Progress is reported in the development of hardware and software for an experiment to detect and measure sound transmission through air

Badavi, F.

Becher, J.

Meredith, R.

English

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By
Roger Meredith
	
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Francis Badavi
and
Jacob Becher, Principal Investigator
Progress Report
For the period January 1 - June 30, 1981
Prepared for the
National Aeronautics and Space Adaunistrato.,
Langley Research Center
Bmpton, Virginia
Under
Research Grant NAG1-142
Harlan K. Holmes, Technical Monitor
Instrment Research Division
	
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THROUGH THE EAR'TH'S A'1MCSFF'EHE Progress
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August 1981
DEPARTMENT OF PHYSICS
SCHOOL OF SCIENCES AND HEALTH PROFESSIONS
OLD DOMINION UNIVERSITY
NORFOLK, VIRGINIA
Technical Report PTR-81-12
PROPAGATION OF SOUND THROUGH THE EARTH'S
ATMOSPHERE
I. MWASUREMENT OF SOUND ABSORPTION IN THE
`	 AIR AND
E	 II. DETECTION OF INFRASOUND GENERATED BY
f	 CLEAR AIR TURBULENCE
By
Roger Meredith
Francis Badavi
and
Jacob Becher, Principal Investigator
Progress Report
For the period January 1 - June 30, 1981
Prepared for the
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia 23665
Under
Research Grant NAG1-142
Harlan K. Holmes, Technical Monitor
Instrument Research Division
Submitted by the
nl9
	
	 Old Dominion University Research Foundation
P.O. Box 6369
Norfolk, Virginia 23508-0369
August 1981
P82-13829
PROPAGATION OF SOUND THROUGH THE EARTH'S ATMOSPHERE
I. MEASUREMENT OF SOUND ABSORPTION IN AIR AND
II. DETECTION OF INFRASOUND GENERATED BY CLEAR AIR TURBULENCE
By
Roger Meredith l , Francis Badavi2 , and Jacob Becher3
SUMMARY
This report summarizes work performed under research grant NAG1-142
since January 1, 1981 in the following areas: (I) Measurement of Sound
Absorption in Air and (II) Detection of Infrasound Generated by Clear Air
Turbulence.
I. MEASUREMENT OF SOUND ABSORPTION IN AIR
Cooling System for the Resonant Tube
The large temperature gradient in each tube section resulting from the
liquid nitrogen coolant (see ref. 1) necessitated a design modification to
the cooling system. The new design incorporates a timer and four solenoid
valves so that the liquid nitrogen coolant flow can be reversed periodi-
cally. The necessary parts were ordered in mid-January and arrived in early
July. When the installation of the valves and timer are completed, the
system will be retested.
Automatic Data-Processing System
The automatic data-processing system centers around the Apple II com-
puter and a 12-bit Datel Intersil model ADC-HC12B analog-to-digital
l Research Associate, Old Dominion University Research Foundation, P.O. Box
'	 6369, Norfolk, VirginiP. 23508-0369
2 Graduate Research Assistant, Department of Physics, Old Dominion Univer-
sity, Norfolk, Virginia 23508
3 Associate Professor, Department of Physics, Old Dominion University,
Norfolk, Virginia 235080
converter. The hardware and software for controlling the converter and
conversion rate have been completed, and the system is operational. a duty
cycle control circuit has been designed and implemen t ed so that or. the six
-teeath conversion a relay shuts off the vibration exciter used to generate
the sound wave. Thus the starting point of each decay curve is exactly
known. This information is necessary for-evaluating the digital decay
information.
It has been found that the slew rate for the digital system is much
faster than that of the old analog system and that the digital decay data is
of better quality than the same analog signal recorded on the B & K 2305
chart recorder. Most of the decr;. turves resemble a staircase where each
stairstep corresponds to one ac _.itical round-trip time in the tube (ref.
2). This type of decay curve was previously observed only at the higher
f requencies.
The software development has been completed in two stages: (1) data
acquisition and (2) digital decay data evaluation. The data-acquisition
program controls the conversion rate, depending upon the decay time constant
from 6 to 500 Hz, via input from the operator. The decay curve is sampled
252 times at the specified rate to compose the digital decay data. The
digital data is displayed on the computer monitor and the operator decides
whether to reset the sampling rate and repeat the sampling or save the dig-
ital decay data. This is done for all frequencies of the measurement set.
The digital data is saved on magnetic floppy disks which can hold 105 fre-
quencies.
Once the data is stored on disk, it can be processed at any time usins
the digital decay evaluation program. This software reads the data from the
disk and plots the decay and the first derivative. The operator chooses the
sample points with the minimum value of the first derivative which determine
the experimental round-trip time. The average step depth, in decibels, is
found from points located an integer number of round trips away from a
starting point. This is repeated for several starting points centered
around sample number 16, the exact starting point of the decay. Based on
these averages, the attenuation in terms of decibels per round-trip time is
calculated and converted to nepers/wavelength.
2
rN82w13830
Software is being developed so that the computer can process a complete
measurement set (- 80 frequencies) without operator assistance. Background
data has been collected at 30' C and is being used to verify and modify the
aforementioned software. The bottled air supplied under contract to NASA
does not meet composition requirements for "standard" air, and consequently
special cylinders of standard air have been ordered.	
n
II. DETECTION OF INFRASOUND GENERATED BY CLEAR AIR
TURBULENCE (CAT)
Introduction
The experimental procedures for this experiment have been divided into
four phases, which are described below. This report focuses on phases 1 and
2.
(1) Detection of Atmospheric Infrasound
This will be accomplished through instrumentation developed under two
previous NASA grants (NGR 36-028-004 and NAS1-11707-29). This is a unified
acoustic data-acquisition system which is an AM carrier system consisting of
a converter signal, conditioning electronics, and pheripheral equipment.
This phase of the experiment also includes calibration and instrumentation
checkout and installation of a microphane array system.
Instrumentation checkout and calibration. --The unified system had to be
checked and calibrated prior to any field testing, and hence it was exten-
sively tested in the lab. The procedures for calibration are described in
the paragraphs which follow.
The distortio- measurements were done by using an HP Model 334A Distor-
tion Analyzer. The procedure was carried out for three different cable
lengths. The data for the distortion level and the system dynamic range is
given in table 1 (see ref. 3). The standard 4 percent distortion level
occurred at approximately 135 dB for all 4 microphone systems. A typical
distortion graph is shown in figure 1.
The amount of crosstalk of the zero-drives due to the leakage of elec-
tronic noise between differ-nt channels was measured by exciting one channel
with an acoustical signal of 1 kHz/120 dB and measuring the SPL of the
3
adjacent channels. In all cases it was decided the amount of crossta?.k was
virtually insignificant in the entire system.
A piston-type infrasonic calibrator, driven at an equivalent SPL of 9U
dB, was used to determine the frequency response of the system. Teble 2
gives the particular frequency at which the amplitude of the SPL fell below
the standard -3 dB over the entire operating frequency range of U.1 to lU
Hz. Figure 2 shows the frequency response of all 4 channels with a 457-m
(1500-ft) long cable as a means of signal transmission (ref. 4).
Installation of microphone array.--An area north of the Lunar Landing
Facility was selected to install the microphone array in the shape of a
244-m (800-ft) equilateral triangle with a microphone at the center sad one
at each apex. The data-acquisition system was installed in a mobile van
near the center microphone. Due to the local terrain, 457-m (1500-ft) long
cables were used to connect the microphones to the data-acquisition system.
(2) Signal Processing
This phase of the experiment includes
(a) design and construction of an A/D converter and development of
necessary software to be incorporated with an APPLE II computer as a means
of digital analysis of the data; and
(b) development of digital correlation techniques to extract the
direction of aa infrasonic source from the phase information contained in
the signal received in the microphone array.
An A/D converter has been procured and the proper software was written
to carry out the conversion. Due to the slow rate of conversion, all of the
software was written in BASIC language. At present, work is being carried
out to develop a multichannel correlation technique to be used in the near
future as a means of input-signal phase studies.
(3) Interpretation of Infrasonic Signature
A prime goal of the experiment is, through experiment, to differentiate
between. CAT and other source: of infrasouid.
4
(4) ',,)rroboration with Other Methods
Once patchys of U._' ire located in the atmoshere, corroboration can be
achieved through +.. sr. fligto;s of aircraft into the suspected region.
►'
4
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5
REFERENCES
1. Meredith, R.W.; and Becher, J.: Propagation of Sound Through the
Earth's Atmosphere: I. Measurement of Sound Ah&orption in the Air
and II. Measurement of Ground Impedance. Final Report, NASA Grant
NAG1-15, Jan. 1981.
2. Zuckerwar, Allan J.; and Griffin, W.A.: J. Acoust. Soc. Am., Vol. 68,
1980, pp. 218-226.
3. Zuckerwar, Allan J.; and Holmes, Harlau N.: A Verified Acquisition
System for Acoustic Data. NASA TND 8327, Mar. 1977, pp. 14-18.
4. Zuckerwar, Allan J.; and Shope, William W.: A Solid State Converter for
Measurement of Aircraft Noise and Sonic Boom. NASA grant NGR-36-028-
004, Nov. 30, 1972, pp. 17-21.
6
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Table 1. Syster distortion and dynamic range.
Cable Range Setting SPL at 4%
Mic	 Length of Zero-Drive Noise Floor Distortion Dynamic Range
Channel	 (ft)a Amplifier (dB)b,c (dB)d (dB)e
1	 1,000c 34 58 134 71
1,500 59 130 66
60 126 61
2	 1,000c	 34 57 132 70
1,500 58 130 67
59 126 62
3	 1,000c	 30 56 135 74
1,500 57 132 70
58 126 63
4	 1,000c	 32	 57	 136	 74
	
1,500	 58	 131	 68
59	 126	 62
al ft a 0.3048 m
bBandwidth of noise floor measurement: 22.4 Hz - 22.4 kHz.
c Reference SPL: 20 uPa.
dFrequency of distortion measurement = 1 kHz.
e 5 dB above noise floor to SPL at 4 percent distortion
7
Table 2. System cutoff frequency of -3 dB.
Microphone
Channel	 -3 dB point
1	 t - 0.5 H&
2	 c-0.2Hz
3	 t - 0.5  Hz
4	 t-0.2Hz
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Figure 1. Typical distortion curve of the microphone with 4 percent distortion
leve? at approximately 135 dB.
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Propagation of sound through the Earth's atmosphere

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