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 is however penetrated at low frequencies by a ground wave, to an extent that depends principally on distance and ground impedance, and at higher frequencies by interference between direct and ground-reflected waves to an extent that depends also on source and receiver heights. These phenomena, well established at ranges up to about 1000 ft and in some aspects to over 3000 ft, have been extrapolated theoretically to the order of 10 000 ft so that simple effects of topography and meteorology can be added to show how reflection or refraction acts in conjunction with ground impedance to result in penetration of the shadow region. Subject Classification: [43]28.40; [43]20.55.
Transportation noise, and motor-vehicle noise in particular, account for the steady or slowly varying ambient noise level, particularly in urban areas. Being so numerous, motor vehicles can be treated statistically, and this establishes consistent noise emission characteristics. Vehicles are categorized as: passenger cars [including light delivery trucks up to 6000 lb gross vehicle weight (GVW)], light, medium, and heavy trucks with GVW ranges of 6000-15 000, 15 000-30 000, and over 30 000 lb, respectively, tractor trailers, buses, cement-mixer trucks, and motorcycles. Speed ranges are 20-29, 30-39, 40-49, 50-59, and 60-69 mph. The sound level of the average vehicle of a given category increases consistently with speed and weight. The average vehicle is defined as the hypothetical vehicle having a sound level (A, B, or C weighting) and band-pressure levels equal to the weighted-mean decibels in the statistical distributions. The rate of increase with speed is such that in going from 30-39 mph to 60-69 mph, the sound level (A weighting) increases 8.5 dB for passenger cars, 9 dB for trucks and buses, 7 dB for tractor trailers, and 12 dB for motorcycles. For an estimated doubling of weight (motorcycles not included), sound levels increase 3.5 dB. For motorcycles, maximum noise occurs for full throttle setting, independent of road speed, gear, or engine loading. The octave-band spectrum of the average vehicle has a shape which is characteristic of each category and shows progressive change in level and shape with increasing speed. The octave-band spectra of four motorcycles, as examples, indicate dependence of level and shape on such parameters as type and size of engine, muffler configuration, and throttle setting. The sound level for acceleration is equivalent to about 40-49 mph cruising speed for tractor trailers and heavy trucks, and 30-39 mph for cement-mixers, but the octave band spectra for the two modes of operation exhibit consistent differences. Sound levels recorded continuously for 24-h periods at five locations, and analyzed for percentile distributions in each 1-h interval, have a diurnal cycle that follows a consistent pattern in response to motor-vehicle traffic.
The city is treated as a plane surface with many identical sound sources (motor vehicles) randomly distributed over its area. The mean energy density at any point in the plane is expressed in terms of the individual source strength, the average number of sources per unit area N, the atmospheric absorption constant c•, and a shielding factor F associated primarily with obstacles in the transmission path. To obtain the steady-state (median) energy density, a central cell containing a single discrete "local" vehicle is identified and treated separately from the rest of the distribution. Graphs and tables of steady-state level and energy density as functions of N and c• are given for the homogeneous infinite city, the city of finite size, and the traffic-free zone within a city. The theory indicates that the spreading of urban noise is determined by a characteristic distance with a typical value of 0.25 km. The observed octave-band sound-pressure levels from 31.5 to 4000 Hz at one location in Ottawa are compared with calculated levels based on statistical data for vehicle source strength, estimates of vehicle density, and known atmospheric absorption constants. The differences are consistent with a shielding factor of 15 dB which is in good agreement with measurements of sound transmission in urban areas reported by others. The shielding factor has an effective value substantially independent of frequency.
Measurements have been made on the performance of an anechoic chamber built for the National Research Council of Canada, adopting the inverse square law as the criterion of performance. Some deviations from the inverse square law were observed, and these were correlated with vibrational modes in the wedge bearing inner walls. It was shown that maximum amplitudes of vibration occur in certain regions of the walls and occupy frequency ranges which coincide with frequency ranges for maximum deviations from the inverse square law. The phase of the vibrations along the walls varies in the same manner as the phase of the incident sound wave in the room. Blocking all experimental points in one of the walls improved room response by a small but consistent amount. Treating regions of maximum vibration in the walls as extended sources and combining contributions from these with the sound field of the source in the room, it was possible to construct room characteristics similar to those observed.
To provide the necessary data for noise-abatement programs, a comprehensive study of motor vehicle noise is being conducted. The concept of noise as a form of environmental pollution is accepted as basic, and the noise output of motor vehicles is regarded as a function of the vehicle-driver combination, rather than of the vehicle alone. The noises from more than 2000 vehicles have been recorded on tape and analyzed for over-all dB, dBA, and in octave bands from 31.5 to 8000 Hz. For passenger cars, the peak of the noise-level distribution lies at about 65, 67, 72, and 73 dbA for the speed ranges of 30–39, 40–49, 50–59, and 60–69 mph, respectively, at a measuring distance of 15 m. The values for dump trucks, the noisiest of the heavy trucks, are about 10 dB higher when empty, and 12 or 13 dB higher when loaded. The values for tractor trailers are about 15 dB higher. The scatter for passenger cars is about 10 dB, and somewhat greater for trucks and tractor trailers. Motorcycles may be the noisest of all.
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