56
57

59
59

61

Figure

29

30

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32

33

34

35
36

37

Plots of frequency vs. depth for
pneumatic sound source

Typical waveform produced by 1. 3-inch
pneumatic sphere

Close-up view of surgical rubber tube
after rupture

Close-up view of rubber sphere after
rupture

Photographs of bubble development from
2-inch sphere

Photographs of bubble development from
2-inch sphere

Original model of acoustic bomb

Acoustic bomb with Mark 15 Mod 0
pressure mechanism

The ''Wolf Whistle"

INTRODUCTION

The advent of deep-running nuclear submarines has
brought about an urgent need for greater detection ranges
and, consequently, a requirement for new and improved
means of generating sound underwater, at increasingly
lower frequencies. This requirement has increased the
size and cost of sonar equipment, and sonar advances to date
have been considerably dependent upon transducer construc-
tion. As the number of transducer engineers is somewhat
limited, it has been necessary to concentrate most of their
efforts on established techniques. However, it has been
recognized that some investigation should be directed
toward unconventional methods of generating sound,
particularly in the low-frequency region. Accordingly,
the Bureau of Ships established a problem to investigate
novel sound sources. The personnel at NEL assigned to
this problem were all experienced in the field of generating
high-power signals, and had previously conducted inter-
mittent studies of the underwater spark as a sound source.
Thus it was logical to use this technique as a point of
departure in undertaking the new assignment. Although
other government laboratories no longer were exploring
this approach, a study was made of previous work by
others before the study was resumed at NEL.

Work with the underwater spark at this laboratory
prior to May 1956 has already been reported. | The early
work had clearly indicated that much of the success of the
spark source was directly related to the electrodes and
the geometry of the gap. It was therefore decided that this
particular phase would be studied thoroughly, leaving major
power increases to later.

A number of other techniques, such as exciting metals
into vibration by contact with solid carbon dioxide, or vibra-
ting wires, reeds, plates, etc., have been given limited
trials, but have not been explored sufficiently to warrant
reporting at this time. The work on novel sound sources is
continuing.

"ijnderwater—Spank Sound Source, '' by L. R. Padberg, Jr.
(Article 32 in NEL Report 698, Lorad Summary Report,
CONFIDENTIAL, 22 June 1956)

I, THE UNDERWATER SPARK SOUND SOURCE

The theory of the underwater spark as a sound source
has been considered for some time without conclusive
findings as to its value and limitations, or the optimum
design of the equipments involved. The principle of this
sound source may be stated briefly as follows: an underwater
electrical discharge causes a Sharp increase in temperature
of the water between the electrodes. The water vaporizes
and forms a gas bubble which expands and collapses, and
radiates acoustic waves. The nature of the radiation is
dependent upon a number of factors. The following sections
discuss some of the major theoretical and mechanical
considerations involved, and the development and testing
of a workable model of the underwater spark sound source.

DESIGN CONSIDERATIONS

Several years of experiment made it apparent that much
had to be learned about the development and construction of
suitable electrodes and gaps before a satisfactory underwater
spark sound source, producing high acoustic levels in the
lower frequency region, could be achieved. Experience at
NEL and other laboratories showed that there was consider-
able variability from one spark discharge to another.

Construction of the Gap

A basic fact about spark gaps, operated underwater, is
that the power into the load is equal to the current squared
times the resistance, or

Therefore, it is necessary to make the resistance of the

gap as large as possible. When two electrodes are short-
circuited by water the resistance is low rather than high

as is desired. It is apparent that the gap should be as wide
as possible to raise this value. A method for getting high
power that at first seems effective is to use a wide gap and
very high voltages to cause the gap to break down. Another
way is to raise the gap resistance by using the best dielectric

material obtainable between the electrodes, slightly imbedding
them in it. When very high electrical energy is discharged
across the gap, the water is vaporized almost instantaneously
and the gap resistance is raised.

In studying the literature and noting the size of the condue-
tors used in some tests it is apparent that few experimenters
have realized the magnitude of the first surge of current. For
example, in discharging 3000 watt-seconds of energy across
the electrodes, the current has been measured in excess of
220,000 amperes for brief periods. (Short, heavy leads are
essential for such an operation.) Very high temperatures at
the gap were also reported and, on one occasion, electrodes
made of titanium showed considerable melting. (Titanium
melts at 1800°C.)

Since it is desired to have the electrical discharge appear
only at the exposed electrode surface, NEL underwater spark
gaps have all been based upon the principle of the ‘horn gap.
The basic design of this gap is shown in figure 1. While the
optimum electrode material is tungsten, it is difficult to
machine and, for experimental purposes, too expensive.

Most of the NEL electrodes were machined from either brass
or copper. Some of the NEL gaps have been fired thousands
of times.

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Figure 1. Basic design of horn gap (before encapsulating).

The narrow slot shown in figure 1 will eventually be
the front of the gap. Note how the gap space widens toward
the rear. Although the picture shows one continuous piece
of brass, the metal will eventually be machined, after
encapsulating in a suitable dielectric material, into two
separate halves. This is an original technique, which
insures a constant gap spacing. Many configurations have
been tried and all incorporate the principle of having the
narrowest part of the gap at the front to prevent the discharge
from occurring inside the material where it will not only
be ineffective but will explode the gap. Figure 2 shows
several types of underwater spark gaps investigated.

Figure 2, Variety of underwater spark gaps investigated.

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Early Types of Underwater Spark Gaps

The first experiments with underwater sparks as a
source of sound utilized special types of automotive and
aircraft spark plugs. Since spark plugs were not designed
for use underwater, in general they were unplated and
rusted very quickly. Several better types were found that
were either chromium or cadmium plated. The types
with multiple discharge points, such as the aircraft variety
designed for igniting jet fuel, proved most satisfactory.

In general it was found that the porcelain insulation near
the firing points blew off on the first firing but the plugs
continued to function.

It was concluded very early in the experimental
program that special spark gaps would have to be designed
to handle any substantial power level. For energy levels
below about 100 watt-seconds, several types of plugs proved
satisfactory. To obtain greater power, attempts were made
to operate plugs in various series and parallel combinations.
Such arrangements usually proved unsatisfactory since
the plugs would seldom fire simultaneously. Firing would
be random and almost unpredictable due to non-uniform
point burning. Various modes of insulating and encapsula-
ting the plugs were tried. A few of the combinations are
shown in figure 3.

Energy Transfer

The sudden discharge of a large amount of stored energy
into a virtual short circuit is a great strain on any type of
electrical switch. A literature search revealed that many
experimenters are still using a three-ball open air gap for
the energy transfer. This method may result in extremely
low efficiency. Hydrogen thyratrons are fairly effective but
are short-lived and have associated ''flash-back'' troubles.

From the beginning of the underwater spark studies at
NEL, specially designed high-vacuum switches have been used
for the energy transfer function. Switching the high voltages
and current in a vacuum solves many problems and is prob-
ably the most practical way of accomplishing the transfer.
Thus all the energy is directed into the gap in water, where
it can be utilized, rather than into the air where its effective-
ness is lost and where it is hazardous to the eardrums of the
personnel in the area. The vacuum switch used (fig. 4) has
a rating of 30 kv at 600 amperes rms, and has been used
successfully to carry peak pulse currents of 220, 000 amperes.

Figure 3. Examples of methods used to insulate and
encapsulate spark plugs.

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14

Figure 4.

High-vacuum switch for

energy transfer.

Energy Storage

Although the principle of energy storage is as old as
the electrical industry, great advances have recently been
made in the construction of energy storage capacitors,
largely as a result of atomic power developments. The type
of job to be done will dictate whether to use very low capa-
city at high voltage, or very large capacity at lower voltage.
The basic formula for energy storage is:

E = Gy
2

In most of the experiments reported here, very large
capacitors were used at moderately high voltages, for exam-
ple 60 microfarads at a working voltage of 15,000. Most
early work used 100 or more microfarads at 4000 working
volts. Corona discharge causes little trouble at these
voltages but can be severe at higher voltages, particularly
in the damp atmosphere at sea.

Few experimenters have made any mention of the
inductance of energy storage capacitors. It is important
to keep this value low for efficient operation. Only recent
designs have this feature. It is equally important to keep
all the ''tank circuit'' leads of heavy copper bus, since the
discharge currents run into many thousands of amperes.

Dielectric Material for the Gap

For many years there has been a continuing search for
a Suitable dielectric material that could withstand the explo-
sive forces at the point of discharge. A number of materials
have been found that are fairly satisfactory for energy levels
up to about 1000 watt-seconds, at 1 atmosphere of pressure.
Pressure increased beyond that level resulted in water
getting inside the gap and causing internal explosion. Many
materials were found that could withstand severe beating
with a hammer without breaking or chipping but fractured
when used in the gap. A certain amount of pliability seems
desirable and also reduces the problem of recoil which is
very great in the underwater spark explosion. Additional
evidence of the tremendous forces at the gap is that the
shock wave generated can be used to form metals such as
stainless steel, titanium, etc. Numerous combinations of

15

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unsaturated compounds of high molecular weight were tried
for the dielectric material. Some have proved far superior
to commercial epoxy resins. One of the latter, known by
the trade name of ''Scotchcast No. 2'' has been satisfactory
up to levels of around 3000 watt-seconds.

In recent experiments using an energy level of 6000
watt-seconds, it was found necessary to abandon the dielec-
tric material and simply fire a well insulated brass rod
against a heavy bronze ground plate. This may be one of
the practical solutions to the problem.

Geometry of the Gap

Since one of the basic aims of the work was to obtain
significant output levels in the low-frequency region below
5000 c/s, many configurations of electrodes were tried.
The importance of the gap geometry became evident early.
This factor can be made to focus sound, can affect the size
and shape of the bubble, and it controls the duration of the
pulse. The many bubble photographs included in the report
illustrate its significance; note the strange, almost ''square"'
shapes of bubbles produced under some conditions. The
phenomena associated with the geometry of the gap are not
fully understood, and should be studied.

THEORETICAL DISCUSSION

Frequencies Generated by an Underwater Spark Discharge

The underwater spark discharge is essentially broad-
band in character and similar to the discharge of a chemical
explosive except that it is more susceptible to energy peaking
at certain frequency bands. From an echo-ranging stand-
point, high acoustic energy in the region between 1000 and
3000 c/s is desirable. Through the use of techniques
reported here, peak acoustic powers greater than those
obtained from large costly sound projectors have already
been achieved. Since we are not interested in wasting
large amounts of power in the region above 15 kc/s, we have
designed underwater spark gaps that generate large, low-
frequency bubbles. The bubble pulse from an underwater
spark can be quite useful, although previous experimenters
have given little attention to this fact. The frequency of the

Avg. Sound Pressure Over the Surface of a 1 meter sphere

bubble pulse is directly proportional to the depth at which it
is generated. In the region of 1000 to 3000 c/s for a non-
directional condition, peas acoustic pressure levels of
about 125 db/dyne/cm¢ at 1 meter have been measured.
This would correspond to 300 kw of acoustic power (fig. 5).

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1000 10, 000 100, 000 1, 000, 000
Total Acoustic Power in Watts

Figure 5. Plot of acoustic power vs. sound pressure.

Add to this the 10 db or more that can be gained by a reflector
of reasonable size, and the result is a megawatt of acoustic
power in a beam that can be directed wherever desired.

By combining such features as resonant discharge loops,
exciting resonant cavities, and operating the spark source, if
possible, at a depth where the bubble pulse is at the same
frequency as the resonators, substantial acoustic levels could
be obtained in the low-frequency region required for long-
distance echo-ranging. The underwater spark is simplicity
itself when compared with complex sophisticated sonars.

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Effect of Pressure on Electrodes

Insufficient evidence has been obtained to form any pos-
itive conclusions on the effect of pressure on electrodes.
However, Several findings can be reported.

In deep-water tests at Lake Pend Oreille it was found
that electrodes that performed very well in many firings at
near atmospheric pressure blew to pieces when used at a
depth of 300 feet below the surface. An example of this is
shown in figure 6. Evidence of many firings can be seen
by examining tne electrode surfaces. Experience has shown
that if water gets inside the electrode it will cause it to
explode from within. In the case shown, apparently water
penetrated the dielectric material via a small crack when
subjected to pressure at this depth.

In one test, using a gap Separation of 1/2 inch, the gap
which fired well at atmospheric pressure would not fire
when the pressure was increased to 200 psi for the same
impressed potential. Decreasing the gap solved the problem.
This result would indicate that there is probably a critical
gap spacing versus depth, at least in fresh water. This
should not be a problem in sea water.

All the problems of deep submergence appear to be
ultimately surmountable, and the underwater spark sound
source appears to be well adapted for use in the deep
ocean.

TESTS AND MEASUREMENTS
Test Procedure
Tank Observations

Pressure and frequency measurements of underwater
sound usually have to be made in an open body of water or
in specially constructed acoustic tanks. For visual and
aural observations, however, a small tank is useful. Two
tanks were used in most of the laboratory experiments
reported here. The smaller of the two was a modified
ammunition box measuring approximately 48 by 40 by 30
inches (fig. 7); the larger tank was a modified barge pontoon
measuring approximately 5 by 7 by 5 feet. Both tanks were
painted white on the inside to provide contrast for photog-
raphy.

Figure 6. Spark gap (A) before and (B) after firing at
300-foot depth.

19

20

Figure 7. Laboratory tank and camera installation. The
second camera has been removed to show end window and
tank construction.

Salt Water vs. Fresh Water

Almost all the underwater spark experiments to date have
been made with fresh water. On occasion salt water was
substituted with no apparent difference except in gap spacing.
Distilled water was also tried and the gaps would not fire,

In most areas, fresh water contains many impurities and
therefore has a sufficiently low resistance. In one exper-
iment the gap was Opened to a point at which there was not
sufficient potential to cause the gap to fire. Increasing the
Salinity of the solution caused the gap to fire as before.

High-Speed Photography

High-speed photography was used to study the phenomena
of the underwater spark and pneumatic sound sources. For
nearly all this work two Eastman Type 3 high-speed cameras
were uSed, photographing the action from two angles. Pic-
ture rates for the underwater spark shots averaged about
2800 per second and for the pneumatic explosions about
2000 per second. Thousands of feet of 16-mm motion pictures
were made, and stills from many of the sequences are shown
in this report. Figures 8A-B are pictures of bubble develop-
ment in the laboratory tank under various conditions. The
deep-water photographs appear in subsequent sections.

(A) From reflector gap.

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(B) "Square" bubbles illustrating effects of
geometry of gap on size and shape of bubbles.

Figure 8. Sequences from high-speed motion pictures
of bubble development produced in laboratory tank under
various conditions.

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Deep- Water Tests at Lake Pend Oreille, Idaho

As the limitations of the small tank precluded meaningful
measurements in the low-frequency region, arrangements
were made for calibration measurements at the NEL Pend
Oreille Calibration Station in Idaho (fig. 9). Months of
preparation were necessary to obtain high-speed underwater
photographs of the bubble development down to a depth of
300 feet below the surface, simultaneously with sound-
pressure and frequency measurements. The problems of
adequate lighting and synchronizing were sizable.

Figure 9. Installation at Lake Pend Oreille for deep-water
tests, showing extension dock used to lower power and control
cables into water. Small float in foreground holds trans-
formers for boosting shore power to barge.

25

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A large framework was constructed on which the necessary
equipment was mounted (fig. 10). The equipment included
two Eastman Type 3 high-speed cameras mounted to
photograph the bubbles from two different angles, eight
1000-watt lamps, the underwater spark capsule, and pneu-
matic source. To accomodate the heavy currents required
for the lights, welding cables were used for conductors.
All the components were enclosed in watertight housings.

Spark Capsule

Since it is impractical to run very heavy currents, such
as those generated in the underwater spark, down long cables,
the entire spark sound source was built into a discarded air
flask from a torpedo (fig. 11). The flask was cut in half and
a flange welded at the center. It contained a 3000-watt-
second energy storage system and 15, 000-volt dc power
supply. Vacuum relays, such as previously described, were
used for keying and safety discharge. Figure 12 is an inside
view of the capsule. Note that the spark gaps were mounted
directly at the bottom of the capsule to keep the lead length
to a minimum. A small, multiple conductor cable connected
the capsule with the surface and supplied 110-volt, 60-c/s, ac
to the unit. A single pair of wires triggered the source from
the surface. This arrangement worked well and would be
suitable for lowering the equipment from a ship or helicopter.
A switch inside the camera would automatically synchronize
the firing with the camera. The spark capsule demonstrated
that an intense sound source can be made into a small unit.
For use aboard a submarine, the capsule could be attached
on the deck, outside the hull, and thus conserve space inside.

Analysis of the Underwater Spark Signals

In the tests reported here, the underwater spark dis-
charge data were obtained by means of three separate
methods for later analysis in the laboratory. One was
high-speed photography of the bubble formation. Speeds
of approximately 2800 frames per second were used. In
addition, the signal, received on a calibrated hydrophone
separated 20 meters from the source, was recorded ona
Magnecord Model PT6J tape recorder. Tape speeds of

15 inches per second were used. Ina third method this
same signal was fed to the vertical deflection amplifier of
a Tektronix Model 512 oscilloscope. This display was
photographed using a 35-mm Fairchild oscillograph camera.
The horizontal sweep or time base was achieved by the
movement of the film past the vertically deflected spot on
the scope face. The film was set to run by the deflected
spot at 25 inches per second; to insure accuracy of final
reading, timing marks were placed on the film at 100-msec
intervals.

During the laboratory analysis of the data it was found
that the tape recordings were of limited value. This was
because the data were amplified and put on the tape as
amplitude changes. The low-frequency response of an AM
recording distorts and loses much of the detail in this type
of complex signal. A frequency- modulated type of record-
ing and playback would have corrected this deficiency; how-
ever it was not possible to conduct further experiments
after this shortcoming was noted. Fortunately the two
types of photographically recorded data appear to contain
the necessary detail. Figure 13, with its associated descrip-
tion, provides pertinent information on a typical underwater
spark signal. It should be noted that the signal from the
underwater spark is very similar to the signal received
from underwater explosions of TNT.

The signal in figure 13 covers a period of slightly more
than 13 milliseconds with the source at 100-foot depth.
During the first 2 msec the shock wave amplitude was so
high that the signal was off the film and cannot be seen. At
about 3.8 msec the bubble has expanded to its greatest
diameter and the pressure is falling. At about 7.6 msec
from the start the pressure rises sharply as predicted,
corresponding in time with the near maximum contraction
of the bubble. The bubble then expands to a second max-
imum at about 9.2 msec and the pressure falls until it
reaches a point at about 10.8 msec, where again the pres-
sure rises sharply, corresponding to another maximum in
the contraction of the bubble. The bubble then tries to
expand again but begins to disperse.

In addition to the oscillograph trace of figure 13,
pertinent frames from the high-speed record are shown
with identification of their time-position relative to the
oscillograph trace.

4 Gilg, R. H., Underwater Explosions, Princeton University

Press, 1948

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Figure 11. Circular spark gap with cluster of underwater
lights to provide illumination for photography. Nose of
camera case in foreground; visual calibration grids in

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Figure 12.

Inside view of spark capsule.

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Sound-Pressure Measurements

The sound-pressure levels listed below were measured
at the NEL Pend Oreille Calibration Station in Idaho in
July 1959. The lake is 750 feet deep at this point. (Photo-
graphs of bubble development at NEL POCS appear in
figure 14. )

Using an energy source of 3000 watt-seconds, a peak,
broadband sound-pressure level of 135.2 db/dyne/cm
referred to 1 meter was measured with the underwater
spark source 100 feet below the surface. This was fora
nondirectional condition and corresponds to a peak acoustic
power in the water of about 3 million watts.

Additional omnidirectional measurements, made ata
depth of 300 feet and using low-pass filtering, show the
following results:

Low-pass Acoustic Pressure Level Approximate
Cut- off (c/s) (db/dyne/cm™ ref. 1 meter) Acoustic Level (kw)
200 114.0 295
1000 122.0 150
4000 1272-10 150
9000 225 160

As can be seen, a substantial amount of energy appeared
in the region between 1000 and 4000 c/s. Reference to the
graph in figure 5 will show that the acoustic level in the 1000
to 4000 c/s region is 300, 000 watts. There are few, if any,
existing sound sources generating powers of this magnitude.
Since the source levels were for the nondirectional case, it
appears that additional gains can be made by operating the
spark source at the focus of a reflector. The dimension of
such a reflector at this frequency is practical for many
applications where directivity is desired.

Since these measurements were made, the electrical
storage-transmitter power has been doubled. However, no
sound-level measurements have been made using the new
storage unit.

EXPERIMENTS WITH A MAGNETIC GAP

A few experiments were conducted to determine the effects
of a magnetic field on the spark gap. The first simple measure-
ments used a spark plug inserted between the poles of a large
magnetron magnet (fig. 15). Later experiments employed

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35

36

Oy

Figure 15. Arrangement for studying effect of magnetic
field upon spark gap (keeper in place).

ceramic magnets, ''Mobilarc Gaps, '' manufactured by the
Westinghouse Electric Corporation for protecting high-
tension power lines from lightning discharges. These
magnets spun the arc and thus avoided burning a gap at one
spot. The underwater spark gap which was designed around
the ceramic magnet is shown in figures 16A-B. While the
gap performed well, its over-all improvement over other
designs was questionable. In using the large circular gaps
it was difficult to produce a true ''ring fire, '' as the spark
tended to rotate around the gap in a random manner,

EXPERIMENTS WITH RESONATORS

A limited amount of work at obtaining spectral emphasis
in the 1000-c/s region by the use of water-filled resonant
chambers was described in reference 1. Several chambers
were designed and tested using these techniques. A typical
chamber is shown in figures 17A-B. The resonant frequency
was first calculated as previously described and checked
in water using Lissajous patterns. The agreement with
original calculations was very close (fig. 18). The chamber
was then shock excited into vibration by the underwater
spark, using electrical resonance in the charging circuit
(fig. 19). This combination worked very well.

Dr. William Toulis, while at NEL, constructed a
compliant-tube type of resonant chamber (fig. 20) for
use with underwater spark gaps. Data from tests using
this device are shown in figure 21.

Il. THE PNEUMATIC SOUND SOURCE

BACKGROUND

In a further attempt to generate high-intensity sound
underwater, several simple experiments were performed
with the pneumatic sound source. Its principle of operation
is as follows: An elastic sphere is inflated underwater until
its expansion limit is exceeded and it ruptures. Upon being

37

38

Fle le li [eve lai al \ [

Figure 16. Experimental magnetic spark gap,
and (B) disassembled.

(A) assembled

Figure 17. Typical resonant chamber (A) assembled and
(B) disassembled.

39

7.0 MSEG

200-400 CYCLES 400-800 CYCLES

800-1600 CYCLES 1600-3200 CYCLES 3200-6400 CYCLES

6400-12,800 CYCLES I2,800- 25,600 CYCLES CALIBRATE =!1000 CYCLES

Figure 18. Waveforms produced by resonant spark sound source.

40

Eneuge 19°

Figure 20.

Resonating coil for underwater spark.

Spark gap in compliant resonant chamber.

4]

BROADBAND

END OPEN

1000-3000 CYCLES 2000-4000 CYCLES 2000-10,000 CYCLES

END PARTLY CLOSED

1000-3000 CYCLES 2000-3000 CYCLES 2000-10,000 CYCLES

Figure 21. Waveforms produced by spark source in
compliant resonant chamber.

42

released, the air bubble alternately expands and contracts,
providing a wave train at the resonant frequency of the
bubble. The frequency of a given bubble size is controlled
by the stiffness of the medium, which is proportional to
depth. Thus, the frequency of oscillation can be controlled.
Preliminary tests with this source were encouraging.

Work has continued, and thousands of pneumatic explosions
have been made with uniform results. Frequencies between
5 and 300 c/s have been generated in this fashion.

EARLY EXPERIMENTS

The early trials consisted of inflating many types and
shapes of elastic material underwater. Surgical rubber
tubing was tried in many lengths and wall thicknesses.
Rubber bulbs, plastic bottles, and glass beakers were also
tested and were blown to destruction. Best results were
obtained with surgical rubber because of its uniformity.
For inflation, both compressed air and carbon dioxide were
used--the latter in the form of carbon dioxide cylinders,
with their triggering mechanisms, from standard Navy life
jackets. Figure 22 shows one of the early designs.

Figure 22. Pneumatic high-power/1lf sound source, with
6-8 gram CO, cartridge.

Nowell Low Frequency Sound Sources, by L. R. Padberg, Jr.,
NEL Letter Report 38, CONFIDENTIAL, 14 October 1957

43

DESIGN CONSIDERATIONS

Early measurements made at the Sweetwater Calibration
Station showed that pressure was dependent upon the wall
thickness of the rubber. It was also observed that the best
waveform was obtained from spherical sources. A local
rubber manufacturer was therefore contacted to discuss the
problem of fabricating special rubber spheres. The
manufacturer agreed to develop special formulas, attempting
to find one that would give results equal or superior to those
obtained with surgical rubber. NEL-designed and built molds
for the job (fig. 23). In order to obtain a maximum of data
in a short time, it was decided to concentrate on a sphere
approximately 1 inch in diameter, varying the wall thickness
from one model to another to compare results (fig. 24).
High-speed photographs (approximately 2500 frames per
second) were made of the bubbles produced by these spheres;
figure 25 is a typical sequence. Later, spheres of other
outside diameters and various wall thicknesses were made
by the manufacturer for testing (fig. 26).

Following are the major characteristics of rubber mixtures
used in the NEL tests:

Surgical Reeves Reeves Reeves
Rubber Wikis SOI, 1 Wks SOIR, 5* wWob< 216

Durometer

Hardness 42 47 50 60
Specific

Gravity ? 1.02 1.02 ike iil
Tensile

Strength (psi) 3400 3310 4500 3400
Elongation (%) 600 690 TUS 480

*The greatest number of tests at NEL have used Reeves Mix 3015.5

Directionality at Low Frequency

The problem of obtaining high directivity at frequencies
below 1000 c/s has been a major difficulty for many years
because the necessary reflector, to be effective, must be
of such large physical dimensions.

Figure 23. NEL designs of molds for rubber spheres.

45

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47

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(penulyuoDd) “GZ eansty

48

For years, the explosives industry has sought a direc-
tive explosive charge, with only minor success. The pneu-
matic sound source would provide an intense low-frequency
point source for use with large parabolic reflectors. At
frequencies in the order of 300 c/s the size of a suitable
reflector might not be compatible with a shipboard installa-
tion, although it might be quite suitable for an ocean-floor
installation. As an example, at 300 c/s, if the parabolic
reflector were 200 feet in diameter, then a 5° beam would
be obtained. The directivity index of such an assembly
would be approximately 32 db and it is possible that a
source level of as much as 138 db above 1 dyne/cm#? at
1 meter would be obtainable. This would be a very usable
source level at this low frequency.

gee

6

Cee ee eee PEN IIE Ce eee eel yea ae.

Figure 26. Examples of rubber spheres designed by
manufacturer for testing: (A) 3-inch OD with 0. 75-inch
wall; (B) 1. 375-inch OD with 0. 25-inch wall.

49

50

Inflation of the Sphere

The kind of gas used for inflation will be dictated somewhat
by where and how the pneumatic source is used. In shallow-
water applications, most of the experiments were conducted
using conventional Navy issue carbon dioxide cylinders of
different sizes. For deep submersion work the problem is
more involved, since carbon dioxide is very temperature-
sensitive. In these applications compressed air at around
2000 psi was used. In order to obtain rupture a certain
differential pressure must be achieved depending upon the
elastic material used.

Inflation of pneumatic spheres or tubes might also be
accomplished by an explosive gas, with the rupture induced
by application of a spark. Since there is a marked difference
between the broad-band sound produced by a chemical
explosive alone and the almost monofrequency effect of the
pneumatic source, it would appear that a combination of the
two might well produce a substantial low-frequency output.
However, this possibility has not been explored at NEL, as
considerable study of chemical explosives as sound sources
has already been performed elsewhere.

THEORETICAL DISC USSION

Frequency Control

The frequency of the pneumatic sound source can be
closely controlled. Although it is dependent to some degree
on characteristics of the basic sphere, it is more conven-
iently dependent on the hydrostatic pressure to which it is
subjected at the time of bursting. The composite effect of
diameter and wall thickness is evident by comparing the
data on figures 27, 28, and 29, which are plots of frequency
vs. depth for three different sizes of spheres, all made of
Reeves Rubber Company Mix No. 3015.5. It has been
found that other rubber mixes influence the frequency
characteristic of a given sized sphere, probably because of
differences in elasticity.

10°

=a ]
Se Sgieeen
Avg. 5S, 106.5 db above 1 microbar
& ee is | a a
at 1 meter (peak level) a (e

cH sll

=a Fe

|
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Freq. in Cycles
S
wo
LN
y
te [sees | ee
aaa]
ars SS]
aS SS eal

107 10°

Depth in Feet

Figure 27. Plot of frequency vs. depth for pneumatic sound
source (1-inch OD sphere inflated with compressed air)
Wall thickness 0. 3125 inch.

3
Soe

Frequency in Cycles

Depth in Feet

Figure 28. Plot of frequency vs. depth for pneumatic sound
source (1. 375-inch OD sphere inflated with compressed air).
Wall thickness 0.25 inch.

51

a2

Sound- Pressure Level

The behavior of the bubble produced by the pneumatic
source varies significantly from that produced by the under-
water spark. After rupture of the sphere, the resulting
bubble oscillates without complete collapse, until it finally
disintegrates. The pulse so generated remains at a single
frequency and does not contain the high frequencies that are
produced by chemical explosives or the underwater spark;
their bubbles collapse completely and re-expand during their
oscillation period, thus producing a broad band of sound
frequencies.

In hundreds of measurements involving the 1-inch sphere
with 3/8-inch walls, the average peak sound-pressure level
for an omnidirectional condition was found to be 107 db/dyne/
em2 at 1 meter, at selectable frequencies within the band
25-250 c/s. This corresponds to an acoustic level of about 4000
watts, which is impressive at low frequencies. Much higher
levels appear obtainable in the near future.

Waveform

A typical waveform from a 1.3-inch pneumatic sphere
is shown in figure 30. This form is obtained without any
filtering using an amplifier and hydrophone with a flat
response between 20 c/s and 10 kc/s. The sine wave at
the bottom of the figure is that of a 50-c/s calibrate signal.
It will be noted that the waveform has a very steep front,
probably indicating the presence of many frequencies,
and immediately goes into the low-frequency oscillation
which in this case is approximately 30 c/s. An unusual
effect is shown in figures 31 and 32. Here the rubber
material has ruptured in the form of a decayed sine wave
similar to the actual waveform (fig. 30).

Pulse Length

The acoustic signal generated by the bursting of the
pneumatic source is a damped wave of approximately 100
milliseconds of usable duration. This lends itself to
narrower band filtering in the receiver than can be used
with the much shorter pulse lengths generated by the
underwater spark source.

in Cycles

Freq.

Avg. Sr, 105 db above 1 microbar

at 1 meter (peak level)

Depth in Feet

Figure 29. Plot of frequency vs. depth for pneumatic sound
source (3-inch OD sphere inflated with compressed air).
Heavy wall thickness.

Figure 30. Typical waveform produced by 1. 3-inch OD pneumatic
sphere; no filtering used. The sine wave at the bottom is that of
a 50-c/s calibrate signal.

53

54

Figure 31. Close-up view of 3/4-inch surgical rubber tube
after rupture.

Figure 32. Close-up view of 3-inch OD rubber sphere with
0. 25-inch wall after rupture.

TESTS AND MEASUREMENTS
Shallow Water Tests

All initial tests of the pneumatic sound source were
conducted in shallow water at the Navy Sweetwater Calibra-
tion Station. These tests can be summarized by stating
that the average peak sound level was around 100 db /dyne /cm2
at 1 meter for the early thin-walled spheres and surgical
rubber tubing used. Hundreds of combinations were tried.
Since the resulting frequencies were all around 100 c/s or
below, it was apparent that, to obtain a true picture, much
deeper water would be necessary to study the device. For
this reason all the later measurements were made in deep
water at the NEL Pend Oreille Calibration Station in Idaho.

Deep-Water Measurements

As mentioned earlier, elaborate preparations were made
to measure both the underwater spark source and the pneumatic
device at various depths and to obtain as much high-speed
photographic data as possible. The large rigging to accomplish
this was shown in figures 9 and 10. Photographs of the
phenomena connected with the oscillating bubble are shown in
figure 33 which was taken 200 feet below the surface, and in
figure 34 which was taken at a depth of 300 feet. These are
thought to be the first pictures of such phenomena taken at
such great depth. It will be noted that the sphere reaches
the same external dimension regardless of the depth. This
is because for a given initial diameter, wall thickness, and
type of material, the spheres rupture at a nearly constant
differential pressure. To date, the maximum depth at
which measurements have been made is 600 feet.

One important finding at the greater depths was that
the air initially entrapped in the sphere and inflating line
was compressed allowing the sphere to be forced into the
inflating fitting, thus weakening a small area of the rubber
wall prior to normal inflation. This problem was eliminated
by making the ball on the end of the molding mandrel remov-
able and allowing it to remain in the sphere. This provided
several advantages since it eliminated the entrapped air
and provided support to the inside of the sphere when it
was under compression from the outside. The ball has
not interfered with the inflation process.

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56

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57

58

THE ACOUSTIC BOMB

It is frequently desired to make sound transmission studies
in the deep ocean. Because there are few sources that can
generate substantial power, particularly in the very low fre-
quency region below 1000 c/s, chemical explosives of various
sizes are used for the study. This is not only a costly process,
but involves considerable hazards.

The pneumatic sound source is ideal for use as a sound
bomb. It is safe, cheap to produce, and creates a substantial
sound level at essentially a single low frequency which can be
controlled. The sound pressure generated is a function of
wall material and thickness. The frequency generated is a
function of the depth at which the source is operated and of
its initial physical dimension,

The first model of the acoustic bomb consisted of a very
simple pressure mechanism, a small cartridge of carbon
dioxide, and a small length of surgical rubber tubing sealed
off at the end (fig. 35), This mechanism worked satisfactorily
to a depth of about 150 feet.

Later versions of the acoustic bomb consist of Mark 15
Mod 0, practice depth charge pressure mechanisms attached
to flasks of compressed air or carbon dioxide, combined
with specially designed elastic spheres of various sizes.
Figure 36 shows one type used.

OTHER NOVEL SOUND SOURCES

CONVERSION OF HEAT TO SOUND

With the advent of nuclear submarines there is an
abundance of heat available which might be utilized in
the generation of sound. A few simple experiments
involving the principle of converting heat to sound were
conducted as part of the Novel Sound Sources program
and are reported briefly as a possible impetus to others
to continue this approach.

Carbon dioxide, or dry ice, at a critical point about
-110°F, changes state from a solid toa gas. This
phenomenon is known as ''subliming.'' If a metal object
which is at or near room temperature is brought into

Figure 35. Original model of acoustic bomb.

Figure 36. Acoustic bomb using Mark 15 Mod 0 pressure
mechanism.

59

60

momentary contact with dry ice, it imparts considerable
heat to it. This causes release of gas and considerable
pressure at the point of contact, and the metal object will
be set into vibration. Depending upon the shape and size
of the vibrating object, sounds of various frequencies and
surprisingly high levels are thus produced. Many varia-
tions of this method of generating sound are possible.
(This is a crude but effective method of testing vibrations
in metals at very low temperatures.) The frequencies
can be measured with a calibrated oscillator by means of
Lissajous patterns.

THE "WOLF WHISTLE"

In this sound source a condition which is normally
avoided as undesirable is intentionally put to use. It is
well known to players of string instruments that under
certain conditions beyond their control, a sympathetic
resonance can occur between the vibrating string and the
sound box. This undesired tone is call a ‘'wolf-tone."'

In the ''wolf whistle'' sound source, the vibrating
string is replaced by a more rugged vibrating body such
as a small metal saw blade, steel clock-spring, reeds, rods,
or bars. The vibration is started by placing the body in
front of a jet of air or water. Varying the tension adjusts
the frequency. ‘The vibrator is in turn placed at the open
end of a resonant chamber which is sympathetically
excited by the vibrating element. When the two frequencies
are identical, the resulting sound is loud. The adjustable
deflection plate can be used to control the intensity of the
sound generated. The two variations of the ''wolf whistle"'
shown in figure 37 have been operated in air but no under-
water measurements have been made of their performance.

Figure 37. The ''Wolf Whistle, '' (A) with vibrating blade in
position; (B) with adjustable reeds in position.

61

62

CONCLUSIONS

I. THE UNDERWATER SPARK SOUND SOURCE

1. An underwater spark is capable of providing very high
intensity, broadband, nondirectional short pulses of acoustic
energy.

2. Resonators excited by an underwater spark are capable

of providing selected frequencies or bands of frequencies

of acoustic energy.

3. The apparatus required for a high-powered underwater
spark is suitable for assembly in a capsule for deep submer-
sence:

4. An underwater spark is useful in explosive metal forming.

Il. THE PNEUMATIC SOUND SOURCE

1. Small rubber spheres are capable of providing acoustic
source levels of at least 107 db above 1 dyne/cm® at selected
frequencies within the band 10 - 300 c/s.

2. The frequency of a given pneumatic source can be con-
trolled by selection of its depth at the time of rupture.

3. The nature of the simple pneumatic source makes it
appear useful in a variety of applications requiring a single
pulse of selected frequency within its operating band.

RECOMMENDATIONS

I. THE UNDERWATER SPARK SOUND SOURCE

1. Conduct tests with the underwater spark to determine its
suitability for converting existing passive coastal defense
systems into active systems.

2. Conduct tests with the underwater spark to determine its
suitability for converting existing submarine passive equip-
ment into active echo- ranging equipment.

3. Conduct tests with the underwater spark to determine the
feasibility of obtaining desired directivity by use of suitable
reflectors.

4. Continue effort to determine the utility of the underwater
spark in fields of oceanography, hydrography, and seismology.
5. Continue effort to obtain improved materials for electrodes
and gaps, as well as to determine possible upper power limits,
utility of resonators, etc.

Il. THE PNEUMATIC SOUND SOURCE

1. Conduct tests to determine the usefulness of the pneumatic
source in converting passive coastal arrays into active ones.
2. Conduct tests to determine the usefulness of the pneumatic
source in low-frequency echo- ranging operations from sub-
marines, helicopters, etc.
3. Conduct tests to determine the usefulness of the pneumatic
source in other military applications, such as:

a. Pre-cursor acoustic sweep in mine countermeasures

b. Location of missile nose- cone water entry position

c. Anti-limpeteer operations

d. Submarine signalling

e. Oceanography, seismology, etc.
4. Continue effort to obtain improved materials and to deter-
mine upper power and frequency limits.

63

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Chief, Bureau of Ships (Code 335)
(Code 670) (Code 673) (Code 688) (6)
Chief, Bureou of Noval Weapone (DLI-3) (OLI-31) (2)
(RUDC-2) (2) (RUDC-1))
Chief of Noval Personnel
(Technical Librory)

Chief of Noval Operations (Op-73) (2) (Op-07T)
Op-03EG)

Chief of Naval Research
Code 41? (Code 455) (Code 461)
Code 466

Commonder In Chief, U. S$. Pacific Floet
Commonder in Chief, U. $. Atlantic Fleet
Commander Operational Test and Evaluation Farce,
U. S. Allantic Fleet
Commander: Crulser-Destroyer Force, U. S, Pacific
jee!
Commonder, Destroyer Force, U. $. Atlontic Fleet
Commonder Submarine Force, U. $. Pacific Fleet
Commonder Submarine Force, U. S. Allantic Fleet
Commander Training Command, U. S$. Pacific Flee?
Commander Submarine Development Group TWO
Commander, Service Force, U.S. Pacific Fleet
(Library)
Commonder, Service Force, U. $. Atlontle Fleet
Commonder Key West Test & Evaluation Detachment
Commonder, U.$. Noval Alr Developmen! Center

(Library)

Commander, U. $. Noval Missile Center
(Code 5340)
Technical tibrary)

Commander, U. $. Naval Ordnance laboratory
(library) (2)

Commander, U. $. Navol Ordnance Test Station,
(Pasodeno Annex Libro

Commander, U. S$. Naval
Chino toke (Code 733)
(Technical Director)

Commander, Charleston Noval Shipyard

Commander, Porismouth Naval Shipyard

Commander, Puge? Sound Noval S$ levees

Commanding Officer and Director, David Taylor
Model Basin (brary)

Commanding Officer and Director, U. $. Naval
Engineering Experiment Station (library)

Commanding Officer and Director, U. S. Navy Mine
Defense laboratory (Code 712)

)
tdnonce Test Station

INITIAL DISTRIBUTION LIST

(One copy fo each addressee unless otherwise specified)

Commanding Officer ond Director, U. $. Naval
Training Device Center

Commonding Officer ond Director, U. S. Navy

Underwater Sound laboratory (Code 1450) (3)

Commonding Officer ond Director, U. S. Atlantic Fleet,
ASW Tactical School ,

Director, U. $. Novol Research Laborato:
(Code 2027) (2) (Code 53120)

Director, U. S$. Navy Underwoter Sound Reference
loboratery (Library)

Commonding Officer, Alr Development Squadron
ONE (VX-1)

Commanding Officer, U.$. Fleet Sonar School,
Koy Weal

Commanding Officer, U.S. Fleet Sonar School,
Son Diego

Commonding Officer, U.$. Naval Underwater
Ordnance Stotion

Commanding Officer, Office of Novol Reseorch,
Posaodena Branch

Commonding Officer,
New london

Officer In Charge, U. $. Noval Medical Research
loboratory

Hydrographer, U.$. Navy Hydrographic Office
(Division of Oceanography)
(Library)

Senior Novy Ulalson Officer, U. $. Navy Electronics
Llaison Office

Superintendent, U. $. Naval Postgraduate School
(library) (2)

Novy Representative, Project LINCOLN,
Massochusetts Institute of Technology

Assiston! Secretary of the Navy, Research ond
Development

Depariment of Defense, Director of Defense Research
and Engineering (Tech. Library)

Assistont Chief of Staff, G-2, U.S. Army (Document
Ubrary Branch) (3)

Commonding General, Army Electronic Proving
Ground echnical librory)

Commonding General, Redstone Arsenal
(Technicol Library)

Rosident Member, Beach Erosion Board, Corps of
Engineers, U.S. Army

Commander, Alr Defense Command
(Office of Operations Analysis)

U.S. Novol Submorine Bose,

Commander, Air University (Alr University Library,
CR-5028)

Commonder, Alr Force Cambridge Research Center

(CRQSL-1)

Commonder, Rome Alr Development Center
(RCRES-4C)

Commonder, Holloman Alr Force Bose (MDGRT)

University of California, Director, Marine Physical
loboratory, San Diego, California

University of California, Director, Scripps Institution
of Oceanogrophy (Library), la Jolla, California

VIA BUREAU OF SHIPS:

Natlonol Research Council (Committes on Undersoa
Warfare, Exocutive Secretary, George Wood) (2)
Brown University, Director, Research Analysis Group

Pennsylvania State University, Director,
Ordnance Research Laboratory
The University of Texas
Director, Defonse Research laboratory
Militory Physics Loboratory (Dr, R. B. Watson)
University of Woshington, Director, Applied Phyalcs
laboratory (Or, J. E. Henderson)

weeeThe Director, Woods Hole Oceanographic Institution

Bell Telephone Laboratories, Incorporated, Murray
Hill, New Jersoy

Bendix Aviation Corp., North Hollywood, California

Edo Corporation, long Island

Goneral Electric Compony, Syracuse, Now York

Raytheon Mfg. Company, Wayland, Massachusetts

Songomo Electric Company, Springfield, Illinols

Via Commanding Officer, ONR New York Branch
loboratory of Marine Physics, Now Haven, Conn.
Columbia University, Director, Hudson Laboratorios

Dobbs Ferry, Now York

Via Commonding Officer, ONR Boston Branch
Horvord University, Director, Acoustics Research

laboratory, Dr. F. V. Hunt

Via ONR Resident Representative, University
of Michigan

University of Michigan (Director, University of
Michigon Research Institute)