Henry E. Bass

Atmospheric absorption of sound: Further developments

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.

Atmospheric absorption in the atmosphere up to 160 km

This paper describes new algorithms, not previously available, for predicting atmospheric absorption of sound at high altitudes. A basis for estimating atmospheric absorption up to 160 km is described. The estimated values at altitudes above 90 km must be considered as only approximate due to uncertainties about the composition of the atmosphere above 90 km and simplifying assumptions. At high altitudes, classical and rotational relaxation absorption are dominant, as opposed to absorption by molecular vibrational relaxation that is the principle atmospheric absorption loss mechanism for primary sonic booms propagating downward from a cruising supersonic aircraft. Classical and rotational relaxation absorption varies inversely with atmospheric pressure, thus increasing in magnitude at high altitudes as atmospheric pressure falls. However, classical and rotational losses also relax at the high values of frequency/pressure reached at high altitudes and thus, for audio and infrasonic frequencies, begin to dec...

General formulation of thermoacoustics for stacks having arbitrarily shaped pore cross sections

Theoretical treatments of thermoacoustics have been reported for stacks with circular pore and parallel plate geometries. A general linear formulation is developed for gas‐filled thermoacoustic elements such as heat exchangers, stacks, and tubes having pores of arbitrary cross‐sectional geometry. For compactness in the following, F represents the functional form of the transverse variation of the longitudinal particle velocity. Generally, F is a function of frequency, pore geometry, the response functions and transport coefficients of the gas used, and the ambient value of the gas density. Expressions are developed for the acoustic temperature, density, particle velocity, pressure, heat flow, and work flow from knowledge of F. Heat and work flows are compared in the short stack approximation for stacks consisting of parallel plates, circular, square, and equilateral triangular pores. In this approximation, heat and work flows are found to be greatest for the parallel plate stack geometry. Pressure and spe...

ATMOSPHERIC ABSORPTION OF SOUND : UPDATE

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)].

Atmospheric Absorption of Sound: Theoretical Predictions

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.

Acoustically induced seismic waves

When an airborne acoustic wave is incident at the ground surface, energy is coupled into the ground as seismic motion. In a previous publication [Sabatier et al., J. Acoust. Soc. Am. 78, 1345–1352 (1986)] the ground surface was modeled as an air‐filled poroelastic layer overlying a semi‐infinite, nonporous elastic substrate. In this work, the model is extended to include calculations of the normal seismic transfer function (ratio of the normal soil particle velocity at a depth d to the acoustic pressure at the surface). Measurements of the seismic transfer function for three sites are considered and compared to the predicted values. Generally good agreement between theory and experiment is achieved by best fits assuming the soil or seismic attenuation. This is accomplished by specifying the ratio of the imaginary to real part of the measured seismic p‐ and s‐wave speeds. The seismic transfer functions quite typically exhibit minima and maxima which are associated with the seismic layering of the ground su...

The interaction of airborne sound with the porous ground: The theoretical formulation

The surface of the ground is modeled as that of an air‐filled poroelastic soil layer of known thickness overlying a semi‐infinite nonporous elastic substrate. Using a modified form of the Biot–Stoll differential equations for wave propagation in fluid‐saturated porous media, propagation constants for the two possible dilatational waves and the shear wave in the poroelastic layer are determined. The dilatational waves are identified as a fast wave, moving predominately in the solid frame, and a slow wave, moving predominately in the pore air. The elastic moduli in the substrate are assumed to be those of the solid grains of which the poroelastic soil layer is composed. Intergranular friction in the soil and substrate is assumed to be negligible. Boundary conditions at the air–soil interface and at the porous soil–substrate interface are applied to determine, numerically, the displacement amplitudes of the allowed wave motions. From the incident and reflected amplitudes at the air–soil interface, the normal...

Capturing the Acoustic Fingerprint of Stratospheric Ash Injection

More than 100 separate incidents of interactions between aircraft and volcanic ash were documented between 1973 and 2003. Incidents on international flight paths over remote areas have resulted in engine failures and significant damage and expense to commercial airlines. To protect aircraft from volcanic ash, pilots need rapid and reliable notification of ash- generating events. A global infrasound array network, consisting of the International Monitoring System (IMS) and other national networks, has demonstrated a capability for remote detection of Vulcanian to Plinian eruptions that can inject ash into commercial aircraft cruise altitudes (approximately 12 kilometers) near the tropopause. The identification of recurring sound signatures associated with high- altitude ash injection implies that acoustic remote sensing can improve the reliability and reduce the latency of these notifications.

COMPARISON OF COMPUTER CODES FOR THE PROPAGATION OF SONIC BOOM WAVEFORMS THROUGH ISOTHERMAL ATMOSPHERES

A numerical exercise to compare computer codes for the propagation of sonic booms through idealized atmospheres is reported. Ground waveforms are calculated using four different codes or algorithms: (1) weak shock theory, an analytical prediction; (2) SHOCKN, a mixed time and frequency domain code developed at the University of Mississippi; (3) ZEPHYRUS, another mixed time and frequency code developed at the University of Texas; and (4) THOR, a pure time domain code recently developed at the University of Texas. The codes are described and their differences noted. They are then used to calculate propagation for two different source waveforms, for both a uniform and isothermal (varying density) atmosphere, with and without the presence of molecular relaxation. In all cases the results of THOR, SHOCKN, and ZEPHYRUS are in excellent agreement. Because the weak shock theory algorithm does not include the effect of ordinary absorption, it does not predict the shock structure provided by the other codes. This l...

Listening to the secret sounds of Earth's atmosphere

A new global network is breathing life into a dormant branch of geophysics. The study of infrasound, or long-period acoustic signals in the atmosphere was bustling in the 1950s and 1960s. Prior to 1963, almost all nuclear tests occurred in the atmosphere. After 1963, the USSR and U.S. signed the Limited Test Ban Treaty (LTBT), which eliminated all atmospheric nuclear tests. During the era of atmospheric nuclear testing, infrasound research was in demand, since the massive explosions produced strong, long-period acoustic waves that were globally observed and could be used to locate and describe the nuclear tests. Interest in this branch of geophysics waned with the end of atmospheric testing.