摘要:

The aim is a standard method of calculation with the credibility necessary to support major compromises between economic and environmental benefits desired by society. A two‐relaxation model for the absorption of sound in air is fitted to a maximum of available laboratory and field measurements, as well as the detailed knowledge of basic mechanisms described in the previous paper. In this way, a set of seven fairly simple equations are generated whose form is determined by the physical nature of the mechanisms. These formulas may be used to compute the absorption of a pure tone over the following range of variables to an estimated accuracy of ±(10 + 1.5|t− 20|)%: frequency/pressure 102to 105Hz/atm, temperature (t) 0° to +35°C (32°−95°F), relative humidity 10%–100%, pressure 2 atm or less. A method of corrected the result to apply to bands of noise is proposed.

ISSN:0001-4966    

DOI:10.1121/1.1914056

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

We set out to model theoretically the spectral features of infrasound observed in the ionosphere and believed to be radiated by severe thunderstorms. We explain the dominant 2–5‐min wave period as the effect of atmospheric filtering; shorter periods are excessively attenuated by absorption in transit to the ionosphere, and longer periods are attenuated in portions of the atmosphere where the waves are evanescent because their frequencies are below the acoustic cutoff. An observed spectral “fine structure” within the 2–5‐min band is explained in terms of resonant interactions between the waves and the atmospheric temperature structure. Accurate quantitative modeling of all these details of the storm‐to‐ionosphere transmission coefficient requires numerical integration of the acoustic‐gravity wave equation, including the effects of ground reflection and partial reflections in atmosphere.

ISSN:0001-4966    

DOI:10.1121/1.1914106

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

A small perforation (diam 1 mm) was placed in the postero‐superior quadrant of the feline tympanic membrane (TM). Together with the voltages across the transducer, sound pressures (amplitudes and phases) required for a 10‐μV CM response (round window) were measured over a range from 200 to 4000 Hz (a) separately in front and behind the TM, (b) in open and closed sound systems, and (c) before and after perforation. Sound‐pressure changes in front of the TM after perforation revealed low‐frequency losses, identical in shape and magnitude for open and closed systems: 12 dB/octave slopes below 1600 Hz. The voltage changes paralleled the sound‐pressure changes only in the open system. Considerably larger voltages were required in the closed system, especially between 630 and 2000 Hz (maximally 18 dB at 1250 Hz). Furthermore, from the sound pressures measured on both sides of the TM, the net force acting upon the TM was calculated. The change in net force after perforation followed an approximate slope of only 6 dB/octave. Implications of these findings will be discussed. [Supported by NIH and HEW grants.]

ISSN:0001-4966    

DOI:10.1121/1.1914165

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Bone‐conduction (BC) thresholds measured in terms of acceleration had shown large systematic variations, mainly with contact area [J. Acoust. Soc. Am.55, S62 (1974)]. Sound pressures generated in the occluded ear canal by BC signals of threshold strength were now found to be independent of contact area and, below 2 kHz, equal to those required for AC thresholds. Air/bone cancellation experiments revealed that, below 2 kHz and with the ear canal occluded, the BC stimulus is really given by the sound pressure generated in the ear canal, i.e., one measures the response to an air signal instead of that to a bone signal. [Essentially the same conclusion was reached for the cat; cf. Acta Otolaryng. Suppl. 132 (1966).]The cancellation experiments permitted the assessment of BC components unaffected by the sound pressure in the ear canal. These true inner‐ear BC responses also varied with the contact area, demonstrating once more that acceleration is a poor indicator of BC responses. Experiments now in progress will determine if the above findings apply also to the unoccluded ear canal. [Supported by NIH and DRF grants.]

ISSN:0001-4966    

DOI:10.1121/1.1914212

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Spatial mean‐square sound pressure due to steady sine‐wave excitation was measured in a room whose volume is 65 m3(2300 ft3) and reverberation time about 0.6 sec. A multi‐direction sound field was measured by rotating a12‐in.random‐incidence electret microphone one revolution in one minute on a circle of 1‐m radius, at the center of the room, mounted either on a boom or on the experimenter, who walked around the circle just back of the boom. Measurements of mean‐square sound‐pressure level were quite consistent (often within 0.5 dB) despite relative interference minima like −40 dB and movement of the experimenter during a rotation. A noise‐average meter made by Computer Engineering Ltd. was used, modified to measure the level of the time integral of squared sound pressure called exposure level. When the microphone was at the breast pocket of a thin shirt worn by the experimenter walking around the circle, the mean‐square sound‐pressure level was greater than it was on the same circle in the absence of the experimenter, in round numbers, by 1, 2, 3, 2, 0, and −1 dB, respectively, at 125, 250, 500, 1000, 2000, and 4000 Hz. The result was nearly the same for a heavy wool jacket and hat, except −4 dB at 4000 Hz. On a shoulder or at an ear, the buildup at the six frequencies was quite similar.

ISSN:0001-4966    

DOI:10.1121/1.1914263

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

A review of miniature audio‐frequency microphones and earphones will be presented. A discussion of the mechanical and electroacoustic characteristics will be given for a variety of transducer types. Typical constructions will be illustrated.

ISSN:0001-4966    

DOI:10.1121/1.1914291

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The signal‐processing gain by digital beamforming in Doppler sonar was investigated in terms of the enhancement of signal‐to‐reverberation ratio (SRR) at the post‐beamforming Doppler filter output with an objective to determine the minimum number of bits necessary for the quantization of beamformer channel input signals without appreciable degradation of sonar performance. It is shown that, in the present case, the simplified method of counting spectral lobes originally due to Davenport [W. B. Davenport Jr., J. Appl. Phys.24, 720 (1953); for application to multichannel multibit signal processors, see H. S. C. Wang, J. Acoust. Soc. Am.53, 929 (1973)] can only be applied with certain modifications in the evaluation of various signal and reverberation interacting power terms. Extensive numerical calculations of “Doppler processing gain” were then carried out to demonstrate that four‐bit quantization of array transducer signals is theoretically sufficient to yield a performance essentially identical to that of a linear beamformer, but severe loss of gain could result if fewer than four bits are taken at the quantizer output. It is further demonstrated numerically and substantiated analytically that, for small input SRR, the processing gain of a digital beamformer with Doppler filters cannot exceed that of a linear (or optimized four‐bit digital) beamformer even if its channel signals are quantized into more than four bits.

ISSN:0001-4966    

DOI:10.1121/1.1914297

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The relation between noise level, force (thrust), and inlet pressure has been investigated for 20 different blow‐gun nozzles or ejectors. This variety of design was a result of an Occupational Safety and Health Administration ruling that all hand‐held blow guns must be provided with pressure regulation and relief such that the maximum blocked static pressure could not exceed 30 psig. At a supply pressure of 100 psig a noise level variation of 63 dBA was found between the quietest and noisiest guns. Nearly the same degree of variation was demonstrated when the supply pressure was adjusted for each gun to yield equal force downstream. Guide lines will be given for the selection of nozzle design to yield minimum noise exposure consistent with downstream force requirements.

ISSN:0001-4966    

DOI:10.1121/1.1914302

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The analysis of vibrating plates with edges elastic. ally restrained against rotation is of interest in many engineering situations since perfect supports or clamps constitute, in general, ideal limiting conditions. The present paper deals with the calculation of some of the lower natural frequencies of a rectangular plate havingw= 0 and uniform slope restraint along pairs of opposite edges. The plate is subject to uniform, in‐plane tensile stressesNx,Nyand the mode shapes are approximated by simple polynomials which identically satisfy the boundary conditions. The procedure is simple and straight‐forward, and accurate frequency expressions are obtained by means of the Galerkin method. It is important to point out that as the rotation boundary parameters φx, and/or φyapproach zero, one obtains expressions corresponding to ideal clamps. On the other hand, if φx, and/or φyapproach infinity, the expressions degenerate properly into those corresponding to simple supports.

ISSN:0001-4966    

DOI:10.1121/1.1914307

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

In a reexamination of the cochlear microphonic and summating potential of the avian cochlea, it has been found that their magnitude and characterization are somewhat at variance with that previously reported. It was found that the CM intensity function is linear with a slope of 1 and that the microphonic amplitude may be as large as 1000 μVp‐p. Both positive and negative SP were measured in,the basal scala vestibuli of the pigeon. The properties of the SP are similar to those in the mammal inasmuch as the SP is positive at lower frequencies and negative at higher frequencies. The maximum positivity occurs at 500 Hz and amounts to about 200 μV at 100 dB SPL. The maximum negativity occurs at 3–4 kHz and amounts to about 250 μV at 100 dB. Differences in the findings from those perviously reported can be accounted for by noting that the present measurements were made in either the scala tympani proper or in the scala vestibuli. Intracochlear recordings from the recessus tympani tend to agree with previous findings. [Supported by National Institute of Neurological Diseases and Stroke.]

ISSN:0001-4966    

DOI:10.1121/1.1913823

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP