Charles I. Malme

Sound Propagation in Urban Areas

Outdoor sound communication systems for urban areas have been traditionally designed using relatively few sources of sound, located high above the buildings. Results have generally not been satisfactory because of sound attenuation in the atmosphere and diffraction by buildings. As an alternative, a block by‐block coverage system using many sound sources placed in the streets has been investigated. Extensive acoustical and meteorological field tests carried out in city streets showed that sound‐propagation conditions are generally favorable there. Typically, the mean wind and temperature gradients are small; consequently, no appreciable sound refraction effects were observed over distances of a city block or two when sound was propagated from a source 10–20 ft above the pavement. Reverberation from the building walls was not detrimental to speech intelligibility when a directive sound source was used with its axis oriented along the street. However, the intelligibility of speech received in a side street ...

Nonlinear Response of a Simple Clamped Panel

Measurements are reported of some characteristics of the response of a simple panel to a steady periodic sound pressure intense enough to cause nonlinearity in the response. The panel is a 2‐in. wide, thin aluminum strip, clamped at its ends with the clamp faces 12 in. apart, and left free on its edges. Attention is focused principally on the fundamental resonance point, which occurred at frequencies as large as twice the low‐level resonance frequency.Measurements of the frequency and strain at resonance, under various levels of excitation, are compared with predictions from a simple one‐mode nonlinear dynamic equation. For good agreement it is found necessary to include not only a nonlinear stiffness but also a nonlinear damping term. Theoretical predictions of the nonlinear stiffness from the physical characteristics of the panel are in error by a factor of 4.7 when based on an assumed mode shape for transverse deflection which is identical with the linear characteristic function of a clamped‐clamped beam.

The influence of sound propagation conditions on the behavioral response of whales to underwater industrial noise

Results of recent studies involving gray, humpback, and bowhead whales show that whales tend to avoid areas with high underwater noise levels. Data obtained from whale behavioral observations during controlled exposure to representative industrial noise sources permit determination of the probability of avoidance Pa of the source region as a function of the noise level Ln. The zone of influence of a source may be defined as the region where Pa > 0.5. While the Ln required to produce this degree of avoidance has been found to depend on whale species and source type, some generalizations may be made. For low‐frequency continuous noise, 50% of the whales exposed have been observed to avoid regions where the overall Ln is higher than 115 to 125 dB (re: 1 μPa). Sound transmission conditions at a specific site determine the distance from the source, where Ln falls below the Pa = 0.5 criterion level. The zone of influence thus has been found to vary considerably for the test sites investigated. For example, a dr...

Supersonic naval missile sounds over San Nicolas Island

Vandals and other missiles are launched occasionally from San Nicolas Island, CA, during Naval exercises and tests. Pinnipeds on the island beaches are exposed to the flight sounds, some of which are sonic booms from directly overhead. Environmental concerns led the Navy to support acoustic studies of the missile sounds at the beaches. The results show flat‐weighted sound pressures from Vandals as high as 150 dB re: 20 μPa(peak) [140 dB re: 20 μPa(rms)] at a near‐vertical distance of 400 m. Other flat‐weighted pressures from Vandals were as low as 107 dB re: 20 μPa(peak) [95 dB re: 20 μPa(rms)] at a beach 3.9 km horizontally behind the launcher. Pulse durations and sound exposure levels were also measured. One‐third octave band sound exposure levels were measured. All parameters (except one‐third octave band levels) were also measured with A weighting. Other missiles measured include Tomahawk cruise missiles, Rolling Airframe Missile, Advanced Gun System, Terrier, and the Supersonic Sea‐Skimming Target. [Work supported by U.S. Navy.]Vandals and other missiles are launched occasionally from San Nicolas Island, CA, during Naval exercises and tests. Pinnipeds on the island beaches are exposed to the flight sounds, some of which are sonic booms from directly overhead. Environmental concerns led the Navy to support acoustic studies of the missile sounds at the beaches. The results show flat‐weighted sound pressures from Vandals as high as 150 dB re: 20 μPa(peak) [140 dB re: 20 μPa(rms)] at a near‐vertical distance of 400 m. Other flat‐weighted pressures from Vandals were as low as 107 dB re: 20 μPa(peak) [95 dB re: 20 μPa(rms)] at a beach 3.9 km horizontally behind the launcher. Pulse durations and sound exposure levels were also measured. One‐third octave band sound exposure levels were measured. All parameters (except one‐third octave band levels) were also measured with A weighting. Other missiles measured include Tomahawk cruise missiles, Rolling Airframe Missile, Advanced Gun System, Terrier, and the Supersonic Sea‐Skimming Target. [...

Prediction of potential disturbance of baleen whales by low‐frequency acoustic transients

High‐energy, low‐frequency sound sources are useful tools for geophysical surveying, submarine detection, and long distance acoustic tomography. These sources produce impulsive, narrow‐band and swept‐tonal signals at high levels in the oceanic environment. This study was made to estimate the received level‐time duration characteristics of acoustic transients that can potentially influence baleen whale behavior, the species of particular concern. Findings of acoustic disturbance studies show that continuous sound levels ≥120 dB re: 1 μPa produces ≥50% avoidance by gray and bowhead whales. However, for impulsive airgun sounds of duration ≤0.5 s, effective pulse levels 30 to 50 dB higher are required to produce 50% avoidance for the same species. Little information is available on whale response to intermediate sound durations representative of some sonar and tomography source operations. Consequently, the literature on human response to acoustic transients was examined for response prediction methodologies ...

A method for ranking the acoustic disturbance potential of noise sources in the environment of marine mammals

A standardized noise contribution model (SNC) has been developed to provide a means of comparing the contributions from diverse noise sources to facilitate assessment of their potential environmental impact. The output of the SNC model is a logarithmically scaled number proportional to the acoustic energy density produced by a specific type of source operating in a defined reference area. An associated standardized exposure rating model (SER) has been developed to rate the potential response of marine mammals to noise exposure. The SER model is designed to evaluate the degree of potential impact of a specific source on a specific species by producing a log‐scaled number proportional to the degree of matching between a noise source output bandwidth and a species hearing sensitivity characteristic. The SNC value for a given source type is used together with information on population density to weight the SER values for specific source and species encounter probabilities. [Work supported by the Minerals Mana...

Acoustic testing procedures for determining the potential impact of underwater industrial noise on migrating gray whales

A procedure was developed for observing the track patterns and behavior of migrating gray whales off Soberanes Point south of Monterey, California. A series of acoustic playbacks were made at realistic levels from an anchored vessel to simulate the presence of oil and gas development and production activities. In another series of tests, a seismic survey vessel with a 4000‐in.3 air‐gun array was navigated at progressively decreased ranges from the migrating whales. This was followed by a test sequence using a vessel with a single 100‐in.3 air gun. Source characteristic and acoustic transmission loss measurements were made to permit prediction of the noise exposure for the nearby subject animals. Exposure levels for which observed behavioral changes occurred were determined. [Work supported by Dept. of Interior, Minerals Management Service.]

The acoustic environment of humpback whales in Glacier Bay and Fredrick Sound, Alaska

The number of humpback whales frequenting Glacier Bay has been decreasing in recent years while the number of whales in the Fredrick Sound and Stevens Passage area has been increasing. A study was made to determine the acoustic characteristics of both areas and measure the influence of man‐made noise sources on ambient levels. ‘Transmission loss (TL) in selected regions of both areas was measured and ambient noise spectra obtained. The TL characteristics for a shallow source and receiver were observed to have a 20 log (range) dependence below 1 kHz in both areas. In Glacier Bay a shallow surface duct was observed which changed the TL to approximately 10 log (range) dependence above 1 kHz. Low ambient noise levels were observed in the absence of boat and ship traffic. Thus the sound from louder vessels dominated the ambient out to a range of up to six miles in some measurements. A significant low‐frequency noise contribution, believed to be due to glacier motion, was observed at some locations in Glacier B...

Fog signal baffle design

As part of its automation program, the U. S. Coast Guard has been replacing multifrequency air‐driven sound signals with electric air oscillators. The pure‐tone character of this signal has been a cause of noise complaints by residents near the installation. For example, an unbaffled ELG 300/02 dual emitter is capable of producing levels of the order of 80 dBA 1/2 mile to the rear of the signal. Thus, to be effective, a baffle design must provide at least 30–40 dB of attenuation in its dark sector. A program was conducted to develop high attenuation baffles for ELG 300‐Hz emitters using the principles of diffraction source interference, acoustically soft baffle edges, and end‐fired array lenses. The program utilized 1/8‐scale models to test the effectiveness of experimental configurations. The most effective baffle designs were then verified by full‐scale field tests. [This research was supported by the Coast Guard under Contract DOT‐CG‐30200‐A.]

Airblasts from Shallow Underwater Explosions

A shallow underwater explosion causes a supersonic plume to rise into the air. The plume shockwave produces the major peak in the ensuing airblast signature. An experimental investigation of the interaction between water motion and gas was conducted, wherein the parameters of the gas above the explosion were varied. Helium and “freon‐114” were substituted for air, giving a variation of 8 to 1 in sound speed and about 1 to 5 in specific acoustic impedance. The dominant parameter in determining whether the explosion is considered shallow or deep proves to be the Mach number of the plume motion with respect to the gas. Plume velocity, though correlated with gas density, is not inversely proportional to it: a 42‐fold increase in density resulted in halving the plume velocity. Some results of a calculation method to predict off‐axis airblasts from shallow explosions are also presented. These calculations, which show good agreement with experimental data, consider the plume as a decelerating supersonic blunt bo...