摘要:

In dealing with wave propagation over land below 1000 Hz, ground attenuation is an important consideration. Very often, the actual ground surface is covered with a thick layer of vegetation that has a very high over‐all void‐to‐volume ratio. Such a composite boundary should be represented as layered media in order to assess the ground attenuation effect correctly. The wave attenuation characteristics under such conditions are studied in this paper. It is found through theoretical analysis that a layered boundary gives rise to strong selective absorption in certain frequency bands. Experimental data has been taken under laboratory conditions over a wide range of normalized parameters, and the results quantitatively confirm many aspects of the predictions. Both theory and experiment indicate that significant attenuation to very low frequencies can be expected for sound propagation over a deep layer of vegetation. Other results include the confirmation that there is a strong sound‐pressure gradient in the vertical direction near the layered boundary, that the excessive attenuation can increase at most 6 dB per doubling distance, and that the effectiveness of ground attenuation depends critically on the elevation angle of the sound source.

ISSN:0001-4966    

DOI:10.1121/1.1912461

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The effects on perceived noisiness of spectral and temporal combinations of stimuli at varying durations were determined by 20 college students in an anechoic chamber. Several recordings of turboprop, turbofan, turbojet, and helicopter flyovers were also included in the list of stimuli. The results indicated that the most accurate predictor of the judged noisiness was perceived noise level with tone and duration corrections as outlined by the Federal Aviation Administrations (1968) aircraft certification procedure. To illustrate the responsiveness of effective perceived noise level (EPNL) over perceived noise level (PNL) in predicting the noisiness of stimuli, the results of the duration test revealed that, at judged equal noisiness, 75% of the data were within 4 dB of the standard signal for EPNL with the FAA tone and integrated duration measure as compared to 11 dB for PNL.

ISSN:0001-4966    

DOI:10.1121/1.1912462

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The method described by Bryn [J. Acoust. Soc. Amer.34, 289–297 (1962)] and Vanderkulk [J. Brit. IRE26, 285–292 (1965)] for analyzing optimum passive detection of weak stationary Gaussian plane waves in additive stationary Gaussian noise is applied to a linear array of hydrophones which may be closely spaced. The advantage in optimum over conventional delay‐sum‐square‐integrate processing is assessed in various situations. At a particular temporal frequency, the noise is characterized by a spatially derived spectrum. If this noise spectrum is reasonably flat over the principal response band of the steered array and if there are no effects from outside this band, then conventional processing will produce almost as high an array gain as the optimum detector. This situation is expected in reasonably isotropic noise fields at steerings near broadside. For long arrays of closely spaced hydrophones, a quantity termed theBLproduct roughly expresses the flatness of the spatial noise spectrum in the response band. When theBLproduct is high, the maximum nonsuperdirective array gain is simply the inverse of the spatial spectrum properly scaled. This gain is achieved by conventional processing the absence of strong interference; otherwise it is achieved by a weighting of the spatial spectrum, e.g., by a partial nulling.

ISSN:0001-4966    

DOI:10.1121/1.1912463

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The presence of isolated acoustic noise sources in the ocean produces an anisotropic noise field. Hence, a hypothesis test for detection of these sources can be formulated as a test ofH0(isotropic noise field) versusH1(anisotropic noise field). The structured approach taken in this paper is to formMbeams fromNhydrophone elements and then measure the power in each beam. The test for isotropy reduces to acceptance ofH0if the spatial variance of the measured power is less than a predetermined threshold, and acceptance ofH1otherwise. This so‐called spatial test is shown to be asymptotically uniformly most powerful invariant for a very natural group of invariance requirements, provided the measured powers are independent through design or suitable transformation. Strengths of the spatial test are that (1) no detailed specification of the anisotropic condition is required, and (2) the predetermined threshold is easily set without prior knowledge of noise levels. Performance curves for the spatial test are presented for specific models of anisotropy and are compared with performance curves of a conventional temporal test.

ISSN:0001-4966    

DOI:10.1121/1.1912464

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

In this paper, the time‐varying transfer function of the surface scattering channel is computed from first principles, using an extended Eckart model for the scattering mechanism. By assuming that the probability distribution of the surface is Gaussian with a correlation function having Gaussian form, first and second moments of the transfer function are computed. The effect of various surface parameters, particularly the relative surface roughness, on the channel autocorrelation function can then be easily demonstrated.

ISSN:0001-4966    

DOI:10.1121/1.1912465

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A problem of interest in signal processing using arrays is to design a processor that steers a null in one direction while steering a main lobe in another. In this paper, a solution to this problem for a monochromatic plane‐wave signal is found by first transforming the gain equation and then choosing a particular basis forK‐dimensional complex vector spaces. Other problems where the principle may be applied are mentioned.

ISSN:0001-4966    

DOI:10.1121/1.1912467

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

A method of calibrating the audiometric TDH‐39 earphone is developed that eliminates the requirement for expensive calibration equipment. This form of reciprocity calibration has been examined and evaluated for 25 pairs of TDH‐39 earphones. Two earphones of a headset are alternately used as transmitters and receivers, acoustically coupled in a small cavity. The electrical, acoustical, and mechanical constants of the transducers are then combined using the absolute averages of the sensitivities of the earphones, where the absolute sensitivity is traceable to the National Bureau of Standards. These sensitivity averages are then integrated with the transmitter‐receiver voltage transfer characteristic to obtain a calibrating K factor. The calibrating K factor enables field determination of average sensitivity. By use of a third earphone it is possible to extend the analysis to determine the absolute sensitivity of any one of the three earphones.

ISSN:0001-4966    

DOI:10.1121/1.1912468

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

摘要:

The feedback system of a Brüel&Kjær level recorder is used to produce an exponentially reducing waveform. Such a waveform is useful for checking reverberation‐time measurement equipment.

ISSN:0001-4966    

DOI:10.1121/1.1912469

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1912470

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1912472

出版商:Acoustical Society of America    

年代:1971

数据来源: AIP