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Bands of noise were propagated over various ground surfaces and sound amplitudes were measured as a function of frequency of the sound source (40 Hz-2kHz), distance of propagation (5-300 m), and surface conditions. By treating the surface impedance as an adjustable parameter, the impedance as a function of frequency was determined from the measured sound amplitudes assuming spherical wave propagation in the vicinity of a locally reacting surface. The surface flow resistance was also experimentally determined in each case. In the frequency range 40 Hz-2 kHz, the results for three distinctly different surfaces suggest that the impedance can be computed from the specific flow resistance [C. I. Chessell, J. Acoust. Soc. Am. 62, 825 (1977)]. Surface vegetation appears to have little effect on the impedance. [ABSTRACT FROM AUTHOR]

The status of current research into the relations between thunder and the lightning process is examined. Nonlinear effects near the discharge strongly influence the pressure waveform and hence the nearfield acoustic spectrum. At distances where measurements are typically made, propagation effects strongly influence the shape of the recorded spectrum. Since most measurements do not reliably determine propagation distances or the physical characteristics of the propagation path, they are of little value in deducing the nearfield pressure waveform. Even with these problems, the gross features of thunder can be explained qualitatively and there is hope that future experiments will provide information necessary to correct for propagation effects making the study of thunder a useful tool for testing theoretical models of lightning. [ABSTRACT FROM AUTHOR]

Simultaneous measurements have been made of sound pressure above the ground and seismic velocity below the ground surface resulting from a source suspended in the air a variable distance from the surface. The ratio of seismic velocity to acoustic sound pressure has been determined; there are peaks in the ratio in the vicinity of 45 and 90 Hz depending on the height of the speaker. The source-receiver distance was 10, 30, and 60 m; the source height was varied between 1 and 10 m. The frequency of maximum acoustic coupling was found to agree well with theory; the first and third shear modes appear to be excited. Results for vertical, horizontal, and radial motion indicate the coupled seismic signal is greatest for vertical, next greatest for radial, and least for transverse, though the difference between radial and vertical displacement velocities was not great and depended on the speaker altitude. [ABSTRACT FROM AUTHOR]

As a band of noise propagates through the atmosphere, atmospheric absorption attenuates the higher frequencies in the band more rapidly than the lower frequencies. For large propagation distances, atmospheric absorption shapes the spectrum such that most of the acoustic energy is in the lower part of the band. When this occurs, the effective atmospheric absorption coefficient for this band of noise is less than when the energy is evenly distributed over the band. When a real filter is used to examine the received spectrum, the problem becomes even more complex since the filter does not cut off completely at the nominal lower frequency limit for the band. The equations governing the attenuation and spectrum analysis of noise under these conditions have been derived and a numerical integration process has been used to obtain quantitative results. It is shown that predicting observed values of atmospheric absorption loss for bands of noise involves two correction factors which are added to single-frequency absorption loss to define the effective attenuation for the band of noise. The first correction factor ΔI accounts for the difference between the attenuation predicted over a given path in a homogeneous atmosphere for a pure tone at the geometric center frequency of an ideal filter band, and the actual attenuation over the same path for the same ideal band of noise. The second correction factor ΔF accounts for the difference between the band level measured at one point with a practical filter with finite transmission outside the nominal filter pass band and the true band level measured at the same point with an ideal (rectangular) filter. Representative calculations show that the sum of these corrections become substantial when the product of distance, in km, and the square of frequency, in kHz, exceeds about 4 to 40 for octave and one-third octave band filters, respectively. Although the net effect of such corrections will generally be small for overall frequency weighted-noise levels (e.g., perceived noise level), the errors in diagnosing spectral detail in noise signatures of distant sources, such as aircraft, can be important. [ABSTRACT FROM AUTHOR]

High power drives for thermoacoustic refrigerators are being investigated by several groups. Theoretical calculations have been performed of the feasibility of parametrically driving a longitudinal resonance tube by uniformly modulating the temperature of the gas with a high-power laser. Parametric drives are promising since the drive mechanism is distributed over the entire volume of gas and because the response may become large before saturation occurs. Although it is demonstrated that laser drive is not a practical means of high-power drive for longitudinal resonators, much interesting physics has been considered in the analysis of the proposed system and is discussed. © 1996 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

The computational tools available for prediction of sound propagation through the atmosphere have increased dramatically during the past decade. The numerical techniques include analytical solutions for selected index of refraction profiles, ray trace techniques which include interaction with a complex impedance boundary, a Gaussian beam ray trace algorithm, and more sophisticated approximate solutions to the full wave equation; the fast field program (FFP) and the parabolic equation (PE) solutions. This large array of computational approaches raises questions concerning under what conditions the various approaches are reliable and concerns about possible errors in specific implementations. This paper presents comparisons of predictions from the several models assuming a complex impedance ground and four atmospheric conditions. For the cases studied, it was found that the FFP and PE algorithms agree to within numerical accuracy over the full range of conditions and agree with the analytical solutions where available. Comparisons to ray solutions define regimes where ray approaches can be used. There is no attempt to compare calculated transmission losses to measurements. [ABSTRACT FROM AUTHOR]

This Letter is an extension of an earlier Letter by Bass et al., ''Atmospheric absorption of sound: Update'' [J. Acoust. Soc. Am. 88, 2019-2021 (1990)]. Errors in a formula for saturation vapor pressure are corrected, and an alternative, much simpler formula is given. The role of atmospheric pressure is emphasized by giving formulas in which the absorption, frequency, and relative humidity are all scaled with respect to atmospheric pressure. Also presented are new, more readable and useful figures showing atmospheric absorption as a function of frequency, relative humidity, and atmospheric pressure. The new figures make it possible to estimate accurately the absorption at any value of atmospheric pressure. [ABSTRACT FROM AUTHOR]

The absorption of sound in air at frequencies from 4 to 100 kHz in 1/12 octave intervals, for temperatures from 255.4° K (0° F) to 310.9° K (100° F) in 5.5° K (10° F) intervals, and at 10% relative-humidity increments between 0% and saturation has been measured. The values of free-field absorption have been analyzed to determine the relaxation frequency of oxygen for each of the 92 combinations of temperature and relative humidity studied and the results are compared to an empirical expression. The relaxation frequencies of oxygen have been analyzed to determine the microscopic energy-transfer rates. [ABSTRACT FROM AUTHOR]

At audible frequencies, the vibrational relaxation of nitrogen contributes significantly to the absorption of sound in still air. The accurate measurement of the humidity dependence of the relaxation frequency of nitrogen, as yet, has eluded careful measurement due to the difficulty in measuring small absorption at low frequencies. Recently, equipment has been constructed for measuring sound absorption in air as a function of humidity over the frequency range from 4 to 100 kHz. In the experiment described here, the temperature and humidity range of this equipment has been extended so that it can be used to study the relaxation absorption in nitrogen at temperatures from 311° K to 418° K. The results indicate that, over this temperature range, the frequency of maximum absorption in moist nitrogen, fr,n, can be given by fr,n/P=260×h, Hz/atm, where h is the percent mole fraction of water, and P is the pressure in atm. To the accuracy of the measurements reported here, fr,n is independent of temperature over the range of temperatures at which the measurements were made. [ABSTRACT FROM AUTHOR]

Equipment had been constructed for measuring sound absorption in the frequency range from 4 to 100 kHz in a large tube 25.4 cm in diameter and 4.8-m long. The technique employs a large, moveable, solid-dielectric capacitance transducer that completely fills the tube cross section and generates pulses of plane waves. An identical transducer terminates the other end of the tube and serves as a microphone to detect and reflect the sound pulses. Measurements in argon, nitrogen, and air indicate that the attenuation of the sound pulses differs by less than 1% from values calculated for the zero-order mode for frequencies up to 44 times the cutoff frequency for the first ''nonplane'' mode. Above that frequency, the measured values are less than those predicted by theory by an amount that is approximately proportional to the wavelength to the -3.1 power. In a smaller tube (5.72-cm inside diameter) of similar construction, the high-frequency deviation from theory is absent. The equipment has been used to measure absorption in moist air at relative humidities from 0% to 100%, and at temperatures from 0° to 100° F. The results of these measurements will be reported in a later paper. [ABSTRACT FROM AUTHOR]

Experimental measurements of sound absorption in air, measurements of ultrasonic absorption in H2O/O2 mixtures at 500 °K reported here, and experimental measurements in mixtures of H2O with other impurities have been analyzed to determine the energy transfer rates in mixtures of O2 and H2O as a function of temperature. A set of energy transfer rate coefficients was found that give results consistent with all the experimental data examined and with vibrational energy transfer theory. The rotational relaxation time for H2O/O2 mixtures was determined from the 500 °K ultrasonic measurements. Subject Classification: [43]35.20, [43]35.35. [ABSTRACT FROM AUTHOR]

Ultrasonic absorption has been measured in mixtures of H2O with 15%, 35%, and 55% Ar and N2 at 500 °K. The classical absorption was subtracted from the measured values and the remainder was analyzed in terms of parallel vibrational and rotational relaxation. By extrapolating to 100% H2O, the rate of self deactivation was found to be (2.0 ± 0.4) × 108 sec-1 atm-1 for vibration and (8.3 ± 0.5) × 108 sec-1 atm-1 for rotation. Extrapolation to 0% H2O yields a rate of (4.0 ± 2.0) × 106 sec-1 atm-1 for H2O vibrational deexcitation by N2 or Ar. The isothermal-isometric rotational relaxation time for H2O in one atm of Ar was found to be (1.2 ± 0.2) × 10-9 sec. These results at 500 °K are compared with other work at higher and lower temperatures in order to form a basis for extrapolating to room temperature. Subject Classification: [43]35.20, [43]35.35. [ABSTRACT FROM AUTHOR]

Measurements of the sound absorption in O2/H2O mixtures have been made at 400° and 500°K. The results have been compared with measurements made by others at room temperature and analyzed by the Bauer, Shields, and Bass method to determine values and limits of errors for the rate constants. The v-t deexcitation rate in H2O/H2O collisions was found to vary from 1.0 to 0.75 × 109/sec-atm and the v-v rate between O2 and H2O was found to vary from 3.8 to 1.5 × 107/sec-atm as the temperature increased from 300° to 500°K. [ABSTRACT FROM AUTHOR]

A set of analytical expressions has been developed which will predict the absorption of sound in air at 20°C. The results agree within 3% of previous work in the frequency range of 100 Hz 1 MHz for all humidities. Below 100 Hz accuracy cannot be judged because of lack of knowledge about the vibration-vibration energy transfer between CO2 and O2. [ABSTRACT FROM AUTHOR]

By assuming that air is composed of four gases (i.e., nitrogen, oxygen, water vapor, and carbon dioxide) and applying energy transfer rates for the binary collisions inherent in such a system, absorption of sound in the atmosphere has been predicted. The calculated curves based upon 24 energy transfer mechanisms are compared with experimental data over the humidity range of 0-100% relative humidity. Agreement between theory and experiment is very good. By including classical absorption and rotational relaxation effects, the total atmospheric absorption is also predicted at 20°C. Calculations made for various concentrations of CO2 indicate that low CO2 levels (less than 0.1%) do not significantly affect absorption of audible sound at high humidities. At very low humidities, however, CO2 is an important factor. [ABSTRACT FROM AUTHOR]

Ultrasonic absorption has been measured in H2S in the temperature range 300°-683°K. These data were analyzed to obtain vibrational and rotational relaxation times. The vibrational collision numbers seem to increase with temperature from 300° to 473°K and then decrease with temperature. This behavior can be explained by the v-r theory of Shields and Burks. The rotational collision numbers compare favorably with thermal transpiration measurements. [ABSTRACT FROM AUTHOR]

Best current expressions for the vibrational relaxation times of oxygen and nitrogen in the atmosphere are used to compute total absorption. The resulting graphs of total absorption as a function of frequency for different humidities should be used in lieu of the graph published earlier by Evans et al. [J. Acoust. Soc. Am. 51, 1565-1575 (1972)]. [ABSTRACT FROM AUTHOR]

Zuckerwar and Griffin [J. Acoust. Soc. Am. 68, 218-226 (1980)] have recently used a resonance tube to extend measurements of sound absorption in N2/CO2 mixtures to much lower frequency to pressure ratios (f/p) than previously obtained. Their results, when compared with previous measurements at higher CO2 concentrations, confirm that the nitrogen relaxes by a series process involving two v-v and one v-t exchange. A value of 7.5×10+4±10% s-1 atm-1 for the rate of the reaction CO2(00 °1)+N2?CO2(0310)+N2+416 cm-1 is obtained. [ABSTRACT FROM AUTHOR]

The effect of vibrational relaxation processes on the rise times and overpressure decay of weak shock waves from explosions is investigated. Calculations have been performed which incorporate finite amplitude wave effects, spherical spreading, classical acoustic attenuation, and attenuation due to vibrational relaxation of oxygen and nitrogen. These calculations are compared to data and to the results of an empirically based calculation. [ABSTRACT FROM AUTHOR]

The attenuation of a 1 MHz ultrasonic signal has been measured in 78%N2/22%O2 at temperatures of 302 °K, 467 °K, and 689 °K and pressures from 1 atm to 1/60 atm. The measured attenuation was corrected for classical effects to obtain the attenuation due to rotational relaxation. In addition, the total absorption has been compared to computed values based on a linear combination of viscous, thermal conduction, diffusion, and relaxation losses to check the constants used for each of these terms. The rotational relaxation time of the mixture was found to be 7×10-10, 10×10-10, and 21×10-10 sec atm at 302 °K, 467 °K, and 689 °K, respectively. Subject Classification: 35.20, 35.35. [ABSTRACT FROM AUTHOR]

The effect of finite bandwidth filters on measured reverberation times of enclosures has been numerically simulated. The reverberation time measured with a real filter will differ significantly from that of a pure tone with a frequency at the geometric center of the band if air absorption is larger than wall absorption or if the wall attenuation is a very strong function of frequency. [ABSTRACT FROM AUTHOR]

Measurements of sound amplitude in the vicinity of a ground plane have been made as a function of frequency of the sound source (50-2000 Hz), distance of propagation (5-300 m), and surface conditions. By treating the impedance as an adjustable parameter, the surface impedance as a function of frequency was determined from the measured amplitudes using a theoretical treatment of a spherical wave in the vicinity of a locally reacting surface. The impedance measurements covered the frequency range 50-1000 Hz. In this frequency range, the results for three distinctly different surfaces suggest that the impedance can be computed from the specific flow resistance and that grass has little effect on the surface impedance except for decreasing the flow resistance due to the root structure. The results of the values of impedance calculated from sound measurements and flow resistance measurements will be compared, and the applicability of the single parameter model of Chessell to the data will be discussed. [Work supported by U.S. Army Research Office.] [ABSTRACT FROM AUTHOR]

A detailed review of the experimental data for the rotational collision number Zrot for air, O2 and N2 has been made to finalize a temperature correction for the absorption due to rotational relaxation for incorporation in a draft standard on air absorption being prepared by the Sl-57 Committee on Propagation. Subject Classification: [43]35.20, [43]35.35. [ABSTRACT FROM AUTHOR]

Absorption of sound in air has been measured for temperatures from 0° to 100°F, relative humidities from 0% to 100%, at frequencies from 4 to 100 kHz. Measurements were made by propagating pulses of plane waves down a large cylindrical tube. The results have been compared to the value predicted using a new standard for sound absorption in still air proposed by the S1-57 Working Group. [Work supported by NASA-Lewis Research Center.] [ABSTRACT FROM AUTHOR]

Previous measurements of sound absorption due to vibrational relaxation in CO2/D2S mixtures indicate a vibrational-vibrational (v-v) coupling between the bending mode of CO2 and the stretching modes of D2S. A theory by Tanczos was used to analyze the results, but this theory did not allow for quantum changes greater than two in either one of the colliding molecules. Theoretical curves were calculated, therefore, for a v-v coupling in which a two-for-one quantum exchange took place between the bending mode in CO2 and the stretching modes in D2S. The theory has now been extended to treat cases where the v-v coupling involves a three-for-one quantum exchange between modes. Using this theory, there is a slight improvement in the theoretical fit to the experimental absorption data on CO2/D2S mixtures. This is to be expected because of the near resonance between the 0310 level of CO2 and the 100 and 001 levels of D2S. [ABSTRACT FROM AUTHOR]

When using ultrasonic absorption to study molecular relaxation processes in gases, it has been the practice of our group and many other investigators to assume that the dependence of classical absorption on the frequency/pressure ratio is given (αλ)c1 = kf/ρV2, where k is a proportionality constant, f is the frequency, ρ is the density, and V is the velocity of sound. This assumption neglects the frequency dependence of the absorption due to thermal conductivity and results in a classical absorption that is too low at high f/p. This assumption can lead to serious error whenever classical absorption is an appreciable fraction of the total absorption. [ABSTRACT FROM AUTHOR]

The amplification of sound in a reacting gas mixture has been observed. Energy for the amplification was provided by a photo-induced, exothermic H2/Cl2 reaction. A sound wave traveling through the gas can be regarded as a modulation of temperature about some steady-state value. As most chemical reaction rates increase with temperature, the regions of higher temperature experience an increase in the rate of the H2/Cl2 reaction. Since the H2/Cl2 reaction is exothermic, this increase in reaction rate causes an additional increase in temperature. Conversely, in regions of low temperature, the H2/Cl2 reaction is slowed causing a relative decrease in temperature. Thus the initial acoustic signal is amplified. The measured gain was 1.8 at 2 kHz compared to a gain of 4 predicted by Ellis and Gilbert [J. Acoust. Soc. Am. 62, 245-249 (1977)]. The source of this difference has not been resolved. [Work supported by NSF.]. [ABSTRACT FROM AUTHOR]

Ultrasonic attenuation has been measured in neat UF6 and in mixtures of 5% and 10% UF6 in Ar and in N2 at 273 of K and 323 of K for f/P values in the range of 10 kHz/atm to 2 Hz/atm. These attenuation measurements were analyzed to determine the relaxation time for vibrationally excited UF6. Isothermal relaxation times were in the range 4-15 × 10-9 atm-s. Argon and N2 were found to be slightly less efficient collision partners (by factors of ∼1.5-3) than UF6 itself. [ABSTRACT FROM AUTHOR]

A new American National Standard to be issued shortly on calculation of the absorption of sound in the atmosphere (Proposed ANSI Standard S1.26) provides a specific method for computing absorption loss coefficients for pure tones and outlines general methods for evaluating absorption losses of filtered bands of noise. This paper considers this latter aspect in more detail and outlines practical considerations of the problem with emphasis on analysis of aircraft noise. Typical aircraft source noise spectra are used along with accurate models for absorption loss and filter response characteristics to predict absorption losses at large distances. It is shown that substantial differences begin to occur between actual band losses and values computed at the center frequency of a filter band when the product of distance, in km. and frequency, in kHz, exceeds about 5 to 15 for octave and one third octave bands respectively. However, frequency weighted overall noise levels are not nearly as sensitive to such errors. The paper also briefly considers the more difficult problem of synthesizing a true source spectrum from measured spectra at a distant receiver. In this case, errors in measured levels due to filter response characteristics require a trial and error approach to accurately compute source spectra. [ABSTRACT FROM AUTHOR]

The coupling of airborne sound into the ground has been measured as a function of frequency (100-500 Hz), sound source height (1-10 m), and propagation distance with a regularly spaced linear array of microphones and geophones. The results suggest that the largest seismic signal is due to energy coupled into the energy near the geophone locations, however, the ranges available would not allow us to rule out a seismic signal arriving much later than the primary which one might expect if a Rayleigh wave was generated near the source. Other experimental results will also be presented in an attempt to spark discussions which might help us to resolve the behavior of the observed coupling coefficient with frequency and angle of incidence. [Work supported by the Army Research Office and Waterways Experiment Station.] [ABSTRACT FROM AUTHOR]

Two series of measurements of sound attenuation by various ground covers have been completed. Surface impedance values have been determined for clay soil, sand, and prepared grass fields of various heights. These values were obtained for frequencies from 50 to 2000 Hz. In addition seismic sensors were used to measure the transmission properties of sound into clay soil as a function of the angle of incidence. Results of the impedance measurements are consistent to previous results [J. E. Piercy, T. F. W. Embleton, and L. C. Sutherland, J. Acoust. Soc. Am. 61, 1403-1419 (1977)] with the largest surface effects observed at low frequencies. [Work supported by ARO.] [ABSTRACT FROM AUTHOR]

The propagation of weak shock waves through the atmosphere has been modeled by a calculation incorporating finite amplitude effects, classical acoustic attenuation, and attenuation due to vibrational relaxation of nitrogen and oxygen. This calculation has been used to study the effect of varying atmospheric conditions on the overpressure decay, rise times, and spectral composition of weak shock waves. The necessity of including vibrational relaxation processes in calculations of shock wave parameters is demonstrated. [Work supported by Office of Naval Research, Atmospheric Sciences Division.] [ABSTRACT FROM AUTHOR]

Recent measurements of sound absorption in moist air at temperatures between 0°F and 100°F have been analyzed to extract that part of the absorption due to the vibrational relaxation of oxygen. The measurements extend to higher water-vapor concentrations than most previous measurements and show the shift in frequency of maximum relaxation absorption per wavelength with percent water vapor to be less than that extrapolated from previous measurements. The fmax versus percent-H2O curve has been analyzed in conjunction with previous measurements in binary mixtures of H2O and O2 to give more accurate values for the rate of v-v exchange between H2O and O2 and for the v-R, t rate in H2O-H2O collisions. Measurements have also been made of the vibrational relaxation absorption in moist nitrogen at 100, 200, and 300°F. In these measurements fmax was found to increase linearly with H2O concentration and a more accurate value for the v-R, t (or v) rate for H2O deexciting N2 vibration has been determined at each temperature. [Work supported NASA-Langley Research Center and NASA-Lewis Research Center.] [ABSTRACT FROM AUTHOR]

Solid-dielectric capacitance transducers, 25.4 cm in diameter, have been constructed and used to generate and detect pulses of sound waves in the frequency range from 4 to 200 kHz. The waves are propagated inside an aluminum tube, 25.4 cm i.d. and 4.5 m long. One of the transducers generates the sound pulses and can be moved axially along the tube; the second serves as a microphone, detecting the sound pulse and reflecting it at one end of the tube. The transducers are made by stretching metalized Mylar across a polished backing plate. Effects of diffraction, transducer nonalignment, and transverse modes within the tube are discussed. The sound absorption is obtained by the measurement of either the attenuation of the first pulse or the increase in the echo decay rate with distance. Pulse heights are measured with a multichannel pulse height analyzer and an on-line computer. The system has been used to measure sound absorption in humid air. [Work supported by NASA-Lewis Research Center.] [ABSTRACT FROM AUTHOR]

Ultrasonic absorption in mixtures of H2O and an impurity M=O2, N2, and Argon has been measured at 500 °K for frequency to pressure ratios from 2 to 50 MHz/atm. Measurements were made for mixtures of 85% H2O/15% M, 65% H2O/35% M, and 45 H2O/55% M. From these absorption measurements the vibrational and rotational relaxation times in pure water vapor were deduced as well as the rate at which Argon, O2, and N2 vibrationally de-excite H2O. In addition, by using earlier work at low H2O concentrations, the rate at which H2O de-excites O2 by vibration-translation and vibration-vibration energy transfer have been determined. [This work supported by the U.S. Army Research Office, Durham, NC.] [ABSTRACT FROM AUTHOR]

The possibility of an acoustic analog of a gaseous molecular laser has been investigated theoretically. It has been found that the mechanisms which lead to relaxation absorption in a gas can, in the presence of vibrational energy level populations far from equilibrium, give rise to sound amplification. This results from the fact that the relaxation times decrease with temperature; hence, as the sound wave causes the local temperature to increase, energy in the vibrational modes is converted to translation rapidly, giving rise to a further temperature rise. The opposite effect occurs when the sound wave causes the local temperature to decrease. For illustrative purposes, a 90% CO-10% H2 numerical example has been developed. This example demonstrates that sound amplification should be experimentally observable using a laser as the source of vibrational excitation. [ABSTRACT FROM AUTHOR]

The aim is a standard method of calculation with the credibility necessary to support major compromises between economic and environmental benefits desired by society. A two-relaxation model for the absorption of sound in air is fitted to a maximum of available laboratory and field measurements, as well as the detailed knowledge of basic mechanisms described in the previous paper. In this way, a set of seven fairly simple equations are generated whose form is determined by the physical nature of the mechanisms. These formulas may be used to compute the absorption of a pure tone over the following range of variables to an estimated accuracy of ±(10 + 1.5|t - 20|)%: frequency/pressure 102 to 105 Hz/atm, temperature (t) 0° to +35°C (32°-95°F), relative humidity 10%-100%, pressure 2 atm or less. A method of corrected the result to apply to bands of noise is proposed. [ABSTRACT FROM AUTHOR]

Utilizing kinetic information obtained from acoustical, optical, and opto-acoustical experiments, a comprehensive description of molecular relaxation in the atmosphere has been developed as the theoretical basis for a method of calculating the absorption of sound in air discussed in the following paper. The effects of atmospheric pressure and sound frequency on absorption evolve easily from the theory. The limitations imposed on theoretical calculations by insufficient energy transfer rate information are discussed. Many of the rates required to predict sound absorption are most easily measured using optical techniques. The similarity of acoustic and high-energy laser beam propagation through the atmosphere is discussed. [Work supported by the Army Research Office, Durham, North Carolina.] [ABSTRACT FROM AUTHOR]

The attenuation of 25- to 250-Hz sound waves has been measured over a homogeneous ground cover. One-third-octave bands of pink noise were transmitted across a field of soy beans and sound-pressure levels were recorded as a function of frequency, distance of absorber, and height and density of the absorber. The total absorption was compared to theoretical predictions of attenuation of sound waves layered boundaries. [Work supported by the Army Research Office, Durham, North Carolina.] [ABSTRACT FROM AUTHOR]

The attenuation of a 1-MHz ultrasonic signal has been measured in 72% N2/28% O2 at temperatures of 300°, 500°, and 700 °K and pressures from 1 atm to 1/100 atm. The measured attenuation was corrected for classical effects to obtain the attenuation due to rotational relaxation. In addition, the total absorption has been compared to computed values based on a linear combination of viscous, thermal conduction, diffusion, and relaxation to check the constants used for each of these terms. [Work supported by Army Research Office, Durham, North Carolina.] [ABSTRACT FROM AUTHOR]

Seven thunder signatures resulting from cloud-to-ground lightning strokes have been analyzed in third-octave bands spanning the frequency range from 25 to 500 Hz. Atmospheric absorption was found to contribute significantly to the change in the signature from source to receiver. Although atmospheric absorption is not large, it is frequency dependent, and hence can not be ignored when attempting to reconstruct the source spectrum from measurements taken several thousand feet away. A program has been developed which converts received sound pressure level (SPL) to initial power level (IPL) for given frequency bands. No attempt was made to determine an absolute measure of the acoustic energy released during each event. These initial results indicate that the spectra of different thunder events are sufficiently consistent to allow thunder to be used as a source for studying low-frequency absorption and atmospheric effects which lead to absorption. [ABSTRACT FROM AUTHOR]

Sound absorption in mixtures of O2/H2O has been measured at 400°K and 500°K for 0.29%, 0.58%, 1.2%, and 2.4% mole fractions of H2O. Curves of the absorption as a function of frequency divided by pressure have been compared with theory to provide rate constants for both vibration-to-translation and vibration-to-vibration energy exchange processes. These results have been compared to room temperature measurements by others. [This work was supported by the Office of Naval Research.] [ABSTRACT FROM AUTHOR]

A capacitor microphone using the ceramic boron nitride as the diaphragm has been built and tested at room temperature. This material was chosen because it offers promise of maintaining suitable strength and flexibility to 1000°C. Design and the frequency response at room temperature are given. Development was halted before testing at elevated temperatures. [ABSTRACT FROM AUTHOR]

The acoustic absorption in mixtures of CO2 and O2 has been measured for 90%, 80%, and 70% concentration of O2 at 600°K and 450°K and 80% and 10% concentration of O2 at 300°K. The absorption has been analyzed to extract the rate of energy transfer between the vibration of the bending mode of CO2 and the O2 virbation and the rate at which O2 deexcites the bending mode of CO2 in a v-t process. [Work supported by the Office of Naval Research.] [ABSTRACT FROM AUTHOR]

Measurements of sound absorption due to vibrational relaxation in SO2/O2 mixtures indicate a vibration-vibration (v-v) coupling between the asymmetric stretching mode of SO2 and the vibrational mode of O2. The theory of Tanczos, modified to allow for four-quantum interchanges, was used to analyze the results. The sound absorption measurements of the 5% SO2/95% O2 and 10% SO2/90% O2 mixtures enabled the distinction between the series and parallel relaxation schemes in SO2. The measurements indicate that SO2 relaxes by way of the series process with the energy passing through the stretching modes into the bending mode by v-v energy transfer. SO2/O2 collisions are of the order of five times as effective as SO2/O2 collisions in producing v-v exchanges within the SO2 molecule. The theoretical fit to the experimental absorption data on 5% SO2/95% O2 and 10% SO2/90% O2 mixtures is improved slightly by including rotational relaxation absorption of the two molecules. [ABSTRACT FROM AUTHOR]

Ultrasonic-absorption measurements have been used to study the vibrational and rotational relaxation in SO2 from 290° to 1090°K. This work extends the temperature range of earlier investigations by 500°K and is in agreement with the other studies in the low-temperature region. The vibrational data has been analyzed by assuming that the 518-cm-1 mode exhibits a relaxation time τ1, that the 1151- and the 1361-cm-1 modes relax together with a relaxation time τ2,3, and that the two relaxations occur in parallel. It is found that τ1 increases with increasing temperature while τ2,3 decreases with increasing temperature in the normal manner. The two vibrational collision numbers are a factor of 10 apart at room temperature but approach each other at high temperature. This is the first ultrasonic measurement of rotational relaxation in SO2, and the results confirm those of a previous thermal-transpiration measurement but conflict with an earlier thermal-conductivity measurement. [ABSTRACT FROM AUTHOR]

Ultrasonic absorption has been measured in H2S in the temperature range 300° to 683°K. This data was analyzed to obtain vibrational and rotational relaxation times. The vibrational collision numbers seem to increase with temperature from 300° to 473°K and then decrease with temperature. This behavior can be explained by the v-r theory of Shields and Burks. The rotational collision numbers compare favorably with thermal-transpiration measurements. [ABSTRACT FROM AUTHOR]

The best working gases for thermoacoustic refrigeration have high ratios of specific heats and low Prandtl numbers. These properties can be optimized by the use of a mixture of light and heavy noble gases. In this paper it is shown that light noble gas-heavy polyatomic gas mixtures can result in useful working gases. In addition, it is demonstrated that the onset temperature of a heat driven prime mover can be minimized with a gas with large Prandtl number and small ratio of specific heats. The gas properties must be optimized for the particular application of thermoacoustics; it cannot be assumed that high specific heat ratio and low Prandtl number are always desirable.