T. F. W. Embleton

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...

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.

Fluctuations in the Propagation of Sound near the Ground

The fluctuation in the propagation of a pure tone over a flat grassy surface from a source on the ground has been studied over ranges of frequency (300–4000 Hz) of the source as well as height (0–5 m) and distance (15–150 m) of the receiving microphone. Preliminary analysis of the records indicates that the standard deviation of signal level about the mean initially increases nearly linearly with distance as found by previous experimenters, but for longer distances saturates at ∼6 dB as found previously only in the propagation of light. The dependence on frequency found for the unsaturated region is considerably greater than the 7/12 power predicted for large‐scale isotropic turbulence, in keeping with the reduced scale expected near the ground. Signal correlation along a wavefront has also been measured. The results place limitations on the use of coherent acoustic theory for the calculation of the attenuation of noise in practice by barriers and ground interference, as well as the use of modeling techni...

Impedance of Soft Ground and Its Effect on Practical Measurements

This paper is a continuation of paper AA5, 84th Meeting of The Acoustical Society, Miami, 1972 [J. Acoust. Soc. Amer. 53, 340 (A)(1973)]. The propagation characteristics of the ground wave are influenced by the ground impedance. The amplitude and phase of the ground‐reflected sky wave (and hence the ground impedance ratio at oblique angles of incidence) have been measured at short ranges up to about 20 ft over grass by interference with the direct wave, and used (a) as an independent check of the value from the ground wave obtained in other geometric configurations and (b) to predict the sound field at greater distances for which direct measurement may be difficult. Such predictions and also measurements at ranges up to 1000 ft indicate shadow zones due to ground impedance of up to 40 Db. In a configuration of interest to measurement standards and community noise, via., source height 1–2 ft, receiver height 4–5 ft at a range of 50 ft over grass, there is a transmission dip of 10–15 dB near 1000 Hz: this dip would be more marked and at lower frequencies (due to “Lloyds mirror” interference between the two sky waves) were the low‐frequency energy not transmitted via the ground wave.

Effect of the Ground on near Horizontal Sound Propagation

The propagation of sound from a point source over short grass on flat ground has been measured for the relatively small grazing angles (<20°) normally encountered in practice. The distances ranged from 1 to 1000 ft at several locations. Shadow zones caused by the ground impedance have been found in situations of importance to community noise problems. With the microphone 4 ft above the ground and 50 ft away from a source 1 ft above the ground, a configuration relevant to snowmobile legislation and also exhaust noise from vehicles in suburban settings, attenuation was ≃15 dB in excess of inverse square law for frequencies near 1 kHz. For longer distances, up to 30 dB of excess attenuation was observed at frequencies above 500 Hz. Two methods have been developed for determining the ground impedance at glancing angles. From these values, the attenuation in the shadow zones may be predicted using the theory developed by Rudnick (1947) and Ingard (1951). It is found that the concepts of ground and sky waves us...

Acoustical standards calibration at the physics division of the National Research Council of Canada

This paper describes some recent research and development in acoustical standards calibration at NRC. Topics included are instrumentation and apparatus for precise reciprocity, and comparison methods of calibration of condenser microphones. Also, a two‐microphone cavity technique for the overall calibration of sound level meters is discussed together with various methods of verification of acoustical calibrators.

Noise Testing of Vehicles - Acoustic Propagation Phenomena

A number of propagation phenomena which cause variability in the noise testing of vehicles using standard procedures have been isolated and studied. Included are the effects of interference produced by reflection from various ground surfaces, the effects of refraction caused by wind and temperature gradients in the atmosphere, changes in sound level due to the wake of the vehicle, and the effects of normal atmospheric turbulence.

Variability in the noise testing of light motor vehicles

There have been complaints about the variation from day to day and site to site in the noise levels of light motor vehicles when measured with standard test procedures. A study aimed at the resolution of the sources of this variation is described. A light‐, a heavy‐, and a medium‐weight automobile were each tested (repeatedly) on three different paved sites under a variety of atmospheric conditions. The levels from a calibrated point source simulating the noise from an exhaust pipe was measured under the same conditions with the source stationary, and also with the source carried through the standard pass by procedure on a coasting vehicle. Vertical gradients of ambient wind and temperature were monitored. Preliminary analysis of the results indicates that the major source of the variation is refraction in the wake of the moving vehicle.

Ground reflection and standard procedures for measuring noise

The effect of interference between direct and ground reflected rays is examined for standard methods of measurement of the noise emitted by road vehicles, snowmobiles, and other devices powered by internal combustion engines, such as chainsaws and lawn mowers. It is shown that cancellation leads to particularly erratic measurements for accelerating large trucks where the propagation is over asphalt, and for all devices where the propagation is over a porous surface, such as grass or snow. For propagation over asphalt, placing the microphone on the asphalt surface eliminates these effects, best considered as transmission artifacts. No such simple cure is available over porous surfaces. Good agreement is achieved between experiment and theory.