J. E. Piercy

Review of noise propagation in the atmosphere.

A general review is presented of most areas of sound‐propagation outdoors that are of interest for the control of community noise. These areas are geometrical spreading, atmospheric absorption, ground effect, (near horizontal propagation in a homogenous atmosphere close to flat ground), refraction, the effect of atmospheric turbulence, and the effect of topography (elevation, hillsides, foilage, etc.) The current state of knowledge in each area is presented and suggestions made concerning research activities, applications of existing research, and practical problems which arise in the prediction of noise levels.

Outdoor sound propagation over ground of finite impedance

There is an extensive body of theory, and some laboratory measurements, of sound propagation over a surface of finite impedance. There are also reliable measurements of outdoor sound propagation in near‐horizontal directions over the ground. In an attempt to relate these more closely, we have made carefully controlled measurements at ranges from 1 to 1000 ft, in most cases over grass‐covered flat surfaces, to demonstrate the several phenomena that are involved. These phenomena depend on, and conversely provide a means of estimating, the values of ground impedance for waves at near‐grazing angles of incidence. Such values obtained for grass‐covered surfaces are in reasonable agreement with each other and with values obtained by conventional means at other angles of incidence. It is suggested that simple but accurate predictions of noise levels can be made by assuming that an excess attenuation due to finite ground impedance would always exist in a certain shadow region near the ground. This shadow region i...

Effective flow resistivity of ground surfaces determined by acoustical measurements

Following earlier work by Chessell [J. Acoust. Soc. Am. 62, 825–834 (1977)] it is shown that his single‐parameter theory can be used to predict the measured transmission spectra between a source and receiver located above ground surfaces having a wide range of acoustic impedance—or effective flow resistivity. Surfaces behaving essentially as locally reacting range from new‐fallen snow, effective flow resistivity σ=10–30 cgs rayls, through grass‐covered ground, σ=150–300 rayls, to mature asphalt, σ=30 000 rayls. The thermal‐conduction and viscous boundary layer of the surface limits the effective flow resistivity of even the hardest and most impervious surface to the range 105–106 rayls, depending on frequency: this value is appropriate to evaluate the complex reflection coefficient of the paint‐sealed surface of a thick slab of reinforced concrete.

Line‐of‐sight propagation through atmospheric turbulence near the ground

Line‐of‐sight measurements of the log‐amplitude and phase fluctuations of pure tones between 250 and 4000 Hz propagated over distances between 2 and 300 m in the turbulent atmosphere close to the ground are compared quantitatively with simple theory using simultaneously measured meteorological variables. The theory is based on the assumption of homogeneous and isotropic turbulence and approximates the availability of eddy sizes in the source region of turbulence by a Gaussian spectrum. In particular the transverse or mutual coherence function (the coherence in a plane perpendicular to the direction of propagation) and the coherence in the direction of propagation which we call the longitudinal coherence, are also calculated and compared with the measurements. When the measured mean square phase fluctuations are compared with the theory using the meteorological measurements, good agreement is obtained. However the measured mean square log‐amplitude fluctuations are in general substantially smaller than pre...

Effects of atmospheric turbulence on the interference of sound waves near a hard boundary

The mean sound levels resulting from the interference between direct waves and those reflected from the ground are strongly influenced, especially at frequencies near interference minima, by fluctuations in phase and amplitude of the sound waves induced by propagation through atmospheric turbulence. Since it was found experimentally that the correlation length (∠1.1 m) of the meteorological fluctuations is comparable to the separation between the interfering sound paths, previous theoretical work by Ingard and Maling [J. Acoust. Soc. Am. 35, 1056–1058 (1963)] has been extended to allow for partial covariance between the two waves. The theory has been further extended to use the calculations of fluctuations in phase and amplitude of spherical waves, and to include the explicit calculation of the fluctuating acoustical index of refraction from the fluctuating values of temperature and wind velocity. Measurements (1–6 kHz) have been made of the interference spectrum at 15, 30, and 45 m from a point source 1....

Propagation of sound in the presence of gradients and turbulence near the ground

Sound‐pressure levels of pure tones between 1 and 16 kHz, generated by a point source on hard ground, were measured as a function of height at horizontal distances up to 21 m. In another experiment, measurements were made above hard ground and ground of finite impedance at horizontal distances up to 250 m using frequencies between 250 and 4000 Hz. Five frequencies were generated simultaneously and measured at four (and sometimes five) heights. The sound‐pressure levels below the refractive shadow boundary are compared with a simple theory that assumes a linear change of sound speed with height. This vertical gradient was obtained by approximating the temperature and wind velocity profiles measured during the acoustical experiment. Good agreement between theory and experiment was obtained at all distances and frequencies. However, at the largest distances and highest frequencies, there was evidence of additional energy penetrating the refractive shadow from scattering by atmospheric turbulence. Therefore, ...

Tactile perception in hands occupationally exposed to vibration

The sensory changes that occur in hands occupationally exposed to vibration have been assessed clinically by conventional neurologic tests and, independently, by improved techniques for the determination of tactile spatial resolution (gap detection) and vibrotactile perception thresholds at frequencies from 2 to 400 Hz. Data from 10 forest workers who were exposed to chain saw vibration and seven laboratory workers of similar age, all of whom were screened to exclude confounding factors, revealed, for the first time, three patterns of response, two of which are associated with a vibration-induced neuropathy. The first appears to be characterized by normal or better than normal thresholds in SAI, FAI, and FAII mechanoreceptor types, while a second, extreme response involves elevated thresholds in all three receptor systems (and abnormal two-point discrimination). The third pattern appears to be characterized by elevated thresholds in SAI and/or FAII receptor types.

Association between a quantitative measure of tactile acuity and hand symptoms reported by operators of power tools

An association between a quantitative measure of tactile acuity at the fingertips and symptoms of reduced manipulative function, as established by responses to a questionnaire, was demonstrated in a population of 81 manual workers from the mining industry (62 power-tool operators and 19 nonusers). Mechanoreceptor-specific vibrotactile thresholds were determined for the slowly adapting type I (SAI) and fast-adapting types I and II (FAI and FAII) receptors at the fingertip of the third digit of each hand. Statistically significant threshold shifts in SAI and/or FAII acuity were found in persons responding affirmatively to questions concerning finger/hand numbness, blanching, and difficulty buttoning clothing. The best predictors of a quantitative change in tactile acuity were questions relating to difficulty manipulating small objects and buttoning clothing, yielding positive predictive values of from 90% to 100% and false positive rates of from 0% to 2.8%. The demonstration of an association between a quantitative measure of tactile acuity at the fingertips and some symptom reports, obtained by means of a questionnaire, provides the basis for the development of a screening procedure for persons at risk of such disturbances in hand function.

Age‐related changes in mechanoreceptor‐specific vibrotactile thresholds for normal hands

Mechanoreceptor‐specific vibrotactile perception thresholds have been measured at the fingertips using the tactometer [A. J. Brammer and J. E. Piercy, in Proc. UK Informal Group Meeting on Human Response to Vibration, Buxton (September 1991)]. Thresholds were obtained from the left and right hands of healthy male white‐collar workers, aged from 25 to 73 years, without signs, symptoms, or history of neurological disease, or of exposure to vibration. Groups were formed from: (1) oriental professionals (mean age 34±5.9 years, N=24); (2) occidental professionals (mean age 40±5.0 years, N=18); and (3) oriental workers (mean age 62±5.6 years, N=24). A preliminary analysis of the data suggests that there is little effect of age on thresholds mediated by the SAI and FAI receptors. Thresholds mediated by the FAII receptors decreased in sensitivity at a average rate of 2.6 dB per decade increase in age. There are also a tendency for the SAI and FAI receptor thresholds in groups 1 and 3 to be more sensitive than tho...

Review of sound propagation in the atmosphere

Advances in our understanding of the mechanisms of outdoor sound propagation during the last five years which are relevant to community noise problems will be discussed, and an attempt made to fit them into a consistent overall picture. One aspect is studies of ground impedance and the relevance of modelling the ground plane by a semi‐infinite porous medium. Another is the contribution of theoretical papers on propagation from a point source through a homogeneous atmosphere over a plane of finite impedance. A third is the effect of atmospheric inhomogeneity—most notably scattering by turbulence and refraction by the thin (∼10 cm) thermal boundary layer close to the ground. The attenuation of barriers will also be discussed including the application of modern theory to diffraction over the top, interference effects produced by reflection from the ground, and scattering down into the diffractive shadow zone by turbulence.