F— 
- 
___
-, o—*ob ’e 118 
NAVAL SURFACE WEAPONS CEN1ER WHITE OAK LAS 
SILVER SP—ETC 
F/s 17/1 
‘N
THE EFFLCT OF SOURCE SPEED ON THE TIME SCALE OF ACOUSTIC FLUCTIJe—tyctu)
SEP 76 I .1 ROSENSAIM
UNCLASSIFIED 
NSWC/WOL/TN—74—95 
NI.
END
OAT
6 —78
VI
NSWC/WOL TR 7O 95 
~~~~
THE EFFECT OF SOURCE SPEED ON THE TIME SCALE OF
~~ ACOUSTIC FLUCTUATIONS
BY DR. I.J. ROSENBAUM
ORDNANCE SYSTEMS DEVELOPMENT DEPARTMENT
4
I5SEPTEMBER 1976
w-J
Approved for public release; distr ibution unlimited.
NAVAL SURFACE WEAPONS CENTER
Dahigren, Virginia 22448 I Silver Spring, Maryland 20910
UNCLASSIFIED
SECURITY CLASSIFICATION OF THIS PAGE (*7t•n Data Ent.r.d) 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
DED~~~1. fltIEIIUEPJTATIAIJ DA~ E 
READ INSTRUCTION S
“
 
“‘ ~~~~~~“ “I” 
~~~~~ I 
BEFORE COMPLETIN G FORM
( p~ 
~~~~~~~~~~~~~~~~ 
ulII9FP 
2. GOVT ACCESSION NO 
3. RECIPIENT’S CATA LOG NUMBER
~~ rNswcMLJrF(_76_95]’ 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _
-~ P 
‘~~~~ ~~~~~~~~~ 
T . 
U I
 - 
100 COVERED
The Effect of Source Speed on the Time Scale 
Final v e.p+ 7
of Acoustic Fluctuations. 
J
8. PERFORMING ORG. REPORT NUMBER
THOR ~) 
S. CONTRACT OR GRANT NUMBER(a)
~~~ i~~~~jRosenba
~~ J 
_ _ _ _ _ _ _ _ _ _ _ _ _
9. PERFORMING ORGANIZATION NAM E AND ADDRESS 
IC, PROGR~~~~ELEMENT. PR~~ iCT TASK
Naval Surface Weapons Center ~ 
A R E A & I ~~RK UNIT NU~~~ ERS ’
White Oak 
627llN;!Flll2lj~~~iiu2l7llJ
Silver Spring, Maryland 20910 
A370370A~OI23;
II. CONTROLLING OFFICE NAME AND ADDRESS 
12 
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~~~~~~~~~~~~~~~
1 4 
MONITORING AGENCY N A ME & AD DRESS(I difforwi from Con,roflln~ Offlc.) 
IS. SECURITY CLASS. (of hi. r*port)
UNCLASSIFIED~~~~~
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~~~~~
ISa. DECLASSIFICA 
/0 
~~~~ flIJ~~~
IS. DISTRIBUTION STATEMENT (of thia R.port)
Approved for public release; distribution unlimited.
I?. DISTRIBUTION STATEMENT (of fAa abatract .nt.r.d in Block 20, ii dlf t.,.n t from R.port)
IS. SUPPLEMENTARY NOTES
IS. KEY WORDS (Continu, on r.v.ra. aid. ii n.c..aat,’ mid idantify by block numb.r)
underwater sound
fluctuation
transmission
correlation
~~ coherence
ABSTRACT (Ccettnu. an r.v.ra. aid. ii n.C.aWy mid ld.niy b~ block numb.,)
The time scale of acoustic fluctuations has been found to depend on source
speed for a shallow receiver—shallow sound source configuration in deep
water off the coast of St. Croix, U. S. Virgin Islands. Measurements were
made of a cw 135 hz source towed at L~ and 8 1a’iots on a 5—mile tangential
track through a point 7.5 miles from the receiving hydrophones. Using the
correlation time, the time it takes an autocorrelo~ ’am to decay to (lie)
of its maximui~ value as a noni~ or standard, the higher source speed
DO ~~~~ 1473 
EDITION OF I NOV 81 IS OBSOLETE 
UNCLASSIFIED 
~~ IP? ~~~~~~~~~
S/N 0102-014- 660 1
1ECU~~I1Y CLASs IFICATION OP THIS PAGE (IS,mi bat 
*m d)
UNCLASSIFIED
..LLUN1TY CLASSIFICATION OF THIS PAGEI1Th.n Data Znt.r.d)
consistently gave rise to shorter correlation times. As the data
integrat ion period was increased from 5 seconds to 100 seconds, the
measured correlation times increased as well , covering the range of 14
to ~462 seconds for various source speed—receiver depth combinations. As
a function of integration t ime, the difference between the correlat ion
times at the two speeds became greater for a receiver located in the
thermal layer than for a receiver beneath the layer.
Ils 
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NSWC/WOL/TR 76—95
PREFACE
(U) This report presents the results of a field test that attempted to
measure the time scale of acoustic fluctuations. The report should be of
interest to those concerned with sonar performance and sonar systems analysis
for the case of moving sound sources.
(U) The results reported herein were derived with the support of Task
No. WF11121711.
E. C. WHITMAN 
., : ‘
,
By direction
I
NSWC/WOL/TR 76—95
COWIENI’S
P~g~
INTRODUCTION 
1
EXPERIMENT 
2
RESULTS AND DISCUSSION 
3
CONCLUSIONS 
4
ACKNOWLEDGEMENTS 
4
Ii
_
_
_
 
_ 
_
_
_
_
_
_
_
--
NSWC/WOL/TR 76—95
ILLUSTRATIONS
Figure 
Page
1 
Experimental arrangement .
 
 
6
2 
Sound velocity profile — St. Croix—Spring  
7
3 
Ray diagram — f ine grid 
8
4 
Ray diagram — coarse grid.  
. .  
9
5 
Bottom profiles  
. . 
* . . . . 
. .  10
6 
Signal power vs Time — 50 foot receiver depth....  
11
7 
Signal power vs Time — 475 foot receiver depth 
12
8 
Autocorrelogram — 50 foot receiver depth — 5 second
integration period 
13
9 
Autocorrelogram — 475 foot receiver depth — 5 second
integration period 
. . . . .  14
10 
Autocorrelogram — 50 foot reciever depth — 25 second
integration period 
* 
. . . . .  15
11 
Autocorrelogram — 475 f oot receiver depth — 25 second
integration period . 
.
 
 
16
12 
Autocorrelogram — 50 f oot receiver depth — 50 second
integration period 
. .  
17
13 
Autocorrelogram — 475 foot receiver depth — 50 second
integration period 
.
 
 
18
14 
Autocorrelogram — 50 foo t receiver depth — 75 second
integration period 
19
15 
Autocorrelogram — 475 foot receiver depth — 75 second
integration period 
. . . . . . . 
. 
* . . 
* 20
16 
Autocorrelogram — 50 foo t receiver depth — 100 second
integration period  
. 
* . . . . . . ..  21
17 
Autocorrelogram — 475 foo t receiver depth — 100 second
integration per iod 
. . . 
* 
. 
. . . . . .  22
TABLES
Table 
Page
1 
Correlation Time 
5
Iii
NSWC/WOL/TR 76—95
INTRODUCTION
1. Under the most ideal conditions, the measurement of physical quantities will
display some variability. Even in the laboratory one expects differences in
successive measurements of “identical” phenomena. Indeed this variation
determines the precision with which a quantity may be measured. The causes of
these differences fall into three basic categories: (1) variations in the
physical quantity itself, (2) variations in the measurement systen , and (3)
variations in the physical environment. There is usually no way a priori
to determine which of these factors will be the dominant contributor to the
experimental scatter.
2. Once the confines of the laboratory environment are left behind, the major
contributor to experimental scatter is often the variability of the physical
environment. The measurement of the fluctuations in an acoustic signal
transmitted through the ocean is a case in point. Depending on the time
scale of the measurements, a variation of 10 or 20 dB is not uncommon.
In fact, the lack of fluctuation in a rec ived signal is usually interpreted as
an indication that something went wrong with the measurement. The causes of
the variability are many. Included in a list of causes would be: the internal
tides and waves in the sea and their effect on the temperature structure of the
medium, the time variant acoustic scattering properties of the sea surface,
and variations in receiver—source geometry.
3. The fluctuations are characterized by both amplitude and temporal
distributions. Over short periods of time the motions of the sea surface
could be a dominant contributor to the variability whereas the diurnal changes
in the temperature structure of the medium often account for long term
fluctuations. An experiment is detailed below which attempts to sort out one
of the causes of fluctuations from the rest, in a case which is of considerable
interest for sonar applications , i.e., the case of relative source—receiver motion
.1
NSWC/WOL/TR 76-95
EXPERIMENT
4. Acoustic energy is often simultaneously transmitted over a large number
of paths between source and receiver. The existence of these various paths
has been proposed to be one of the prime sources for signal fluctuations in
the ocean.1 
In the case of single frequency transmission , the contributions
from the various multipaths will interfere with each other both constructively
and destructively, producing a fluctuating signal. One can attempt to control
the rate at which multipaths interfere with each other by controlling the
geometry under which the sound is transmitted and received. Furthermore, one
might hope to control the rate of the fluctuations produced by multipath
interference by controlling the rate at which geometry is changed, i.e., by
changing relative source—receiver velocity .
5. The experimental arrangement used to demonstrate this effect is shown in
Figure (1). Sound travelling over the paths marked B and SB will produce
interference effects which cause signal fluctuations. (Only two of the many
paths are depicted.) If there is no source motion, the signal will fluctuate
at a rate which is characterstic of the rate at which the two acoustic path
lengths vary with time. For the case of source motion , an additional
fluctuation due to the rate of change of source—receiver geometry will be
present. The purpose of the experiment was to see if this change in
fluctuation rate is a measurable quantity as a function of source speed.
6. The test was conducted off the coast of St. Croix, in the U. S. Virgin Islands.
The Navy barge YSN1126 was brought to its deep mooring site at l7°5V7”N
and 64°41’l”W for the duration of the test. Three hydrophones were deployed
from the barge at depths of 50 feet, 100 feet , and 475 feet. The data from
the 100 foot phone is not included in the report due to its similarity with
the 50 foot phone. The sound source, in a streamlined body , was towed by
the research vessel PAUL LANGEVIN III on an east to west path through the
test datum 7.5 milesdue north of the barge. The first of two runs was made
at 4 knots with the sound source at 500 foot depth. A DECCA navigation
system was used to return the towing vessel to the identical path for a second
pass at 8 knots. The sound source emitted a CW signal at 135 Hz. The source
driving power was monitored and maintained constant aboard the towing vessel.
Sea state 3 conditions held for the duration of the test.
7. A sound velocity profile is given in Figure (2). The layer depth was 360 feet.
A ray plot diagram for this profile is shown in Figure (3). The ray bundle
shown leaves the source between the horizontal ray and 50 above the horizontal
at increments of .2°. The region occupied by the hydrophones in ray space is
shaded in. The range interval for the test was from 7.5 to 7.9 miles.
Little if any direct energy reaches the hydrophones in this shallow source—shallow
receiver configuration. Most of the sound will arrive via bottom bounce paths
or through scattering mechanisms.
‘R. J. Urick, “A Statistical Model for the Fluctuation of Sound Transmission
in the Sea,” NSWC/WOLITR 75—18, 1975.
2
NSWC/WOL/TR 76-95
8. In Figure (4) is seen the ray paths which would have existed had the bottom
been absolutely flat. The rays shown are between ±30° at 2° increments.
The bottom bounce mode is the major contributor in this idealized case.
9. The bottom profile was measured along two orthogonal paths through the
test datum , and is shown in Figure (5). A north—south track ten miles long
and an east—west track five miles long were surveyed. Though the fathometer
had an 8 kHz transducer while the test frequency was 135 Hz, it is not unreasonable
to assume that the bottom appeared flat at the test frequency as well.
RESULTS AND DISCUSSION
10. The data were converted from analog to digital format and analyzed with
a fast Fourier transform spectrum analyzer having 1 Hz resolution. In Figure (6)
are displayed 5 second averages for the 4 and 8 knot runs received at the
50 foot hydrophone for 1 Hz window at 135 Hz. In Figure (7) similar data
are shown for the 475 foot receiver. The power averages are shown on a
relative scale. A cursory review of these figures does n~ reveal any significant
trends with speed or depth.
11. An analysis of these data in the time domain is somewhat more revealing.
A convenient way to characterize the fluctuation rate is with the autocorrelation
function. The autocorrelation function c(i) for a discrete set of data values
a4 is defined as
n-i
1
c(i) = (N—i) 
a~a~+i
:1=1
12. The correlation time is a convenient measure of the degree of correlation.
It is defined as that t ime which the correlogram takes to fall to (if e)
of its maximum value at i=0. For the normalized autocorrelation function ,
this is the time it takes to fall to .~ 68. The correlation time concept
has been used by previous investigators and is particularly useful in quantifying
the temporal scale of signal fluctuations.
13. In Figures (8) and (9) are shown the autocorrelation function for the
4 and 8 knot runs for the 50 and 475 foot hydrophones. Only the first
5 minutes of the correlograxns are shown although the calculations were made
for the full 33 and 75 minute data set in each instance. Using the correlation
time as a measure of the time scale of the fluctuations , we see that the
8 knot runs have a shorter correlation time than the 4 knot runs at both
receiver depths, i.e., the higher speed correlogram decays more rapidly .
~R. J. Urick, “The Time—Scale of the Fluctuations of a Bottom Bounce Narrow—Band
Signal from a Moving Source in the Sea,” NSWC/WOL/TR 
75—83.3
NSWC/WOL/TR 76-95
14. As the data is integrated over longer periods of time, one might
expect to see different trends in the time scale of the fluctuations.
One would tend to associate the longer term fluctuations with the change in
geometry during the runs and the shorter period fluctuations with the sea
surface motion although at the low test frequency used there is probably not
a large contribution from sea surface motion. In Figures (10) through (17)
autocorrelograms are displayed for data integration periods of 25 to 100
seconds in steps of 25 seconds. In every one of these cases the higher speed runs
have correlograms which decay to (l/e) more rapidly than the lower speed
runs. So despite two diverse causes of fluctuations , the trend for longer
correlation times at lower speeds remains.
15. In Table I are listed the correlation times for all integration periods
at both speeds and both receiver depths. For each set of data we see an
increase of correlation time with increase of integration period. Additionally,
we see that the difference in correlation times between the two speeds becomes
much more pronounced with increasing integration time at the shallower
receiver.
CONCLUSI ONS
16. The conclusions of this study may be sumnunarized as follows:
a. Under controlled experimental conditions, the effect which
speed has upon the time scale of acoustic fluctuation is measurable
using the correlation time as an indicator.
b. Higher source speed give rise to shorter correlation times
for sound transmission in the bottom bounce mode.
c. For a sound source beneath a thermal layer, the effect which
source speed has upon correlation time is more pronounced for receivers
whithin the layer than beneath the layer as data integration time is
increased .
d. Correlation time increases uniformly with data integration
periods at fixed source speeds.
ACKNOWLEDGEMENTS
17. The author wishes to thank Mr. R. J. Urick of Tracor , Inc. for his
valuable advice and counsel on this project , Mr. C. M. Colvin of NAVSURFWPNCEN
and the personnel of Tracor Marine who operate the NAVAIR facility in St. Croix
for their able assistance in the data collection , and Mr. R. S. Hebbert of
NAVSURFWPNCEN for his help with the data analysis.
1~,
NSWC/WOL/TR 76—95
TABLE I
CORRELATION TIME
Correlation Time (see)
Integration Period 
Receiver Depth: 50 ft 
Receiver Depth: 475 ft
(sec) 
Source Speed 
Source Speed
4 k t 
8 k t 
4 k t 
8 k t
5 
4 
11
25 
23 
22 
39 
19
50 
190 
42 
56 
40
75 
354 
57 
85 
58
100 
462 
61 
91 
74
5
—*
.
NSWCNJOL/TR 76-95
I .
TOWING VESSEL 
BARGE
SEA SURFACE
HYDROPHONES
LAYER DEPTH
SOUND SOURCE
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
SEA BOTTOM
FIG. 1 
EXPERIMENTAL ARRANGEMENT
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-______________________
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FIG. 8 AUTOCORRELOGRAM -50 FOOT RECEIVER DEPTH 5 SECOND INTEGRATION PERIOD
13
NSWC/WOL/TR 76-95
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FIG. 9 AUTOCORRELOGRA M - 475 FOOT RECEIVER DEPTH 5 SECOND INTEGRATION PERIOD
- - 
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14
NSWC/WO L/TR 76-95
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FIG. 10 AUTOCORRELOGRAM — 50 FOOT RECEIVER DEPTH 25 SECOND INTEGRATION PERIOD
15
NSWC/WOLITR 76-95
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FIG. 11 AUTOCORRELOGRAM - 475 FOOT RECEIVER DEPTH 25 SECOND INTEGRATION PERIOD
16 
i
NSWCMOLITR 76-95
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AUTOCORRELOGRAM -50 FOOT RECEIVER DEPTH 50 SECOND INTEGRATION PERIOD
17
NSWCIWOL/TR 76-95
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AUTOCORRELOGRAM -475 FOOT RECEIVER DEPTH 50 SECOND INTEGRATION PERIOD
18
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NSWC/WOL/TR 76-95
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8 kno t source speed
!
~~~
00 
50.00 
100.00 
150.00 
200.00 
250.00 
300.00
C’
C’
C’
Titl E 
( S EC )
FIG. 14 
AUTOCORRELOGRAM -50 FOOT RECEIVER DEPTH 75 SECOND INTEGRATION PERIOD
19
-- 
--- ~~~,- 
- 
_
_
_
 
_
_
_
NSWC/WOL/TR 76-95
C’
C’
C’
LI)
knot source speed
0
4.-I
I.-a:
I 
I
.00 
50.00 
100.00 
150.00 
200.00 
250.00 
300.00
C.)0
Z~C’
I 
—
C’
C’
8 kno t source speed
4-1
100 .00 
15’ . 10 
2~~~~~0 
2
1
5 0 .00 
3~ 0 .00
.013
a:u~
C’
C’
C’
-4
I —
TitlE ( S EC )
FIG. 15 
AUTOCORRELOG RAM - 475 FOOT RECEIVER DEPTH 75 SECOND INTEGRATION PERIOD
20
---
~~~~~-
NSWCIWOL/TR 76-96
C’
C’
4 knot source speed
I 
I 
I 
I
~~~~.00 
50.00 
100.00 
150.00 
200 .00 
250.00 
300.00
C’
C’
C’ 
8 knot sourc e speed
.01) 
50.00 
100.00 
150.00 
2 00 .00 
250.00 
300.00
C.)0
:J c’
C’
C’
C’
-4
Titl E ( SE C )
FIG. 16 
AUTOCORRELOGRAM -50 FOOT RECEIVER DEPTH 100 SECOND INTEGRATION PERIOD
21
NSWC/WOL/TR 76-95
C’
C’
C’
source speed
0
0
-4
I—a:
I 
I 
I
.00 
50.00 
100.00 
150.00 
200.00 
250.00 
300.00
Li0
I 
—
C’
C’
8 knot source speed
0
4-1
a:
.00 
50 .00 
1 0 0.0 0
C
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 
300 . 00
U0
a:”H
ci
D I
0 1
TI t lE ( SEC . )
FIG. 17 AUTOCORRELOGRAM - 475 FOOT RECEIVER DEPTH 100 SECOND INTEGRATION PERIOD
22
___________
.- 
___•-1w 
- .