The data collected at a pressure of one atmosphere for the different temperatures and relative humidities of the air-water vapor mixtures is summarized. The dew point hygrometer used in these measurements did not give reliable results for dew points much above the ambient room temperature. For this reason measurements were not attempted at the higher temperatures and humidities. Viscous wall losses in the resonant tube at 0 C so dominate the molecular relaxation of nitrogen, in the air-water vapor mixture, that reliable data could not be obtained using the free decay method in a resonant tube at one atmosphere. In an effort to obtain viable data at these temperatures, measurements were performed at a pressure of 10 atmospheres. Since the molecular relaxation peak is proportional to the pressure and the viscous losses are proportional to the inverse square root of the pressure the peak height should be measurable at the higher pressure. The tradeoff here is that at 10 atmospheres; the highest relative humidity attainable is 10 percent. The data collected at 10 atmospheres is also summarized

Becher, J.

Meredith, R. W.

English

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DEPARTMENT OF PHYSICS
SCHOOL OF SCIENCES AND HEALTH PROFESSIONS
OLD DOMINION UNIVERSITY
NORFOLK, VIRGINIA
TECHNICAL REPORT PTR-83-2
PROPAGATION OF SOUND THROUGH THE EARTH'S
LL
	
ATMOSPHERE
U
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	 By
w	 Roger W. Meredith
w	 and
Jacob Lacher, Principal Investigator
Final Report
For the period July 1, 1982 to May 31, 1983
Prepared for the
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia 23665
Unde r
Research Grant NAG1-234
Harlan K. Holmes, Technical Monitor
Technology Utilization and Application Division
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DEPARTMENT OF PHYS ICS
SCHOOL OF SCIENCES AND HEALTH PROFESSIONS
OLD DOMINION UNIVERSITY
NORFOLK, VIRGINIA
TECHNICAL REPORT PTR-83-2
PROPAGATION OF SOUND THROUGH THE EARTH'S
ATMOSPHERE
By
Roger W. Meredith
and
Jacob Becher, Principal Investigator
Final Report
For the period July 1, 1982 to May 31, 1983
Prepared for the
National Aeronautics and Space Administration
Langley Research Center
Hampton, Virginia 23665
Under
Research Grant NAG1-234
Harlan K. Holmes, Technical Monitor
Technology Utilization and Application Division
Submitted by the
Old Dominion University Research Foundation
P.O. Box 6369
Norfolk, Virginia 23508 -0369
0:sl9d
March 1983
PROPAGATION OF SOUND THROUGH THE EARTH'S ATMOSPHERE
By
Roger W. Meredith l
 and Jacob Becher2
SUMMARY
This report summarizes the work performed under research grant NAG1-
234, during the period from June 1, 1982 to May 31, 1983, concerning the
Measurement of Sound Absorption in Air.
DATA ACQUISITION
Table 1 summarizes the data collected at a pressure of one atmosphere
for the different temperatures and relative humidities of the air-water
vapor mixtures of this study. The dew point hygrometer used in these meas-
urements did not give reliable results for dew points much above the ambient
room temperature. For this reason measurements were not attempted at the
higher temperatures and humidities.
Viscous wall losses in the resonant tube at 0° C so dominate the molec-
ular r P J.axation of nitrogen, in the air-water vapor mixture, that reliable
data could not be obtained using the free decay method in a resonant tube at
one atmosphere. In an effort to obtain viable data at these temperatures,
measurements were performed at a pressure of 10 atmospheres. Since the
molecular relaxation peak is proportional to the pressure and the viscous
losses are proporticnal to the inverse square root of the pressure the peak
height should be measurable at the higher pressure. The tradeoff here is
that at 10 atmospheres the highest relative humidity attainable is 10
percent. Table 2 summarizes the data collected at 10 atmospheres.
l Research Associate, Old Dominion University Research Foundation., P.O. Box
6369, Norfolk, Virginia 23508.
2 Associate Professor, Department of Physics, Old Dominion University,
Norfolk, Virginia 23508.
Data collected in air-water vapor mixtures at one atmosphere.
RELATIVE HUMIDITY %
Temperature °C
10°C	 20°C	 30°C	 40°C	 50°C
Table 2. Data collected in air-water vapor mixtures at 10 atm
RELATIVE HUMIDITY
Temperature °C
0°C
	 10'C
3.25%	 1.25%
5.28%	 4.76%
3
1
DATA EVALUATION
For the low frequency, straight line decay curves, the absorption is
computed from the slope determined by the least squares best fit to a third
degree polynomial. For the higher frequency, stair step decay curves, the
absorption is computed from the average step depth.
The software and documentation has been completed and the data is cur-
rently being evaluated at NASA/Langley. Appendix A is a reprint of the
manuscript describing the evaluation and acquisition in more detail.
Y{
1
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APPENDIX A
DIGITAL DATA ACQUISITION SYSTEM FOR MEASURING
THE FREE DECAY OF ACOUSTICAL STANDING WAVES
IN A RESONANT TUBE
Roger W. Meredith
Old Dominion University Research Foundation
P.O. Box 6369
Norfolk, Virginia 23508
Allan J. Zuckerwar
NASA/Langley Research Center
M/S 238
Hampton, Virginia 23655
Presented at the 104th Meeting of the
Acoustical Society of America
November 1982
INTRODUCTION
The widespread availability of powerful low-cost microcomputers in
recent years has led to their increasing utilization in the control,
acquisition, and processing of experimental data obtained in the laboratory.
Such a system based on an 8-bit Apple II microcomputer has been designed to
achieve on-line evaluation of sound absorption measurements in gases. The
measurements are conducted in a resonant tube, whereby an acoustical stand-
ing wave is excited in the tube, the excitation removed, and the sound
absorption evaluated from the free decay envelope. The response of a micro-
phone to the sound pressure at one end of the tube is the source of the
analog data. Following a brief review of the analog instrumentation, this
paper will describe the digital hardware, software, and system performance.
Some of the peculiarities of the free decay of standing waves in a resonant
tube will be discussed, and measurements of sound absorption in air will be
presented as an example. 1h e digital data acquisition system can be used to
evaluate the damping of any resonant system in which it is feasible to use
the method of free decay.
I. INSTRUMENTATION AND ACQUISITION SYSTEM
A block diagram of the analog instrumentation and the digital data
acquisition systan is shown in figure 1. A detailed description of the
resonant tube and operating procedure has been given previously.' The exci-
tation signal originates at the frequency synthesizer and passes through a
relay and a power amp. ifier to a vibration exciter, which displaces a rigid
piston inside the tube. The acoustical signal is received by a high-sensi-
tivity piezoelectric q uartz microphone, and amplified and filtered by the
lock-in amplifier. The synthesizer provides the reference signal to the
lock-in. The do output of the lock-in is applied logarithmically to an
analog-to-digital converter. Due to the exponential nature of the free
decay envelope, the logarithmic converter is employed to distribute the
digital sampling intervals equally per dB increment over the full range of
the free decay. Thus, a given voltage drop corresponds uniformly to the
same number of decibels independently of voltage level.
6
. 0Q
The 12-bit A/D converter provides a resolution of 1:4096 or 0.024%. It
is a CMS level device and requires the non-inverting hex buffers to make
the A/D output TTL-compatible. The analog instrumentation, on the other
hand, has a dynamic range of 40 dB.
After the acoustic standing wave is set up in the tube, a keyboard
command calls a machine language program that controls the A/D converter.
The timer is used to increase the start convert pulses to levels sufficient
to trigger the A/D. On the 16th start convert pulse the counter causes the
relay to open, shutting off the excitation, and allowing the remaining 240
samples to monitor the free decay. As a result, the reference level of the
decay is well defined by the first 16 samples, and the sample number at the
start of the decay is exactly known. 7h e reset switch, which de- energizes
the relay, and the on/off switch together permit manual control of the
excitation.
II. PROGRAM CONTROL
The digital data acquisition is accomplished by a combination of
machine language and BASIC programming. A flow chart for controlling the
A/D converter and for acquiring the digital data is shown in figure 2.
After initialization of the peripheral interface adapter, the computer sends
the first start-convert pulse to the !,/D onverter. The duration of the
timing loop--an input parameter assigned during initialization--affords
software control of the digital sampling rate SR. Thus, the time between
start convert pulses, i.e. the digital sampling period, can be varied from 2
milliseconds to 10 seconds. The results of the analog-to-digital conversion
are stored in hexadecimal format in successive memory addresses in the
computer. The process repeats itself until the 256th conversion, at which
time the program returns to BASIC. The digital data is converted to decimal
format by means of the PEEK command and stored in an array.
III. DECAY CURVE EVALUATION
A typical low frequency (9.592 Hz) decay curve appears in figure 3.
The oscillographic trace shows the microphone response to the sound pressure
level as the standing wave decays in time. The lower curve is the digital
reproduction of the logarithmic envelope. The sound absorption coefficient
7
is determined from the decav slope, which is computed from a linear regres-
sion analysis. Typically, 180 successive digital samples are used in the
numerical evaluation. Correlation coefficients for these decay curves are
usually greater than 0.9999.
Figure 4 shows an example of a high frequency (992 Hs) decay curve.
Again, the oscillographic trace is the analog signal from the microphone and
the lower plot is the digital reproduction of the logarithmic envelope. The
stairstep phenomenon is inherent in the free decay of standing waves in
continuous media. When the excitation is removed each cycle of the standing
wave initially has the same amplitude as the others and undergoes the same
attenuation while it travels from one endplate to the other. Thus, the
microphone sees an interval of constant amplitude equal to the duration of
the wave train, followed by a sudden amplitude drop corresponding to the
round trip atteruation. The sound absorption coefficient is determined from
the absorption Fer unit round trip time. For each interval of constant
amplitude the average strap depth with respect to the known reference level
is computed to find the average absorption per round trip time. It should
be noted that if the analog input saturates the A/D converter, an error will
result in the value of the reference level.
ACKNOWLEDGMENTS
The authors thank the following individuals at Langley Research Center:
J.W. Lynch for designing the logarithmic converter, and H.K. Holmes and D.
Solloway for assistance in designing the control box.
•
8
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REFERENCE
1. A.J. Zuckerwr and W.A. Griffin, J. Acoust. Soc. Am. 68, 218-226
(1980) .
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13


Propagation of sound through the Earth's atmosphere

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