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.

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)].

Modifications to prior theory yield expressions for the frequency response and equivalent lumped elements of a condenser microphone in terms of its fundamental geometrical and material properties. Results of the analysis show excellent agreement with experimental data taken on B&K pressure microphone types 4134 and 4146.

A piezopolymer pressure sensor has been developed for service in a portable fetal heart rate monitor, which will permit an expectant mother to perform the fetal nonstress test, a standard predelivery test, in her home. Several sensors are mounted in an array on a belt worn by the mother. The sensor design conforms to the distinctive features of the fetal heart tone, namely, the acoustic signature, frequency spectrum, signal amplitude, and localization. The components of a sensor serve to fulfill five functions: signal detection, acceleration cancellation, acoustical isolation, electrical shielding, and electrical isolation of the mother. A theoretical analysis of the sensor response yields a numerical value for the sensor sensitivity, which is compared to experiment in an in vitro sensor calibration. Finally, an in vivo test on patients within the last six weeks of term reveals that nonstress test recordings from the acoustic monitor compare well with those obtained from conventional ultrasound.

The simple Laplace formula for the speed of sound in gases is corrected to account for three real-gas effects: molecular degrees of freedom which in equilibrium are not fully excited, deviations from the ideal-gas law, and dispersion due to relaxation processes. These are called the specific-heat, virial, and relaxation corrections, respectively. The specific-heat correction is based on a power-series expansion ofCp0 (specific heat at zero pressure) with respect to temperature. The virial correction is based on a three-parameter formula by Kaye and Laby for the second virial coefficient and a new five-parameter empirical formula for the third virial coefficient. Both are used to derive the corresponding acoustic virial coefficients. The relaxation correction is based on both Landau–Teller and Arrhenius temperature dependencies and inverse-pressure scaling. Using independent handbook data to obtain values for the above three corrections, the theory is capable of yielding sound-speed estimates to high accuracy over a wide range of temperatures and pressures. The theoretical uncertainty is due to the uncertainty in the original data, smoothing process, and truncation error; these are discussed in detail.

The variational principle of Hamilton is applied to develop an analytical formulation to describe the volume viscosity in fluids. The procedure described here differs from those used in the past in that a dissipative process is represented by the chemical affinity and progress variable ͑sometimes called "order parameter"͒ of a reacting species. These state variables appear in the variational integral in two places: first, in the expression for the internal energy, and second, in a subsidiary condition accounting for the conservation of the reacting species. As a result of the variational procedure, two dissipative terms appear in the Navier-Stokes equation. The first is the traditional volume viscosity term, proportional to the dilatational component of velocity; the second term is proportional to the material time derivative of the pressure gradient. Values of the respective volume viscosity coefficients are determined by applying the resulting volume-viscous Navier-Stokes equation to the case of acoustical propagation and then comparing expressions for the dispersion and absorption of sound. The formulation includes the special case of equilibration of the translational degrees of freedom. As examples, values are tabulated for dry and humid air, argon, and sea water.

Following a description of microphone cartridge construction, the topics to be discussed include: electrical and mechanical microphone sensitivities, effect of the polarization voltage, electrical and mechanical stability, membrane damping, electromechanical equivalent circuit, background noise, and response to infrasound. The discussion will be interspersed with little-known factoids on microphone performance which are not found in the traditional literature.

The variational principle of Hamilton is applied to derive the volume viscosity coefficients of a reacting fluid with multiple dissipative processes. The procedure, as in the case of a single dissipative process, yields two dissipative terms in the Navier-Stokes equation: The first is the traditional volume viscosity term, proportional to the dilatational component of the velocity; the second term is proportional to the material time derivative of the pressure gradient. Each dissipative process is assumed to be independent of the others. In a fluid comprising a single constituent with multiple relaxation processes, the relaxation times of the multiple processes are additive in the respective volume viscosity terms. If the fluid comprises several relaxing constituents ͑each with a single relaxation process͒, the relaxation times are again additive but weighted by the mole fractions of the fluid constituents. A generalized equation of state is derived, for which two special cases are considered: The case of "low-entropy production," where entropy variation is neglected, and that of "high entropy production," where the progress variables of the internal molecular processes are neglected. Applications include acoustical wave propagation, Stokes flow around a sphere, and the structure and thickness of a normal shock. Finally, it is shown that the analysis presented here resolves several misconceptions concerning the volume viscosity of fluids.