A portable, general-purpose underwater sound measuring system

adil

NUWC TP 25
COPY

A PORTABLE, GENERAL-PURPOSE UNDERWATER SOUND
MEASURING SYSTEM

By M. A. Calderon and G. M. Wenz Listening Division
December, 1967 San Diego, California

DISTRIBUTION STATEMENT

Q i THIS DOCUMENT HAS BEEN APPROVED FOR PUBLIC RELEASE AND SALE;
ITS DISTRIBUTION 1S UNLIMITED.

ou.

Sul

Woes
de

NAVAL UNDERSEA WARFARE CENTER

An activity of the Naval Material Command

G. H. Lowe, Capt., USN Wm. B. McLean, Ph.D.

Commander Technical Director

The work described in this report was performed
under SF 101 03 15, Task 8119 (NEL L20471) by members
of the Listening Division. The report covers work from
January 1965 to July 1966 and was approved for publication
ZA Decemiyer hob:

The authors are indebted to R, Cruse for the design
of the pressure measuring system; to H. Carmichael for
much of its construction; to J. Leonard for the design and
construction of the calibrate boxes; and to G. Beilke
for his suggestion semmertance in field tests.

Under authority of
Curtis R. Haupt, Head
Sensor Development Dept.

, San Diego, California.

HANOI

0 0301 0043342 1

PROBLEM

Specify and procure a portable, general-purpose,
underwater-sound measuring system, primarily, but not
exclusively, for the investigation of ambient noise in the
ocean,

RESULTS

1. A portable underwater sound-pressure measuring
system has been developed, and in a variety of situations
has demonstrated the capability of measuring very low
ambient-noise levels.

2. High self-noise levels at frequencies less than 90 to
63 Hz are quite likely to be present if measurements are
made from a drifting ship and the cable is towed at the drift
velocity. These high self-noise levels are believed to be
due to a combination of flow noise and noise arising from
cable vibrations.

3. These high self-noise levels can be satisfactorily
eliminated by using a slack cable system as described in
this report.

RECOMMENDATIONS

1. Use a slack cable system if a tethered system is
used; and use an all-buoyant in preference to a nonbuoyant
cable for a simple system to be used for measurements from
a ship on the open ocean.

2. Perform calibration of the system with the random
noise during the slack period; otherwise the high noise
levels during towing may mask the random-noise calibrate
signal at the lower frequencies.

3, Use a"pink'’ random noise (-3 dB/octave slope) for
calibration, when noise with a negative spectrum slope is
to be measured. With the same broadband level a pink
random noise has a higher spectrum level at low frequencies
than a flat random noise.

4. Use an untethered system, if it is available, to
obtain ambient-noise measurements from a ship platform
in the open ocean. This system should have the capability
of obtaining data at any selected depth,

il
2

CONTENTS

INTRODUCTION, .. page 5

SYSTEM DESCRIPTION,.. 6
Hydrophone and Preamplifier... 7
Cabliever LL
Calibrate Boxes... 14
Sound-Level Meter and Analyzer... 17
Tape Recorders... 18

SELF-NOISE SOURCES.,.. 19

EE ED EE od) sce 20
Hydrophone Suspensions... 20
Cable Systems... 24

RESULTS OF FIELD TESTS... 30
Self-Noise... 30
Suspension and Cable Systems... 33

CONCLUSIONS,.. 33
RECOMMENDATIONS... 34
REFERENCES, . .' 35

APPENDIX A: HYDROPHONE-DEPTH MEASURING
SPY Mier ena

APPENDIX B: HYDROPHONE-PREAMPLIFIER
ASSEMBLY,.. 39

APPENDIX C: CABLE SPECIFICATIONS... 45

TABLES

Electrical Properties of the Cable... page 12

Lower and Upper Cutoff Frequencies and Midpoint
Frequencies for the Octave Bands of the Sound Level
IMIS KEIO, 5 6. I

w

>

ILLUSTRATIONS

Block diagram of the Basic Underwater Sound-Measuring
system... page 6

Basic Sound-Measuring System shown with portable tape
RE COrdeIr an. 7

System noise of the hydrophone-preamplifier... 8

Receiving responses of hydrophone M-H90-A, Serial Nos. 1
and 6...9

Block diagram of M-H90-A hydrophone with cable and
connections... 10

Diagram of hookup of cable conductors to cable
COnNnNECtOGS.. 2 11

Cross talk between calibrate pair and signal pair...13

Block diagram of calibrate box... 14

Equivalent sound-pressure spectrum level of random -noise
calibrate signal... 16

Frequency-response curves of the portable tape recorder... 19

Diagram of two hydrophone suspension systems... 22

Spar-buoy type suspension system... 23

All-buoyant cable system with shock-cord suspension... 25

All-nonbuoyant cable system with a simple float
suspension... 25

Combination buoyant and nonbuoyant cable system with
shock-cord suspension,.. 27

Simplified all-buoyant cable system... 28

Nonbuoyant cable system using spar-buoy suspension... 29

Comparison of noise-spectrum levels for a towed system
and a slack system... 31

Strip-chart records of the one-third octave band levels
centered at 10, 50, and 1000 Hz, and of the broadband... 32

Block diagram of pressure module and associated ship
equipment... 38

External view of pressure module.. .38

INTRODUCTION

In many cases when underwater sound measurements
have been needed, it has been the practice to assemble
available hydrophones, cables, amplifiers, recorders, volt-
meters, etc., making such provisions for calibration as
permitted by the available equipment. This practice was
often time-consuming and sometimes resulted in a less
than effective system.

The need for an underwater sound measuring system
has long been recognized and several such systems have
been designed and built, for example, the ''Suitcase"'
system! and the Noise Measuring Set AN/PQM-1A.2 Such
systems have proved to be very useful tools, but for some
applications the size, weight, and power requirements have
been a problem, and the procedures for accomplishing com-
prehensive calibrations have been tedious and time-
consuming.

Included in the objectives for the system described
herein was the minimization of size, weight, and power
requirements, Other special objectives were:

i Prequency range trom, NO Tizsto 10 KHZ;

2. Minimum self-noise and maximum sensitivity for
the measurement of low background noise levels.

3. Flexibility: capability for use from a rowboat on
the one hand, or from a relatively large ocean-going
research ship on the other.

4, "Off-the-shelf'' components: catalog items or items
of standard manufacture.

5. Simplicity in both operation and calibration,

Since system self-noise of an underwater measuring
system involves not only the equipment used but also the
way in which it is used, the definition of the system must
necessarily include a discussion of the techniques and proce-
dures of its use,

lsuperscript numbers identify references listed at end of report.

SYSTEM DESCRIPTION

The basic underwater sound measuring system is
shown in the block diagram of figure 1, and figure 2 isa
photograph of the system. Each of the elements will be
described in detail. Some auxiliary equipment to determine
the depth of the hydrophone can be used and such a system
is discussed in Appendix A.

SHIP EQUIPMENT

HY DROPHONE
AND
PREAMPLIFIER

TAPE
RECORDER

Figure 1. Block diagram of the Basic Underwater Sound-Measuring System.

BATTERY-OPERATED
CAL SOUND-LEVEL PORTABLE TAPE
BOX METER RECORDER

i

12 INCHES

HYDROPHONE-PREAMPLIFIER

Figure 2. The Sound-Measuring System shown with a portable tape recorder.

Hydrophone and Preamplifier

The hydrophone-preamplifier assemblies (M-H90-A)
were manufactured by Wilcoxon Research Co., Bethesda,
Md., according to the specifications which are included in
Appendix B. The crystal material used in PZT4 and the
construction of the hydrophone is such that the acceleration
sensitivity is reduced to a minimum, The acceleration
sensitivity is given as less than +60 dB/ubar/g. The source
capacitance, sensitivity, and input impedance can be speci-
fied to obtain a hydrophone with a low input noise level and
sufficient sensitivity to measure the lowest ambient noise
levels in the frequency range of interest. The crystal was
specified with a source capacitance of not less than 1000 pF
with a sensitivity of not less than -90 dBV re 1 dyn-cm”~?,
The input impedance of the preamplifier was specified to be

EQUIVALENT SOUND-PRESSURE SPECTRUM LEVEL

(DB RE 0.0002 DYN-CM~2)

40

No
(=)

=)

not less than 100 megohms with less than 30 pF of capaci-
tance. The high input impedance is necessary to have good
low-frequency response, This combination of source
capacitance, sensitivity, and input impedance resulted in
system input noise as shown by the curve of figure 3, at the
preamplifier input referred to an equivalent SPSL in water,
The empirical ambient-noise minimum? and the sea state 0
curve‘4 are given for comparison,

The actual receiving responses of two hydrophones
as determined at an acoustic calibration facility (NUWC:
SDL-TRANSDEC) are shown in figures 4 and 5.

EMPIRICAL MINIMUM Se
OF SEA AMBIENT NOISE SEA STATE 0

(KNUDSEN)
<> ra | Ne

SYSTEM
NOISE

100 1000 10,000
FREQUENCY (HZ)

Figure 3. Comparison of system noise at hydrophone-preamplifier input to the
sea state 0 curve (Knudsen) and to the empirical ambient noise minimum (Wenz).

RON TEMES
‘V-06H-W euoydorpAy 0-2 YoIBasoYy UOXOITIM Jo osuodsal SUTATODOY *¢ INST Y

(ZH) ADNAN Aes
000'0L O00L ool Ol
a= a aaa! = Sais! |e ioe a ee =e ees {ASE aa T 2 06-

"L “ON [eHag
‘V-0GH-W ouoydompay ‘0d YoIResoeY UOXODTIM JO osuodsal SUIATODOY “pf aMsSTy

(ZH) ADNANDAYS
000'01 0001 Oot Ol

DSc) Cs =
IISd 0

06-

08-

OL

yva"“~ t ay Ga

uva"/A | au aa

10

ELEMENT

The amount of voltage gain required for the hydro-
phone preamplifier is determined by the gain necessary to
insure that the voltage levels corresponding to the lowest
expected ambient noise levels would be above the input noise
level of the remainder of the system. The desirability of
maintaining as large a dynamic range as possible for the
hydrophone-preamplifier requires that the minimum amount
of gain necessary to accomplish the above purpose be speci-
fied. In this case the specified voltage gain is a nominal
30 dB +1 dB (28 dB as measured).

The output stage of the preamplifier was especially
designed to prevent high frequency oscillation when used
with long cables, This required the addition of two output
transistors to the basic preamplifier and necessitated at
least 12 mA of current at about 12 volts. The output imped-
ance is less than 50 ohms, with a maximum output voltage
not less than 2 volts rms.

The hydrophone and preamplifier unit with connec-
tions to the Marsh and Marine connector is shown in the
simplified schematic of figure 6.

Cae POWER

Figure 6. Block diagram of Wilcoxon Research Co. hydrophone M-H90-A with
18-inch cable and connections to a Marsh and Marine eable connector.

Cable

The underwater cable consists of three twisted
pairs, one pair covered with a shield, and the group of
three pairs covered with an overall shield. The two shields
are tied to each other at only one end of the cable. The
cables were procured in lengths of 50, 500, and 1000 feet.
Two types of cable, nonbuoyant and buoyant, were available.
Their electrical characteristics are identical. The non-
buoyant cable has a neoprene jacket with an OD of 0, 440
inch. The cable connectors used are Marsh and Marine's
XSL-6-CCR and XSL-6-CCP, one at either end, connected
as shown in figure 7. A separate return lead was used for
the calibration circuit to minimize common-mode voltages
when long cables are used, The buoyant cable is similar
but with a thinner neoprene jacket and an added flotation
jacket of foamed polyethylene for a slight positive buoyancy
in seawater.

OVERALL SHIELD
SIGNAL
PAIR #1 AND SHIELD

COMMON

PAIR #2

POWER (+12V)

PAIR #3
CALIBRATE PAIR

CCR CGR

Figure 7. Diagram of hookup of cable conductors and shields to Marsh and
Marine connectors XSL-6-CCR and XSL-6-CCP.

Lat

The two types of cable were obtained in different
lengths to provide some degree of diversification in the
kind of system that can be put together, depending on the
desired use, Variations of cable systems actually used
will be covered in the section, ''Field Tests.'' The specifica-
tion sheet for the cable is presented in Appendix C. The
measured electrical properties are given intable 1. The
cross-talk output of the signal pair when a signal is placed
across the calibrate pair was measured and is shown in
figure 8. About 1000 feet of cable were used for the cross-
talk measurements.

TABLE 1, ELECTRICAL PROPERTIES OF THE CABLE*

Resistance
(ohms/ ft)

1 al

0, 062
. 056
5 Ae)
«5 (OK!
058

orwhNd rr
eis) (2) (=)

Capacitance
(pF / ft)

Pins

1&2 25
2&3 26
3&4 28
4&9 25
1&6 38
2&6 43
3 & 6 68
5 & 6 38

*Values are the averaged measurements of three 1000-foot
cables. Measurements were made with a GR type 1650A
Impedance Bridge,

SIGNAL LEVEL AT OUTPUT OF 1/3-OCTAVE FILTER (DBV)

INPUT
SIGNAL

UNTERMINATED
x

“= 502 TERMINATION

100 1000 10,000
FREQUENCY (HZ)

Figure 8. Cross talk between calibrate pairs and signal pair using arrange-
ment shown.

13

Calibrate Boxes

The purpose of the calibrate boxes is to provide
calibration signals to the hydrophone-preamplifier input
using the insert resistor method, The calibrate boxes
(fig. 9) contain a 1000-Hz sine-wave generator and a broad-
band, flat-spectrum, random-noise generator. It also
contains a monitoring circuit for measuring the current in
the calibrate pair leads and a meter provides a Visual indica-
tion of the amount of current, Provision for inserting exter-
nal calibrate signals is included,

The calibration signals are applied through cable
leads 4 and 5 to the 10-ohm resistor at the input of the
hydrophone-preamplifier. The calibration signal thus
appears in series with the hydrophone signal. This method

CAL BOX

oe = ee, ee ee a 4

SIGNAL PAIR

HYDROPHONE
AND
PREAMPLIFIER

CAL SIGNAL
CURRENT
MONITOR

OSCILLATOR
1000 HZ
WHIT E-NOISE
GENERATOR

|
|
|
|
|
|.
|
| of
|
|_|

EXTERNAL

CAL
SOURCE

Figure 9. Simplified block diagram of the calibrate box and connections to
the hydrophone and sound-level meter.

14

of calibration, using two easily inserted calibration signals,
allows the entire electrical system to be calibrated very
simply and as often as desired, The calibrate signals are
recorded on tape along with the ambient-noise signal, and

so also serve to calibrate any subsequent analysis equipment
as well.

In practice the calibration signals are recorded just
prior to the ambient-noise measurement. The level of the
tone is adjusted precisely to the same known level each
time by monitoring the current in the calibration circuit.
Since the ambient-noise signal is present when the calibrate
signals are applied, it is necessary to apply the calibrate
signals at a much higher level than the ambient-noise
voltage level.

To use this method of calibration it is only necessary
to know the sensitivity of the hydrophone as a function of
frequency, the insert level of the tone (as monitored) and the
relative spectrum shape of the random noise as applied at
the 10-ohm resistor. The sensitivity, 9p, of the hydro-
phone is measured periodically at an acoustic calibration
facility. The spectrum shape of the random noise is mea-
sured periodically using a one-third octave band analysis,
and the relative spectrum level, N“(f), is obtained by sub-
tracting 10 log,, (bandwidth) from the band level. The spec-
trum level of the random noise is determined by the differ-
ence, D, between the known tone level, T, and the one-third
octave band level centered at the tone frequency.

N(1000) = T - D - 10 log (bandwidth)

where N(1000) is the absolute spectrum level of the random
noise at 1000 Hz. The absolute spectrum level at other
frequencies is determined by

N(f) = N(1000) - [N (1000) - N (11
Conversion to an equivalent SPSL is made using the appropri-
ate hydrophone sensitivity curve (fig. 4 or 5) as measured at
an acoustic calibration facility.

Equivalent SPSL = N(f) - S(f) + 74
where the 74 is a conversion factor to a reference pressure
of 0.0002 dyn-cm~’. Figure 10 shows the equivalent SPSL
of the tone and random noise of the two calibrate boxes as
normally used. If the broadband level of the random noise
should change, this will show up as a change in the difference,

15

EQUIVALENT SPSL RE 0.0002 DYN-CM"?

16

—

D,and an adjustment in the equivalent SPSL N(f) would be
made, (It is assumed that while the random-noise level may
change somewhat, the spectrum shape of the random noise
signal does not.) Ambient-noise spectrum levels are then
determined by a direct comparison with the random-noise
calibrate signal at each frequency.

The use of this tone and random-noise calibration
method eliminates the need for time-consuming calibration
using many different frequencies to measure the frequency
response of the system, although such calibrations can be
made from time to time for comparison, using the external
calibrate signal input.

120 T aa a FG T Ya ces CS T ln
e-— SPSL OF TONE |
110 F 5
00 7 1/3-OCTAVE BAND LEVEL 7]
/ AT 1000 HZ
@
90 F =
al MOD 2
eee ee ee ee
— es
L \ —— ee
19 MOD 1
7 ee pepe P| l ae ae era om ee We ea Ee
10 10? 10? 104
FREQUENCY (HZ)
Figure 10. Equivalent sound-pressure spectrum level of random-noise

calibrate signal.

Sound-Level Meter and Analyzer

The sound-level meter, General Radio Type 1558-AP
Octave Band Noise Analyzer, is a portable, battery-operated
audio-frequency spectrum analyzer. The analyzer contains
a high-impedance microphone preamplifier, selectable one-
octave bandwidth filters, an output amplifier, and an indicat-
ing meter. The gain of the analyzer can be set (depending on
the sensitivity of the receiving transducer) so that the meter
directly indicates the sound pressure level in any of its
bands. The frequency response of the analyzer is flat +1 dB
from about 20 Hz to 15, 000 Hz in the ALL PASS position.
The output of the analyzer is normally used in the ALL PASS
position to provide an input signal to the tape recorder.
Table 2 lists the lower and upper cutoff frequencies at the
3-dB down points and center frequency for all the octave
bands of the analyzer.

TABLE 2, LOWER AND UPPER CUTOFF FREQUENCIES
AND MIDBAND FREQUENCIES FOR THE ONE-
OCTAVE BANDS OF SOUND LEVEL METER

LOWER CUTOFF | UPPER CUTOFF | CENTER FREQUENCY
FREQUENCY FREQUENCY (Hz)
(Hz) (Hz) (Geometric Mean)
3-dB Down point | 3-dB Down point

99.3 7.5
44.6 63
89.2 125
Ley 250
354 500
707 1000
1414 2000
2828 4000
5656 8000
11, 310 16, 000
ALL PASS
A

1 /

18

A portable battery-operated analyzer was chosen
because it can be used in a variety of situations. It can be
used in a semipermanent installation where data are taken
at periodic intervals, It can also be used for a system
requiring a completely portable system, such as would be
needed in a small boat where no ac power is available, and
will operate for about 30 hours on its battery. The battery
can be recharged using the built-in charger. The octave
band filters of the analyzer provide the means of doing an
on-the-spot frequency analysis of the noise. The ''A"'
weighting position is useful for pre-equalization when
recording noise with predominant low frequency content,

Tape Recorders

The kind of tape recorder to be used depends on
where and how the noise-measuring system is to be used,
When the measurements are being taken from a relatively
large ocean-going research ship a good quality ac-operated
tape recorder can be used. For use on a small boat, where
portability is essential and ac power is not available, a
battery-operated tape recorder is necessary. The tape
recorders described below are the ones used in this system
and serve as examples of the two types.

The Ampex model 350-2P is a two-channel recorder
which uses 10-1/2-inch reels of tape. Two tape speeds of
3-3/4 and 7-1/2 ips are available. The frequency response
extends from about 15 Hz to 8000 Hz at the slower speed,
and from about 10 Hz to 15, 000 Hz at the higher speed.

The Uher 4000 Report-S is a portable battery-
operated tape recorder which uses 5-inch reels of tape.
Four speeds are available, 15/16, 1-7/8, 3-3/4, 7-1/2 ips.
The measured frequency response for one of these tape
recorders is shown in figure 11. These curves may be
found to vary from the manufacturer's specifications
depending on the condition and maintenance of the recorders.

The effective response can be extended below the
low-frequency limit by using a high playback-to-record
ratio for the analysis of the data.

RELATIVE LEVEL (DB)

100 1000
FREQUENCY (HZ)

Figure 11. Frequency-response curves of the portable tape recorder.

SELF-NOISE SOURCES

The electronic system noise was discussed in the
section on the hydrophone-preamplifier. This source of
system noise was shown to be considerably below the noise
levels to be expected from the lowest ambient-noise level
likely to be encountered.

Another source of self-noise is due to the motions
of the cable and suspension system. In particular, self-
noise arises from flow noise, cable vibrations caused by
towing the cable, and accelerations and static pressure
changes which are caused by vertical motions of the hydro-
phone due to the action of surface waves on the system. In
the cable systems which are described below the self-noise
is reduced by isolating the hydrophone from the motion of
the ocean and the motion of the ship. This is done through
the use of shock cord in the suspension system and by main-
taining slack in the cable during measurements, The accel-
leration sensitivity of the hydrophone, as mentioned pre-
viously, is minimized by design. The sensitivity of the
system decreases below 10 Hz, so that it is relatively in-
sensitive at the very low frequencies associated with major
wave motion.

1)

20

Another source of self-noise is radiated noise from
machinery on board the platform. This noise must be
reduced in order to obtain valid ambient-noise measure-
ments, It is also desirable to place the hydrophone as far
from the platform as possible when making these
measurements.

FIELD TESTS

Various hydrophone suspension systems and cable
systems were tested at sea to attempt to devise a system or
systems which have low self-noise, are simple to use, and
which are adaptable for measurements in deep or shallow
water. Tests were run using various cable combinations of
buoyant and nonbuoyant cable and suspension systems to
attempt to determine the self-noise of the cable systems
for a taut and for a slack cable, and also to determine which
system of those tested was most suitable. Descriptions of
the suspension systems and of the cable systems follow.

Hydrophone Suspensions

The purpose of a hydrophone suspension is to isolate
the hydrophone from vertical motions which are induced by
surface waves and swells. The isolation should be sufficient
to prevent "blocking" of the system which is caused when
excessively high noise voltages are applied to the preampli-
fier input.

As a starting point for the development of a suspen-
sion system, a system based on one developed by General
Dynamics/Electric Boat was used? (fig. 12A). It consists
of a hydrophone-preamplifier unit, a 50-foot cable, a buoy
(a flexible inflated plastic hose), a 50-foot shock cord

between the buoy and the 50-foot cable, and a 100-foot line,
‘also between the buoy and the 50-foot cable. The shock
cord is meant to isolate the hydrophone from the motion of
the surface waves and the line is to prevent the shock cord
from being extended beyond its elastic limit and possibly
breaking and the float being lost. The inflated buoy was
weighted so that most of the float was underwater, the pur-
pose being to reduce the effect of waves on it. The weight
in water of the 50-foot cable and hydrophone was adjusted
to about 2 or 3 pounds, so as to provide the right amount of
weight on the shock cord for proper operation,

Another suspension system which was tested is
shown in figure 12B. It consists simply of floats attached
to a line which is connected at the other end to the junction
of a 50-foot cable and a 1000-foot length of nonbuoyant
cable. No shock cord was used in this system for isolation.

A suspension used during part of an extended cruise
of USNS Charles H. Davis (AGOR-5) consisted simply of an
inflated plastic hose, which was tied with about a 10-foot
line about 300 feet from the sea end of the buoyant cable
when operating in deep water. No shock cord was used in
this suspension. The float can be attached at any place on
the cable, thus regulating the depth of the hydrophone, The
plastic hose was weighted so that most of it would be under-
water, but with enough buoyancy remaining so that it could
support the system.

A spar buoy was used as a suspension for a system
to be used on a small boat for taking data several miles

from the mother ship. The spar buoy (see fig. 13) was a
hollow plastic tube about 3 inches in diameter and about

20 feet long. One end was sealed airtight. In the operating
position the lower end of the tube was left open and the tube
was allowed to partially fill with water. The upper part of
the tube contained some trapped air which provided the
buoyancy. * The buoy was very stable and did not appear to
be much affected by the wave action, It had enough buoyancy
to support the hydrophone and cable which were attached to
the bottom end,

* The open-end spar buoy was suggested by J.J. Blanchard.

21

A. SYSTEM INCORPORATING SHOCK CORD B. SYSTEM UTILIZING NONBUOYANT CABLE
AND BUOYANT CABLE TO SHIP. AND NO SHOCK CORD.

FLOATS
PLASTIC HOSE
(INFLATED)

WEIGHT

SHOCK CORD

NONBUOYANT

CABLE TO SHIP
BUOYANT CABLE
TO SHIP

— 50-FOOT CABLE

50-FOOT CABLE

rHYDROPHONE ——, — HYDROPHONE

Figure 12. Diagram of two hydrophone suspension systems.

22

SEALED

<— TRAPPED AIR

WATER | | NONBUOYANT CABLE

TO BOAT

OPEN END

50-FOOl CABLE =a

.a— HYDROPHONE

Figure 13. Spar-buoy type of suspension system

as used from a small boat.

23

24

Cable Systems

Figure 14 illustrates a cable system using all-
buoyant cable with the shock-cord suspension system.

The procedure used in this system was to let the suspension
and part of the buoyant cable drift out from the ship a pre-
determined distance, and hold the remainder of the cable
on the deck coiled in a figure-8., The figure-8 was used so
that there would be no twist in the cable as it was let out,
At the start of a run the cable would be payed out fast
enough to keep it slack but not so fast that it would tend to
coil up as it floated on the surface. This system stabilized
very quickly and there was little or no overloading of the
signal from the hydrophone when the cable was slack, The
recording time was limited to about 10 minutes with this
system, since at a typical ship-drift rate of 1 knot a 1000-
foot length of cable would drift out in about 12 minutes,

A system using all nonbuoyant cable with the float
and line suspension is illustrated in figure 15, The float
provides enough buoyancy that the hydrophone would not be
pulled under by the weight of the nonbuoyant cable, In this
method of operation the floats and hydrophone would be
placed overboard with a nylon line attached to the floats,
About 300 feet of nylon line would be payed out, and also
all the cable, so that initially the system would be as shown
in figure 15, When ready for the test more nylon line would
be payed out and the system would slowly drift apart until
all slack in the cable would be taken up and the hydrophone
would start being towed, To repeat a run the nylon line
would be pulled in to about 300 feet as before. This system
took more time to stabilize than did the previous cable
system. ''Blocking" of the hydrophone-preamplifier
occurred for some time atter the release of the nylon lane
at the start of the run because of excessively high voltage
levels probably caused by accelerations of the hydrophone;
however the system settled down and thereafter was quiet.
It was also noted that the combined weight of the cable and
the drag made it necessary to use quite a large force to
pull the cable back on board. It was feared that the cable >
might not stand up to repeated use in this way.

BUOYANT CABLE

PLASTIC HOSE

SHOCK
CORD

HY DROPHONE a

Figure 14. All-buoyant cable system using the shock-cord hydrophone suspension.

NYLON LINE TO FEO

Pie SHOCK GORD

NONBUOYANT CABLE ——~> HYDROPHONE

Figure 15. All-nonbuoyant cable system using a simple float suspension.

20

A third system used a combination of a buoyant and
a nonbuoyant cable with the shock-cord suspension system
(fig. 16). The buoyant cable was used to place the hydro-
phone farther away from the ship, to minimize the inter-
ference from the radiated noise of the ship. A nylon line
was also used for this system and was attached to the float.
The float had enough buoyancy to prevent the weight of the
nonbuoyant cable from pulling the buoyant cable under the
surface, The procedure used for this system is the same
as that for the all-nonbuoyant cable system, The results
obtained are similar to those of the previous system and,
as before, the weights involved produced quite a large stress
On the cable,

A cable system that was used on the Charles H, Davis
(AGOR-5) (fig. 17) employed the simple float system
described earlier. The system consisted of a total of 1500
feet of buoyant cable to which was attached a 50-foot length
of nonbuoyant cable with the hydrophone, A 7-pound weight
was attached to the nonbuoyant cable about 5 feet from the
hydrophone, to sink the system to the desired depth. The
depth was selected by attaching the inflated hose at the
proper distance from the hydrophone, The 7-pound weight
also presents a greater inertia to accelerating forces on
the system. The procedure involved releasing 500 feet of
cable over the side and letting it drift away while keeping
the remainder on the deck in a figure-8, During the run
the cable was payed out just fast enough to keep it slack
throughout the run. After the run the cable was brought
back in and placed in a figure-8 in preparation for the next
run, leaving 500 feet of cable out.

Also used during the cruise of the Davis was a
small-boat system consisting of nonbuoyant cable with the
spar-buoy suspension system described earlier (fig. 18).
About 500 feet was sufficient to provide a long enough slack
period for an ambient-noise sample. A slack condition
was obtained by paying out the nonbuoyant cable slowly
while the spar buoy and the small boat drifted apart.

NYLON LINE TO FLOAT

BUOYANT

PLASTIC
BOAT FLOAT CABLE HOSE
GAG Fi ee SSS LIL
wins BA Ne
NV
SHOCK CORD SHOCK
CORD
\
<— LINE
NONBUOY ANT
—<—— OA BEE
HYDROPHONE

Figure 16. Combination buoyant and nonbuoyant cable system with the shock-cord

suspension.

27

PEASTL@iiOSE

Ne eaOa>owses ,, ee=r>»~>S<—*”?@?@--

— 1000-1500 FT

BUOYANT CABLE

50-FT CABLE

™~ WEIGHT

® <— HYDROPHONE

Figure 17. Simplified all-buoyant cable system.

28

SS

HYDROPHONE

Figure 18. Nonbuoyant cable system using spar-buoy suspension for use from a

small boat.

NONBUOYANT
CABLE

29

30

RESULTS OF FIELD TESTS

Self-Noise

The magnitude of the self-noise of the various sus-
pension and cable systems is illustrated in figure 19,
These results were obtained by a one-third octave band
analysis of the recorded noise from the simplified cable
system used on the Davis (fig. 17). The figure shows the
sound-pressure spectrum level of the noise at frequencies
from 10 to 1000 Hz at three different times during the
same run. One sample was taken before the run had
started and shows the measured level while the cable was
taut and the hydrophone was being towed at a speed of about
1 knot. The next sample was taken a few minutes later
with the cable in a slack condition. The difference in the
two samples is about 30 dB at 10 Hz and decreases with
increasing frequency until at about 40 to 50 Hz there is
practically no difference in the measured level. The third
sample was taken when the cable was again taut. This
high low-frequency noise level during towing was also
observed in all of the other cable systems tested and is
believed to be due to flow noise and cable vibration,

Figure 20 shows four strip-chart records of the
noise sample. Three of them are one-third octave band
levels which are centered at 10, 50, and 1000 Hz, The
fourth is a broadband record of the measured ambient
noise, At 10 Hz the noise level is high at the start and
begins to drop as soon as the cable becomes slack and
then rises again as the cable becomes taut toward the end
of the run, Note that the character of the noise in the 50-
Hz and the 1000-Hz bands during the taut condition is dif-
ferent from that in the slack-cable condition; however the
level remains about the same.

Ol

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31

—/§TART 10 HZ t

“BROADBAND ——=

0 5 10 15
TIME (MIN)

Figure 20. Strip-chart records of the one-third octave band levels centered at
10, 50, and 1000 Hz and of the broadband.

32

Suspension and Cable Systems

The all-nonbuoyant cable system and the system
using both buoyant and nonbuoyant cable will be discussed
together, since in many respects they are quite similar.
These two systems overloaded more during the initial part
of a run and took longer to stabilize than did the other
systems. This effectively reduced the amount of slack
time. Also, the two systems were not suitable for use in
very Shallow water, since the nonbuoyant cable would drag
on the bottom. The stresses on the cable were also greater
for the nonbuoyant systems.

The two remaining cable systems both use all-
buoyant cable. One of them uses the shock-cord suspension
and the other uses simply the inflated hose with no shock
cord. Both of these systems are very quiet and both reach
stability very quickly. The system without the shock cord
has the advantages of not needing careful adjustment of the
suspension system, and of permitting change in the hydro-
phone depth by simply changing the location of the suspen-
sion. In view of these features, the all-buoyant cable
system without the shock-cord suspension was selected for
general use,

CONCLUSIONS

A portable sound-pressure measuring system has
been developed and used in a variety of situations, with the
capability of measuring very low ambient-noise levels.

High self-noise levels at frequencies less than 50 to
63 Hz are quite likely to be present if measurements are
made from a drifting ship and the cable is towed at the drift
velocity. These high levels are believed to be due to a
combination of flow noise and noise arising from cable
vibrations which impart accelerations to the hydrophone,

These high self-noise levels can be satisfactorily
eliminated by using the slack-cable system described in
this report.

33

34

RECOMMENDATIONS

If a tethered system is used, the cable system
should be slack, and for a simple system an all-buoyant
cable system should be used in preference to the non-
buoyant type for measurements from a ship on the open
ocean,

Calibration of the system with the random noise
should be done during the slack period; otherwise the high
noise levels during towing may mask the random-noise
calibrate signal at the lower frequencies,

Use a "pink" random noise (-3 dB/octave slope) for
calibration, when noise with a negative spectrum slope is
to be measured, With the same broadband level a pink
random noise has a higher spectrum level at low frequencies
than does a flat random noise,

If available, an untethered system should be used to
obtain ambient-noise measurements from a platform in the
open ocean. This system should have the capability of
obtaining data at any selected depth.

REFERENCES

1. Woods Hole Oceanographic Institution Reference 52-10,
An Experimental Portable Listening Device for Detection,
Measurement and Recording of Underwater Sound, by

W. Dow, March 1952

2. Bureau of Ships NavShips 92277, Instruction Book for
Noise Measuring Set AN/PQM-1A, 21 July 1954

3. Wenz, G,M,, "Acoustic Ambient Noise in the Ocean:
Spectra and Sources, '' Acoustical Society of America,
Jjounnalsv. 34, p. 1936-1956, December 1962

4, National Defense Research Committee. Division 6
Report No. 6.1-NDRC-1848, Survey of Underwater Sound:
Report No. 3; Ambient Noise, by V. Knudsen and

R. Alford, 26 September 1944

5. Chapman, F,E, and Rothenberg, E,B., ''A Hydrophone
Suspension System for Deep Water Noise Measurement, "'

p. 199-209 in International Buoy Technology Symposium,
Washington, 1964, Buoy Technology; an Aspect of Observa-
tional Data Acquisition in Oceanography and Meteorology;
Transactions [Held | 24, 25 March 1964, Marine Technology
Society, 1964

REVERSE SIDE BLANK

35

APPENDIX A: HYDROPHONE-DEPTH
MEASURING SYSTEM

A block diagram of the pressure module with
associated equipment is shown in figure Al, The pressure
module is a waterproof stainless-steel cylinder designed
to withstand pressures at least as great as any to which

the hydrophone will be subjected (see fig. A2), The module

contains a strain-gage pressure transducer, a differential
amplifier, a VCO, and its own battery supply. The pres-
sure transducer is calibrated by a resistor which is con-
nected in parallel across one of the legs of the bridge.
This lowers the resistance of the leg and corresponds to
a known pressure change.

The cylinder was designed to be inserted between
the 50-foot cable and the longer sea cable. With the pres-
sure module in the system, the hydrophone-preamplifier
obtained its power from the internal battery supply of the
module, This was done so that the output of the VCO
could be placed on the leads which are normally used to
bring power to the hydrophone preamplifier.

The frequency of the VCO (12.5 to 14,5 kHz) was
then measured with an HP model 521E frequency counter
and recorded by a digital recorder model HP 561B. The
digital clock is a model HP 971B.

37

PRESSURE MODULE SHIP EQUIPMENT

CAL
z |
|
STRAIN-GAGE |
PRESSURE | DIGITAL
9V | TRANSDUCER l CLOCK
al !
SEA C
I
|
|
FREQUENCY
| COUNTED PRINTER
|
BRIDGE |
AMPLIFIER l
|
|
|

Figure Al. Block diagram of pressure module and associated ship equipment.

34 INCHES

Figure A2. External view of the pressure module.

38

APPENDIX B: TECHNICAL SPECIFICATIONS,
HYDROPHONE-PREAMPLIFIER
ASSEMBLY

U.S, Navy Electronics Laboratory,
San Diego, California

19 October 1965
1.0 SCOPE

These technical requirements cover a hydrophone-
preamplifier assembly to be used as part of an underwater
sound-pressure-level measuring set which includes con-
necting cables up to 1000 feet in length. An important
feature of this transducer is the capability of measuring
very low sound-pressure levels. The assembly shall
include a hydrophone, preamplifier with calibrate resistor,
case, cable, and connector,

2,0 REQUIREMENTS

2.1 General. The construction shall be modern and of
good commercial practice. The assembly shall have suf-
ficient strength and ruggedness to withstand normal han-
dling aboard ship at sea without suffering damage.

2,2 Hydrophone,

2.2.1 Voltage Response. Not less than -90 dBV for
1 dyn-cm-2 sound pressure, Preference will be given to
that design which minimizes the dependence of voltage
response on static pressure and temperature, The varia-
tion of voltage response with pressure and temperature
shall be stated by the supplier.

2.2.2 Capacitance, Not less than 0.001 microfarad,
2.2.3 Performance Goal. The characteristics are not

specified exactly herein since the supplier should select
that combination which, when used with his preamplifier,

39

40

will minimize the equivalent sound-pressure spectrum
levels of the maximum system noise of the hydrophone-
preamplifier combination (see section 2.6.3), The selected
characteristics shall be stated by the supplier.
2,3 Preamplifier,
2.3.1 Input Resistance. Not less than 100 megohms,
2.3.2 Input Capacitance, Not more than 30 picofarads.
2.3.3 Voltage Gain. 30 dB +1 dB.

2,3,4 Output Impedance, Not more than 50 ohms,

2.3.5 Open-circuit Output Voltage, Maximum. Not
less than 2 V rms, 5 V peak-to-peak.

2.3.6 Power Supply. Not more than 5 mA, @ 10 - 14
Vde. [Changed to 12 mA to accommodate added output
stage. ]

2.3.7 Calibrate Resistor, 10 ohms + 0.1 ohm,
2.4 Case,

The case for housing the hydrophone and preamplifier shall
be cylindrical in shape with rounded ends approximating a
hemisphere (see also section 3,0). The design shall allow
convenient access to the preamplifier and calibrate resis-
tor using only simple tools such as a screwdriver and
small wrench, Remolding or revulcanizing is unacceptable,

2.9 Cable and Connector.

The assembly shall be supplied with 18 inches of water-
proof low-noise cable and connector attached.

2.5.1 Cable. Five conductors (six conductors accept-
able), cable strength sufficiently adequate to support the
hydrophone assembly.

2.5.2 Connector, Six-pin, female, Marsh and Marine
type XSL-6-CCP or equivalent for mating with Marsh
and Marine XSL-6-CCR and XSL-6-BCR.

2.5.3 Connections. The cable and connector shall be
wired as shown in figure 1,

2.6 Hydrophone-Preamplifier Assembly.

2.6.1 Frequency Response, 9.0 Hz to 11,200 Hz +1 dB.

2.6.2 Directionality. Omnidirectional, +1 dB about
Z-axis, +1 dB about X- and Y-axis to 5 kHz, +3 dB about
Me anGd Weaxas bi to di2 Kez. “he Z-axis is the axis of the
cylindrical case (see also section 30).

2.6.3 System Noise, The equivalent sound-pressure
levels of the maximum system noise of the hydrophone-
preamplifier combination at 25°C shall not exceed the
levels shown in figure 3 lin the main text]. Preference will
be given to that design which minimizes system noise,
Information as to the spectrum levels of the limiting noise
shall be furnished by the supplier.

2.6.4 Shielding. The hydrophone and preamplifier
shall be electrostatically shielded with the shield insulated
from the water.

2.6.5 Acceleration Sensitivity. Preference will be
given to that design which minimizes acceleration sensi-
tivity. The acceleration sensitivity of the unit shall be
stated by the supplier.

2.6.6 Size. Not more than 2,5-inch diameter by not
more than 8-inch length, exclusive of the cable and con-
nector, The minimum size consistent with good perfor-
mance and economy of construction is desired.

2.6.7 Weight. Not more than 5 pounds, The minimum
weight consistent with good performance and economy of
construction is desired,

41

42

2.7 Environmental Requirements,

2.7.1 Operational Pressure, The assembly shall
operate as specified at any static pressure up to 250 psi.

2.7.2 Operational Temperature, The assembly shall
operate as specified at temperatures between 0° and 30°
centigrade,

2.7.3 Stowage Temperatures, No permanent change
in performance shall result from long periods of exposure
to temperatures between -30° and 50° centigrade.

2.7.4 Handling. The assembly shall withstand normal
handling aboard ship at sea without suffering damage.

2.7.5 Corrosion, Materials shall be chosen to resist
corrosion in a saltwater environment,

3.0 NOTE

An overall spherical shape will also be acceptable if not
more than 4 inches in diameter and provided all other
requirements are met. The Z-axis for the spherical case
is that diameter of the sphere which includes the point at
which the cable is attached.

4,0 INFORMATION MATERIAL

The supplier shall furnish informational material in the
form of sheets and graphs including, but not necessarily
limited to, the following:

a. Schematic wiring diagram of complete assembly,
including that of the preamplifier, showing all components
and clearly identifying each component,

b. Complete parts list including sufficient information
for procurement of each item, and keyed to the schematic
diagram,

ec. Graphs and/or tabulations showing spectrum levels
_of the voltage response, equivalent system noise, accelera-
tion sensitivity.

d. Characteristics of the hydrophone and of the
preamplifier.

5.0 QUALITY ASSURANCE PROVISIONS
The manufacturer shall be responsible for initial inspec-
tion and testing for compliance with the requirements,

Final inspection and testing will be made by the Navy
Electronics Laboratory after delivery.

REVERSE SIDE BLANK

43

aed

APPENDIX C:

CONDUCTOR:

ASSEMBLY "A":

ASSEMBLY ''B":

CABLE ASSEMBLY:

CABLE SPECIFICATIONS

#24 AWG: 19/0,005" soft tinned
copper wires, concentric stranded,
(1/ 46#/M"') OD 0.0025 in,

Insulation: Copolymer (ethylene-
propylene) color coded, nominal
wall thickness 0, 010 in, (0. 45#/M"')
OD 0.045 in.

Color Code: Solid color insulation.
Four different colors,

Twist: Two CP24U conductors
together with a right lay. (3. 9#/ M')
OD 0.090 in,

Color Code: Each pair to have one
common and one different color,

One CP24CP) assembly "Ay"

Shield: Braid #36 AWG soft tinned
copper wires, 6 ends, 16 carriers
13,2 ppi, 30° angle, 90% nominal
coverage, Bind with adhesive
mylar tape, l-layer, 50% lap.
Nominal OD 0.120 in,

t}

Core: Plastic monofilament.
Nominal OD 0, 020 in.

lst Layer: Cable, two CP24GP
(assembly "'A'') and one SP24TPS
(assembly ''B"') around the core
with a left lay. Fill voids with
rubber compound filler. Bind with
adhesive mylar tape, 1-layer,

50% lap. Nominal OD 0,210 in.

45

46

SHIELD:

JACKET:

FLOTATION JACKET:

PROPERTIES:

Braid: #30 AWG soft tinned copper
wires, 4 ends, 16 carriers, 8 ppi,
32° angle, 82% nominal coverage.
Bind with #113 tape, l-layer, butt
lap. Nominal OD 0. 280 in.
(22#/M'),

Nonflotation Cable: Neoprene
#480B, black, nominal wall thick-
ness 0.080 in. Nominal OD
0.440 in. (51#/M').

Flotation Cable: Neoprene #480B,
black, nominal wall thickness
0.040 in, Nominal OD 0. 360 in.
(23#/M'),

Cellular polyethylene, brown,
nominal wall thickness 0, 120 in,
OD approximately 0, 600 in.
(37#/M').

Cable Weight +10% per 1000 ft:

Nonflotation Cable Flotation Cable

In air: 112# In air: 140#
In water: 50# In water: 12#

UNCLASSIFIED

Security Classification

DOCUMENT CONTROL DATA-R&D

(Security classification of title, body of abstract and indexing annotation must be entered when the overall report is classified)
1. ORIGINATING ACTIVITY (Corporate author) 2a. REPORT SECURITY CLASSIFICATION

Naval Undersea Warfare Center UNCLASSIFIED
| San Diego, California 92152

3. REPORT TITLE

[a PORTABLE, GENERAL-PURPOSE UNDERWATER SOUND MEASURING SYSTEM

f 4. DESCRIPTIVE NOTES (Type of report and inclusive dates)

| Research and Development Report, January 1965 - July 1966

|S. AUTHOR(S) (First name, middle initial, last name)

M.A, Calderon and G,M, Wenz

H6. REPORT DATE 7a. TOTAL NO. OF PAGES 7b, NO. OF REFS i

} 8a. CONTRACT OR GRANT NO 9a. ORIGINATOR'S REPORT NUMBER(S)

b. PRosEctTNO. SF 101 03 15 AME AD
| Task 8119

9b. OTHER REPORT NO(S) (Any oth b ‘A 7
(NEL L2 0471) inistrenort) (Any other numbers that may be assigned

f 10. DISTRIBUTION STATEMENT
This document has been approved for public release and sale; its distribution
is unlimited.

11. SUPPLEMENTARY NOTES 12. SPONSORING MILITARY ACTIVITY

Naval Ship Systems Command
Department of the Navy

B13. ABSTRACT

A portable underwater sound-pressure measuring system was developed and
| tested. The system was shown to be capable of measuring very low ambient-noise
levels. High self-noise levels at frequencies less than 50 to 63 Hz are likely to be
present if measurements are made from a drifting ship, and the cable is towed at
| drift velocity, probably because of a combination of flow noise and noise arising
from cable vibrations. These high self-noise levels can be satisfactorily eliminated
by using a slack cable system as described in the report.

DDAee147 3 ease) UNCLASSIFIED

S/N 0101-807-6801 Security Classification

UNCLASSIFIED

Security Classification

KEY WORDS

Underwater Ambient Noise - Measurement
Underwater Sound - Measurement

DD N..1473 (Back) UNCLASSIFIED
(PAGE 2) Security Classification

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