>'i

US Army Corps

of Engineers

Construction Engineering

Research Laboratories

USACERL Technical Report 98/27

January 1998

Reflector Microphones for Field

Recording of Natural Sounds

by

George W. Swenson, Jr.

QVAITTT

®s

0.1 5 A rocci length

0.20 X focci length

0.50 A fecal length

Directional microphone systems for field

recording of sounds in the air usually involve

either a parabolic reflector to focus the sound

waves on the microphone (transducer)

element, or a linear array of transducers so

phased as to respond preferentially to sounds

from one directional sector. The latter system

(the "shotgun" microphone) can be analyzed in

a fairly straightforward manner. The reflector

system involves a structure comparable to a

wavelength in linear dimension, and is

therefore not susceptible to the conventional

approximate methods of computation.

Recently developed computational techniques

now permit exact calculation of the directional

responses of small reflectors. This work

computed the characteristics of a number of

small microphone reflectors and linear

microphone arrays. Aflat reflector microphone

for the low-frequency range of 10 to 40 Hz was

also described. The study concluded that no

directional microphone can, in practice,

reproduce sounds with fidelity to the sounds as

emitted by the source.

Approved for public release; distribution is unlimited. 19980317 072
The contents of this report are not to be used for advertising, publication,

or promotional purposes. Citation of trade names does not constitute an

official endorsement or approval of the use of such commercial products.

The findings,of this report are not to be construed as an official

Department of the Army position, unless so designated by other authorized

documents.

DESTROY THIS REPORT WHEN IT IS NO LONGER NEEDED

DO NOT RETURN IT TO THE ORIGINATOR
USER EVALUATION OF REPORT

REFERENCE: USACERL Technical Report 98/27, Reflector Microphonesfor Field Recording of

Natural Sounds

Please take a few minutes to answer the questions below, tear out this sheet, and return it to USACERL. As

user ofthis report, your customer comments will provide USACERL with information essential for improving

future reports.

1. Does this report satisfy a need? (Comment on purpose, related proj ect, or other area ofinterest for which

report will be used.)

2. How, specifically, is the report being used? (Information source, design data or procedure, management

procedure, source ofideas, etc.)

3. Has the information in this report led to any quantitative savings as far as manhours/contract dollars

saved, operating costs avoided, efficiencies achieved, etc.? Ifso, please elaborate.

4. What is your evaluation ofthis report in the following areas?

a. Presentation:

b. Completeness:

c. Easy to Understand:

d. Easy to Implement:

e. Adequate Reference Material:

f. Relates to Area ofInterest: _

g. Did the report meet your expectations?

h. Does the report raise unanswered questions?
i. General Comments. (Indicate what you think should be changed to make this report and future reports

ofthis type more responsive to your needs, more usable, improve readability, etc.)

5. Ifyou would like to be contacted by the personnel who prepared this report to raise specific questions or

discuss the topic, please fill in the following information.

Name:

Telephone Number:

Organization Address:

6. Please mail the completed form to:

Department ofthe Army

CONSTRUCTION ENGINEERING RESEARCH LABORATORIES

ATTN: CECER-TR-I

P.O. Box 9005

Champaign, IL 61826-9005
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources,

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this

collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for information Operations and Reports, 1215 Jefferson

Davis Highway, Suite 1204, Arlington, VA 22202-4302, and to the Office of Management and Budget, Paperwork Reduction Project (0704-0188), Washington, DC 20503.

1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE

January 1998

3. REPORT TYPE AND DATES COVERED

Final

4. TITLE AND SUBTITLE

Reflector Microphones for Field Recording ofNatural Sounds

5. FUNDING NUMBERS

BT25

J27

6. AUTHOR(S)

George W. Swensen, Jr.

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

U.S. Army Construction Engineering Research Laboratories (USACERL)

P.O. Box 9005

Champaign, IL 61826-9005

8. PERFORMING ORGANIZATION

REPORT NUMBER

TR 98/27

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES)

U.S. Army Center for Health Promotion and Preventive Medicine (USACHPPM)

ATTN: MCHB-DC-EN

5158 Blackhawk Road

Aberdeen Proving Ground, MD 21010-5422

10. SPONSORING / MONITORING

AGENCY REPORT NUMBER

11. SUPPLEMENTARY NOTES

Copies are available from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161.

12a. DISTRIBUTION / AVAILABILITY STATEMENT

Approved for public release; distribution is unlimited.

12b. DISTRIBUTION CODE

13. ABSTRACT (Maximum 200 words)

Directional microphone systems for field recording ofsounds in the air usually involve either a parabolic reflector to focus

the sound waves on the microphone (transducer) element, or a linear array oftransducers so phased as to respond

preferentially to sounds from one directional sector. The latter system (the "shotgun" microphone) can be analyzed in a

fairly straightforward manner. The reflector system involves a structure comparable to a wavelength in linear dimension,

and is therefore not susceptible to the conventional approximate methods of computation. Recently developed computational techniques now permit exact calculation ofthe directional responses ofsmall reflectors. This work computed the

characteristics of a number ofsmall microphone reflectors and linear microphone arrays. A flat reflector microphone for

the low-frequency range of 10 to 40 Hz was also described. The study concluded that no directional microphone can, in

practice, reproduce sounds with fidelity to the sounds as emitted by the source.

14. SUBJECT TERMS

Reflectors

Microphones

Sound recording

15. NUMBER OF PAGES

18

16. PRICE CODE

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

19. SECURITY CLASSIFICATION

OF ABSTRACT

Unclassified

20. LIMITATION OF

ABSTRACT

NSN 7540-01-280-5500 Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std 239-18

298-102
USACERL TR 98/27

Foreword

This study was conducted for U.S. Army Center for Health Promotion and

Preventive Medicine (USACHPPM) under Project 4A161102BT25,

"Environmental Research—Corps of Engineers"; Work Unit J27, "Remote

Activity Monitoring." The technical monitor was William Russell, MCHB-DCEEN.

The work was performed by the Planning and Mission Impact Division (LL-P) of

the Land Management Laboratory (LL), U.S. Army Construction Engineering

Research Laboratories (USACERL). The USACERL principal investigator was

Dr. Larry L. Pater. Dr. George W Swenson, Jr. is associated with the

Department of Electrical and Computer Engineering, University of Illinois,

Urbana. The computations presented here were performed using code written by

Soon Young Lee and Alona Boag. Some ofthe graphs were prepared by Jonathan

Benson. Dr. Harold E. Balbach is Chief, CECER-LL-P; Dr. William D.

Severinghaus is Operations Chief, CECER-LL; and William D. Goran is the

responsible Technical Director, CECER-LL. The USACERL technical editor was

William J. Wolfe, Technical Resources.

COL James A. Walter is Commander and Dr. Michael J. O'Connor is Director of

USACERL.
USACFRL TR-98/27

Contents

SF298 1

Foreword 2

1 Introduction 5

Background 5

Objective 5

Approach 6

Mode of Technology Transfer 6

2 Microphone Systems 7

The Parabolic Reflector 8

The Flat Reflector Microphone 12

End-Fire Microphones 14

3 Conclusion 16

References 17

Distribution
USACERL TR-98/27

1 Introduction

Background

For observing or recording of sound from a discrete direction, particularly in the

outdoor environment, it is appropriate to use a microphone system that receives

preferentially from that direction, to reduce the influence of extraneous noise

from other directions and to realize a gain in the power received from the desired

source. A widely-used technique for realizing suitable directional receiving

characteristics has been to use the parabolic reflector, a rigid structure, generally

a paraboloid of revolution, which concentrates the incoming sound power at its

focal point where an electroacoustical transducer (microphone element) is

located. For portable use in the field, the reflector must be of a convenient size

to be carried or deployed, generally not much more than a meter in diameter. At

this size, for sounds of frequencies that are efficiently propagated in the

terrestrial atmosphere and are discernible to the human ear, the reflector is

between approximately 0.5 and 30 wavelengths in diameter. In this range of

dimensions the classical high-frequency (geometrical optics) and low-frequency

(small-particle scattering) approximate methods of computation of the diffraction

characteristics of material objects do not apply. However, with the availability of

high speed computers with extremely large memory capacity, computational

methods based on the classical Helmholtz integral equation formulation of the

radiation problem have been developed using discrete-element approximations

that can be made as precise as desired (Harrington 1993; Seybert et al. 1986; Jin

1993). Thus it is now possible to solve the mid-frequency diffraction problem

with great accuracy, and to investigate small reflectors in considerable detail.

Objective

The objective of this study was to investigate the feasibility of using reflector

microphones for field use as an effective and inexpensive alternative to linearray microphones.

Preceding Page Blank
USACERL TR 98/27

Approach

1. For this study, it was assumed that a hand-portable directional microphone

system is desired for recording or observing outdoor sounds, for example, bird

song, in the frequency range from, say, 200 to 5000 Hertz.

2. Three general techniques are examined: (a) the parabolic reflector, (b) the

endfire transducer array or "shotgun" microphone, and (3) a novel system

involving a plane reflector, the "baffle microphone."

3. Consideration was given to the way in which the performance of each ofthese

systems varies with frequency, size of structure and, in the case of the

parabola, with focal length.

4. Finally, attributes common to all three systems are discussed and conclusions

are drawn regarding the feasibility ofreflector microphones as alternatives to

line-array microphones for field use.

Mode of Technology Transfer

It is anticipated that the results of this research will be used to design

equipment configurations for studies that require use of outdoor microphones or

microphone arrays, e.g., in noise abatement research.
USACERL TR-98/27

2 Microphone Systems

The directional attributes of a microphone system depend on the geometrical

shape of its structure, its dimensions measured in wavelengths, and the

disposition of its transducer or transducers. The wavelength depends on the

frequency ofthe sound, in the relation:

c = vX, Eq 1

where:

c = the speed ofsound in air, here assumed to be 344 m/second

v = the frequency in Hertz

X = the wavelength in meters.

A transducer is a device that converts the sound pressure or velocity in air to a

voltage or current to drive an amplifier, recorder, or other output device. (The

technical aspects oftransducers are not addressed here.)

The directional characteristics are displayed as diagrams in polar coordinates,

in which the relative sensitivity of the microphone to the sound amplitude is

plotted as the radius, as a function of the angle with respect to the axis of the

system. Here, the radius of the plot may be either linear in amplitude (sound

pressure) or logarithmic in sound intensity. In the latter case, the unit used is

the decibel (dB). In the computations, it is assumed that the sound emanates

from a point source at a great distance from the microphone. Each diagram

applies only to a single frequency. This diagram gives a graphic conception of

the reception "beam" of the microphone. Note that, in addition to the central

lobe or "main beam" of the directional pattern, there are usually a number of

"minor lobes" of lower strength. It is possible that, in recording sound from a

source of unknown position, the sound may be received in one of these minor

lobes, giving an entirely erroneous perception of both the direction and the

strength of the source.
USACERL TR 98/27

The Parabolic Reflector

Parabolic reflectors are widely used in optical and radio engineering to provide

directional characteristics in such diverse applications as automobile headlights,

radar antennas, astronomical telescopes, and microwave communications. In all

these cases, these reflectors are very large in terms of wavelengths to provide a

high degree of directivity. Under this condition, the directional characteristics of

the device can be calculated with reasonable accuracy by the classical methods

of physical optics (Born and Wolf 1980), assuming specular reflection at the

surface and certain approximations relating to the large size ofthe reflector.

In acoustics in the atmosphere, it is unusual to use a parabolic reflector that is

large in terms of wavelengths, and that therefore possesses a narrow reception

beam. Winds and spatial variations of temperature produce refraction, making

the direction of arrival ofthe sound waves unpredictable and variable with time.

Especially for portable use, it is preferable to use a small reflector with a

relatively broad reception beam. A typical reflector sold for field use might be

0.5 to 1 m in diameter. In this size range, for the frequencies under discussion,

the computation methods used extensively in radio and optical technology do not

apply. Wahlstrom (1985) provides a practical discussion of this phenomenon

that is useful for many purposes. However, to examine the details of the

diffraction pattern of a small reflector, one must use a more precise computational technique.

Theoretically, the Method of Moments can be applied, with any degree of

precision, to diffraction and scattering by arbitrarily-shaped structures. For this

study, it was economical and convenient to use axial symmetry to reduce the

computational burden. A program has been written (Lee 1996) for this purpose.

A curve ofthe form:

y + 2a = -Jx2 + y

2 Eq 2

is assumed, in which the detector (transducer) is at the focal point at the origin

and the surface is generated by rotation of the curve about the y-axis. The

distance a is the focal length. For each case, the source of sound is assumed to

be 1000 wavelengths from the origin and the sound pressure is computed at the

focal point. It is convenient to dimension the problem in terms of wavelength;

then the results for a given case can be scaled to any frequency. For example, a

source distance of 1000 wavelengths the reception pattern is essentially

independent ofthe distance; this is the so-called "far field" situation.
USACERLTR-98/27

Figure 1 shows reception beams for a parabolic microphone one wavelength in

diameter, which would be 50 cm at a frequency of 688 Hz. Results are given for

three different focal lengths, showing the critical nature of this parameter.

Results are normalized so that the on-axis gain for each case is unity. The

paraboloid with focal length 0.2 wavelength clearly has the most desirable

pattern, with the smallest rearward lobe. The "front-to-back ratio" for this case

is 3.9, or 11.8 dB. (The power ratio is proportional to the square of the

amplitude ratio.) For a shallow paraboloid (focal length 0.5 wavelength) the

ratio is only 1.6, or 4.2 dB. The deepest paraboloid shown, focal length 0.15

wavelength, shows a distorted pattern with a lobe at about 120 degrees, 8.8 dB

lower than the main lobe. The optimum focal length depends on the diameter in

wavelengths, so a compromise is necessary for a system that is to accommodate

a range of frequencies. For most applications, the highest front-to-back ratio is

preferable to discriminate against sounds from unwanted directions. Note that

all the patterns have minor lobes in addition to the main lobe, indicating some

sensitivity to directions other than the desired one.

Figure 2 shows the performance of a paraboloid 47.6 cm in diameter with a focal

length 30.3 cm, for frequencies of 1, 2, and 3 kilohertz. Higher frequencies

produce narrower beams. A rough rule of thumb applying to reflectors two or

more wavelengths in diameter is that the amplitude beamwidth is

approximately 60 degrees divided by the diameter in wavelengths. Here

"beamwidth" is defined as the angle between the directions at which the gain is

0.707 times that on axis. Minor lobe structure is highly frequency dependent.

Figure 3 shows how the gain of a parabolic microphone depends on the

frequency, for three different directions within the main beam. Note that a

sound signal containing several different frequency components will not be

faithfully represented in the output of the microphone, even if the reflector is

accurately aimed at the source. For the situation in which the microphone has

been mis-aimed, the lack of fidelity is more pronounced. As the frequency

increases, the beam narrowing accentuates the loss of gain and the increasingly

complex minor lobe structure causes strong fluctuations in gain. Although

Figure 3 is specific to its particular reflector parameters, such considerations

apply in general to all reflector microphones. Clearly, one cannot rely on

directional microphones to yield accurate spectra of sound sources except

possibly under rigidly controlled laboratory conditions. (See also Pye 1993).
10 USACERL TR 98/27

Figure 1. Reception patterns of paraboloidal microphone, one wavelength in diameter,

showing effect of focal length. In each case, the transducer is at focal point.

Figure 2. Reception patterns of a paraboloidal microphone, 47.6 cm in diameter,

showing the effect of frequency on the pattern. Focal length is 30.3 cm; transducer at

focal point.
USACERL TR-98/27 11

es

s

30

20

10

-10

-20

1.8 Meter Paraboloid, Focal Length 8.4 Meters

Cain us Frequency in uarious directions.

0.5 1 1.5

Frequency in KHz.

2.5

Figure 3. Variation of gain with frequency for a paraboloidal reflector 1.0 meter in

diameter, focal length 40 cm, at various directions within the main beam: on the

principal axis, 15 degrees off axis, and 25 degrees off axis.

Figure 4. Reception patterns of flat reflector microphones, square and circular in

shape and 0.89 wavelength in dimension. "Circular" pattern was computed; "Square"

pattern was measured (after Benson et al., 1994).
12 USACERL TR 98/27

The Flat Reflector Microphone

Structures small in terms of wavelength do not scatter waves in narrow beams.

By a well established reciprocity principle (Strutt 1877), it follows that a small

microphone structure cannot have a highly directional reception pattern. From

this it can be argued that, for a sufficiently small reflector, the specific shape of

the reflector has only a small influence on the reception pattern. Thus, a small

parabolic reflector may not be the most economical or convenient choice for field

use, depending on the circumstances. Use of a flat reflector has been

investigated as an easily-constructed means of achieving moderate directivity

(Benson et al. 1994). The logical location for the transducer in such an

arrangement would be directly on the reflector surface, as the sound pressure at

that point is double the pressure in the incident wave. Figure 4 shows the

computed sound pressure reception pattern of a flat circular disk 0.889

wavelength in diameter with a transducer on the surface at the center. Also

shown is the measured pattern of a square, flat panel of the same dimension.

This measured pattern is almost identical to the pattern computed by the "

" Method of Moments (Boag 1994; Lee 1996), confirming the accuracy of the

computational method.

Figure 5 shows the effect of frequency on the reception pattern of the circular

disk, here assumed to be 16 cm in diameter. Ifthe reflectors of Figure 4 are also

assumed to be 16 cm in dimension, their patterns are those obtaining at 1911

Hz. This simple device gives useful directivity and reasonable pattern shapes,

though at the lowest frequency, the scalloping of the principle lobe amounts to

7.0 dB. For the experimental measurements the reflectors were sawed from 1.3-

cm thick particleboard and the transducers were very inexpensive electret

microphone elements fastened directly to the surface.

Figure 6 shows a flat-reflector microphone designed for the frequency band 10 to

40 Hz. Such low frequencies are characteristic of the sounds produced by

explosions, and the device illustrated was constructed to monitor the

environmental noise of a military artillery training facility. The dimensions of

the reflector are 4.5 x 9.0 m, and the microphone is mounted at ground level

immediately in front of the surface. The system, including the ground surface,

which is assumed impermeable at these frequencies, is equivalent (with respect

to its reception pattern) to a 9.0 m square reflector in unbounded air.
USACERL TR-98/27 13

Figure 5. Reception patterns of a flat disk microphone, 16 cm in diameter, showing

effect of frequency. Transducer is on the surface at the center of the disk.

Figure 6. A flat reflector microphone baffle for low frequencies (10 to 40 Hz).
14 USACERL TR 98/27

End-Fire Microphones

The end-fire microphone or "shotgun" microphone, a popular device for portable

use, is briefly discussed here for comparison with reflector microphones. In its

canonical form, it consists of an array of two or more transducers along an axis,

some or all of which are associated with phase-shift or time-delay arrangements

to optimize the response in the axial direction (Olson 1957; 1972). Any end-fire

array has an inherently broader reception pattern than a reflector microphone of

the same linear dimension. The reception pattern of an array is easily analyzed.

Figure 7 (solid curve) shows the pattern of an array consisting oftwo transducers

separated by one-fourth wavelength with an appropriate acoustical time delay in

the input to one of them to optimize the response in the axial direction. Here,

the outputs ofthe transducers are assumed to be weighted equally in amplitude.

At a frequency of 500 Hz, the length of the device would be 17.25 cm. This

microphone would be useful in many situations, since it is compact, easily

portable, and would not require precise aiming. At twice this frequency, the

same device would have a different pattern, as shown in the dashed curve in

Figure 7.

An array of four transducers produces the reception patterns shown in Figure 8.

For the dotted curve, the separation between transducers is 0.2 wavelength; for

the dashed curve, 0.3 wavelength; and for the solid curve, 0.75 wavelength.

Thus, it is apparent that, in the linear-array or "shotgun" microphone, it is

important that the spacing between elements be less than 0.5 wavelength, at the

highest frequency to be observed, if a single-lobed pattern is to be achieved.

In practice, the shotgun microphone may be constructed of a number of tubes

feeding a common transducer, or of a single tube perforated at intervals to admit

pressure components at successive points along the tube, which in turn feeds a

transducer at its end. These arrangements are artfully designed to produce

useful patterns across a broad range of frequencies, and even to optimize the

irregular amplitude response with frequency that is basically inherent with any

(mis-aimed) directional microphone. However, the engineering effort required to

achieve these attributes results in high cost for the manufactured item.
USACERL TR-98/27 15

Figure 7. Reception patterns of an array of two transducers. Dashed curve: separation

one-fourth wavelength; solid curve: one-half wavelength. In each case a time delay

corresponding to the separation is inserted into the signal path of one transducer.

Figure 8. Reception patterns of a four-element array, at frequencies corresponding to

inter-element spacings of 0.2 wavelength (dotted), 0.3 wavelength (dashed), and 0.75

wavelength (solid).
16 USACERL TR 98/27

3 Conclusion

Reflector microphones may provide inexpensive and effective alternatives to linearray microphones for field use; however it should be appreciated that all

directional microphones have definite limitations with respect to fidelity of sound

reproduction. The line-array (shotgun) microphone can be engineered to reduce

its frequency dependent gain and pattern variations, but at substantial increase

in cost.

The physical environment is also a factor in microphone performance. Wind

noise is troublesome although it can be mitigated somewhat by use of

windscreens. Nearby obstacles such as the ground, buildings, persons, etc.,

especially in ad hoc situations, also produce essentially unpredictable variations

in gain and pattern. This study concludes that it is not productive to seek the

ultimate in performance from a microphone system for use in the field, and that

these limitations must be considered in interpreting field data.
USACERL TR-98/27 17

References

Benson, J.W., Y.L. Li, and G.W. Swenson, Jr., "A Baffle-Type Directional Microphone," J. Acoust.

Soc. Am., vol 5 (1994), pp 2536-2538.

Boag, Alona, Unpublished notes (1994).

Born, M., and E. Wolf, Principles OfOptics, 6

th ed. (Pergamon Press, Oxford, 1980).

Harrington, R.F., Field Computation by Moment Methods (IEEE Press, Piscataway, NJ, 1993).

Lee, Soon-Young, Diffraction of Sound by Objects in the One-Wavelength Size Range, Master's

Thesis (Dept. ofElectrical Engineering, University of Illinois at Urbana-Champaign, 1996).

Olson, H.F., Acoustical Engineering, 2

nd ed. (D. Van Nostrand Co., Princeton, NJ, 1957).

Olson, H.E, Modern Sound Reproduction (Van Nostrand Reinhold, New York 1972).

Pye, J.D., "Is Fidelity Futile? The "True" Signal Is Illusory, Especially With Ultrasound,"

Bioacoustics, vol 4, pp 271-286 (1993).

Strutt, J.W (Lord Rayleigh) The Theory of Sound (Macmillan, London, 1877; Dover Edition, New

York, 1945).

Swenson, G.W, Jr., Principles ofModern Acoustics (D. Van Nostrand Co., New York, 1956).

Wahlström, S., "The Parabolic Reflector as an Acoustical Amplifier," J. Audio Eng. Soc, vol 33

(1985), pp 419-429.
18 USACERL TR 98/27

USACERL DISTRIBUTION

Chief of Engineers

ATTN: CEHEC-IM-LH (2)

ATTN: CEHEC-IM-LP (2)

ATTN: CEMP

ATTN: CEMP-CE

ATTN: CEMP-EA (2)

ATTN: CEMP-ZM

ATTN: CERD-L

ATTN: CERD-M (2)

ATTN: CERD-ZA

Commander FORSCOM

ATTN: FCEN-RDF 30330-6000

CEWES 39180

ATTN: Library

CECRL 03755

ATTN: Library

US Army ARDEC 07806

ATTN: SMCAR-ISE

Engr Societies Library 10017

ATTN: Acquisitions

US Army Environmental Center

ATTN

ATTN

ATTN

ATTN

SFTM-AEC-NR21010

SFIM-AEC-CR64152

SFIM-AEC-SR 30335-6801

AFIM-AEC-WR 80022-2108

ACS(IM) 22060

ATTN: DATM-FDP National Guard Bureau 20310

ATTN: NGB-ARI

CECPW 22310-3862

ATTN: CECPW-E US Military Academy 10996

ATTN: CECPW-FT ATTN: MAEN-A

ATTN: CECPW-ZC ATTN: Facilities Engineer

HQ USAREUR & 7th Army

ATTN: AEAEN-EH Naval Facilities Engr Command

ATTN: Unit 29351 ATTN: Facilities Engr Comrr

Code 20YAZ (2)

US Military Academy

ATTN: MAEN-A

ATTN: Civil Div Director

US Army CHPPM

ATTN: MCHB-DC-EEN (5)

ATTN: Dept of Geo & Env Engr

US Gov't Printing Office 20401

ATTN: Rec Sec;Deposit Sec (2)

Nat'l Institute of Standards & Tech

ATTN; Library 20899

Defense Construction Supply Center

ATTN: DCSC-WI 43216-500

Defense Tech Info Center 22060-6218

ATTN: DTIC-0 (2)

47

3/98