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Full text of "Summary of work on acoustic properties of underwater bubble screens"

NPS-61-82-003-PR 



"' IOOL 

J. 

LIBRARY 

RESEARCH REPORTS DIVISION 
NAVAL POSTGRADUATE SCH: 
MONTEREY, CALIFORNIA 93940 



// 



Wm. POSTGRADUATE SCHOOL 

Monterey, California 




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SUMMARY OF WORK ON ACOUSTIC' PROPERTIES OF 
UNDERWATER BUBBLE SCREENS 



O.B. WILSON and J.V. SANDERS - 
Department of Physics and Chemistry 



FINAL REPORT FOR PERIOD JAN 19 81 - DEC 198 2 



Prepared for : 

Commander, Puget Sound Naval Shipyard 

ATTN: Mr. John Kriebel , Acoustic Range Branch, Code 246 

Bremerton, WA 9 8 314 



FEDDOCS 

D 208.14/2:NPS-61-82-003PR 



NAVAL POSTGRADUATE SCHOOL 
Monterey, California 



Rear Admiral J. J. Ekelund D. A. Schrady 

Superintendent Acting Provost 



This report was prepared as a summary of work supported in part by 
funds fron the Puget Sound Naval Shipyard, Job Number N 0025181 WR 
10124, dated 30 January 1981. 

Reproduction of all or part of this report is authorized. 
This report was prepared by: 



A. INTRODUCTION 

The objective of this report is to summarize the work carried 
out this last year by Lieutenants Kelley and Marr as part of their 
thesis research, supported in part by funds from your 
organization, and to provide some comments, conclusions and 
recommendations of our own. The theses (1,2) have been 
transmitted separately. 

In the design of an acoustically insulating bubble screen, 
there are many aspects which must be considered. The work of 
Kelley and Marr addressed primarily only two, the acoustic 
transmission properties of the screen and the noise which is 
generated by the screen itself. 

B. NOISE GENERATION 

A possible problem in the use of a bubble screen for noise 
isolation is the noise generated by the screen. The work of LT 
Kelley (1) was directed toward the measurement of the noise power 
associated with the formation of a bubble screen. The following 
summarizes some of the results in Kelley' s thesis and recasts some 
of the results in a form which may be more useful. Some errors 
noted in Ref. 1 are also corrected. 

The work was carried out in the tanks in the Postgraduate 
School's Underwater Acoustics Laboratory, using a reverberant 
field method. The steady state spatial average of the sound level 
in an enclosure which has at least partially reflective walls is a 
measure of the acoustic power generated by a source in the 
enclosure. An enclosure, in this case, the tanks, and the 
associated hydrophone system were calibrated by using a source 



of known power output. A similar averaging taken when an unknown 
source is in the enclosure can be used with the calibrations to 
estimate the power output from the unknown source. In this way LT 
Kelley was able to make some measurements of the acoustic power 
created or associated with the bubble screen for several 
configurations of bubble generating manifolds and air flow rates. 
Efforts were made to approximate bubble densities which might be 
appropriate for a screen which would be effective at the low 
acoustic frequencies of interest at Carr Inlet. Noise 
measurements were made on three different bubble generators, 
constructed by drilling rows of small holes in two inch diameter 
PVC pipe. They differed in number and sizes of holes. One had 
many small holes, the second had about the same hole area achieved 
by a smaller number of larger holes. The third had one row of 
small holes. The generator pipes were a bit less than five feet 
long and were located at the bottom of the tanks which are about 
seven feet deep. 

Flow rates were controlled by valves and were measured using 
flow meters and by timed capture of air of a known volume from the 
bubbles. Only approximate estimates of bubble size were possible 
using visual and photographic observation. Bubble density was 
estimated from flow rates, bubble rise times and screen 
dimensions. Tests were conducted to discriminate between the 
relative noise generating characteristic of bubble formation, 
bubble rise and bubble venting. There were found some errors and 
omissions in Kelley' s thesis. Some are trivial and obvious, but 



the errors in Table VI are not. Enclosed as an appendix is a 
corrected Table VI with additional information tabulated. 
Kelley's results support the following conclusions: 

(1) The dominant source of noise from the screens produced 
in this lab is the bubble formation. Bubble migration appears to 
be a measurable source and may contribute as much as ten percent 
of the energy. Bubble venting appears to contribute very little 
additional noise in the frequency range we used (20Hz to 10kHz). 

(2) The production of bubbles by a large number of small 
holes is significantly quieter (the order of 10 to 15 dB at some 
frequencies) than when the same air flow rate passes through a 
smaller number of larger holes. This effect may be due, in part, 
to better acoustic shielding provided by the bubble distributions 
around the pipe in the case of the larger number of holes. 
Quantitative measures of the differences are imprecise because at 
many frequencies, the noise generated by the quiet screen was less 
than the ambient threshold of the measuring system. 

(3) Figures 15 and 16 in Ref. 1, give the source level in one- 
third octave bands for the 4.8 foot long screen in dB ref 1 UPa at 
1 meter. It can be seen that for the quieter type bubble generat- 
or, the source level determination is limited by the ambient noise 
threshold. We believe that the peaks in the output from the 
noisier manifold may be due to bubble resonances. However, we can 
not be really sure that these results would be applicable to 
bubble generators at significantly greater depths. 

A worst case is from Figure 16 at 200Hz. The one-third 
octave source level aiven is about 132 dB . If it is assumed that 



the acoustic power is a linear function of the length of the 
manifold and that the spectrum is uniform over a one-third octave 
band, then for one yard length of screen an acoustic power 
generated in a one Hertz band at 200Hz is about two microwatts. 
This is for a bubble screen about ten inches thick, one yard long 
with an air bubble concentration of about one percent and a total 
air flow rate of about 4 SCFM . The quieter screen should be less 
noisy than this by a factor of ten or more in sound power. 

If one desired to predict the level at some distance from 
such a screen, a reasonable source model is to assume that the 
screen generator behaves as a line array of incoherent sources. 
Transmission loss models appropriate to the geometry of the 
problem would have to be chosen and applied. 
C. ACOUSTIC TRANSMISSION 

LT Marr (2) assumed a geometrically ideal screen (plane, 
parallel boundaries) with a uniform distribution of non-resonant 
bubbles having diameters much less that the wavelength of the 
sound. His computer calculations showed that for realistically 
obtainable bubble concentrations (between 10~1 and 10~3 
volume percentage of air), stop bands exist within which the 
transmitted intensity is significantly reduced for a broad band of 
frequencies and a considerable range of incident angles. The 
attenuation in these stop bands exceeds 20 dB . Pass bands of 
nearly dB attenuation have very small frequency extent. 

A real bubble screen is not expected to be geometrically 
ideal nor is it expected that the bubble concentration will be 
uniform. The screen will increase in width as it rises to the 



surface, and its boundaries will be irregular and ill defined. 
Furthermore, the bubble concentration will vary with depth 
(because of both the increased size of the individual bubbles and 
the increased width of the screen) and with distance from the axis 
of the screen. The first effect is difficult to predict because, 
while the bubble size as a function of depth is well known, the 
width of the screen has never been measured except for screens a 
few meters deep. The behavior of bubble concentration with 
distance from the axis is completely unknown; it has been reported 
in the literature that both the concentration and the bubble size 
decrease near the edges of the screen. 

It is difficult to predict the effectiveness of a non-ideal 
screen. While the transmission in the pass bands will undoubtedly 
be reduced, it is equally likely that the transmission in the stop 
bands will be increased, making quantitative prediction of the 
effectiveness of a real screen impossible. Given the distribution 
of bubble concentration, sophisticated theoretical approaches 
exist that could be used to predict the transmission. However, it 
is our belief that the hydrodynamic theory necessary to predict 
the bubble concentration for a given bubble-injection population 
does not exist. 
D. RECOMMENDATIONS 

We believe that the work we have studied so far does not 
provide enough information to permit design and construction of a 
full scale bubble screen with a satisfactory degree of risk. This 
applies to both the noise generation and the attenuation. There 
is no question that a screen of the kinds considered by Marr would 



provide a sizezable amount of attenuation. It is not really clear 
that the bubble generation noise would be a problem. We just do 
not yet have sufficient confidence to design a full scale screen. 

It is our opinion that in the absence of a full-scale basic 
research effort the effectiveness of a real bubble screen would be 
most efficiently determined from in situ experiments. Such exper- 
iments must be carried out on a larger scale that those done prev- 
iously where the screen depth never exceeded more than a few 
meters. Measurements would have to be made of the bubble concen- 
tration as a function of depth and distance from the axis of the 
screen. Acoustical transmission measurements should be made con- 
currently to allow comparison with theoretical results. Some very 
recent bubble screen experiments done in a pond by S.N. Domenico 
at Amaco Production Co . ( 3 ) came to our attention this week. 
We have talked with the author and expect to get a preprint of his 
paper soon. We will send a copy of it as soon as we get it. 

We recommend that full-scale bubble screen experiments not be 
carried out until after more research has been done to better 
define the basic hydrodynamic properties of such screens. Since 
such work would be both expensive and slow, we suggest that other 
approaches to reducing the effects of noise interference in Carr 
Inlet be examined. Signal processing combined with directional 
hydrophone arrays might be a productive approach. 



References 

1. Experimental Study of Noise Produced by an Underwater Acoustic 
Bubble Screen, Clark Thomas Kelley, MS Thesis, NPS, June 
1981. 

2. On the Design of an Acoustically Isolating Bubble Screen For 
Carr Inlet Acoustic Range, Kenneth William Marr, MS Thesis, 
NPS, June 1981. 

3. Acoustic Wave Propagation in Air Bubble Screens in Water, S.N. 
Domenico, Paper presented at the October 1981 meeting of The 
Society of Exploration Geophysics. (To be published in 
Geophysics, 1982.) 







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APPENDIX 



DISTRIBUTION LIST 



Commander 

Puget Sound Naval Shipyard 

ATTN: Mr. John Kriebel 

Code 246 

Bremerton, WA 98314 

Dudley Knox Library 
Naval Postgraduate School 
Monterey, CA 93940 

Professor J.V. Sanders 
Code 61Sd 

Naval Postgraduate School 
Monterey, CA 93940 

Professor O.B. Wilson, Jr. 
Code 61W1 

Naval Postgraduate School 
Monterey, CA 93940 

LT Kenneth Marr 
4717 Thresher Ct. 
Virginia Beach, VA 23464 

LT Clark Kelley 
1812 Long Meadow Dr. 
Montgomery, AL 36106 



U200022 



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