摘要:

A noncontacting displacement probe is described which is linear throughout the audio‐frequency range over a wide displacement amplitude range. The instrument employs commercially available condenser microphone circuitry to measure the displacement of any conducting surface. Entirely conventional audio‐instrumentation may be employed to analyze test data obtained with the probe. A typical output voltage, with the gauge located 0.1 in. from the vibrating member is 25 mv for a 0.001‐in. displacement amplitude. A simple accessory is described for checking the instrumentation sensitivity.

ISSN:0001-4966    

DOI:10.1121/1.1907620

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

Attempts to combine shock spectra, by envelope of fiducial limit types of analysis, to produce curves useful for design purposes have led to design shock spectra which are extremely overconservative. It was noticed that this technique assumes that the impedance of any structure at its foundation for its fixed base natural frequencies is very low, and therefore unable to affect the foundation motion. This was in contrast to test evidence that a dip would appear in the regions of these frequencies in the spectrum plot. This paper examines the theory of shock spectra, Fourier spectra, and mechanical impedance to determine qualitatively the effect upon shock spectra of the dynamic reaction of structures. The concept of shock spectrum dip is presented, explained, and experimental verification exhibited. Several theorems regarding shock and Fourier spectra are advanced. The necessary distinction between shock spectrum values at fixed base natural frequencies of a structure and at natural frequencies of a structure‐foundation combination is demonstrated. The potentially extreme overconservatism in design resulting from incorrect usage of shock spectra is pointed out.

ISSN:0001-4966    

DOI:10.1121/1.1907621

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

Volume scattering the ocean determines the lower limit to what can be achieved in a hydrophone'sin situminor lobe response. This paper makes theoretical examination of the magnitude of minor lobes produced by volume scattering. It is concluded that for a directional hydrophone of moderate‐to‐high directivity, the relative minor lobe response due to volume scattering is given to a close approximation by the product of the scattering cross section and the distance to source divided by the directivity factor. Direction of source has a comparatively small effect. Attenuation constant may be neglected unless the attenuation loss between source and hydrophone is larger than 1 db or so.

ISSN:0001-4966    

DOI:10.1121/1.1907622

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

Berman and Clay [J. Acoust. Soc. Am.29, 7 (1957)] have investigated the directional characteristics of a linear array of omnidirectional receiving elements when the output from these elements are multiplied together and time arranged in certain ways. They conclude that the same directional characteristics may be obtained from these “time‐averaged product” (TAP) arrays having a small number of detectors as with an additive array with a large number of elements; they also show that there is some economy in terms of the over‐all length of the array required to give a specified beam width. Because it is not possible to assess the characteristics of a TAP array solely on the basis of its polar diagram, it is necessary to carry out a detailed analysis to determine its usefulness in applications which require a system with a narrow polar diagram. Three particular applications are investigated, namely the detection of a point source against a noise background, the resolution of the signals from two closely spaced point sources, and the exploration of the distribution of power across an extended source. For comparison purposes a parallel analysis is carried out for an intraclass correlation system. For simplicity, the analyses are confined to investigating the performance of a 4‐element linear array with equal spacings between adjacent elements under elementary and idealized conditions. The results so obtained, however, are of general application. Two class I TAP array systems are compared with the intraclass correlator, the detection performance of which was analysed by Faran and Hills [Acoustics Research Laboratory, Harvard University, Techn. Memo. No. 28 (November 1952).]and which is believed to be the most “efficient” detector in this application. {A class I array is defined as one in which the number of time averaging operations lies between (N− 1) and12N(N − 1), whereNis the number of receiving elements. A class II TAP array, which employs true multipliers, is obviously inferior to a class I array in terms of detection performance and will not be considered. It has been shown by Meltonet al.[see B. S. Melton and P. R. Kerr, Geophysics22, 553 (1957), for example] that a class II type of array employing coincidence detectors rather than multipliers has some useful properties but it can be demonstrated that its detection performance is inferior to that of an intraclass correlator. Since class III TAP arrays have the same detection performance as the system prior to the power and sum circuits there is no need to analyse such arrays here.}

ISSN:0001-4966    

DOI:10.1121/1.1907623

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

A small, closed system for the absolute calibration of a standard hydrophone is described. The method allows control of the static pressure and temperature and is applicable at frequencies up to 5 kc. The system is designed so that the blocked or Thevenin pressure of the sound source is the same as the sound pressure acting on the hydrophone. The use of a hydrophone, calibrated with a pressure phone, as a standard in large closed‐chamber systems is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1907625

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

A series of experiments were performed to compare the efficiency of different diffusing devices in producing a completely random sound field in a reverberation chamber and the effectiveness of some of the methods for judging whether a sound field is completely random. An additional experiment was performed to compare two types of test signals,viz., warble tone and the same nominal band width of random noise. Results of these measurements indicate the general superiority of the rotating vane over other diffusing devices and superiority of random noise over warble tone in producing a smooth decay curve. One of the significant conclusions is that even when the diffusion is reasonably adequate the measured absorption coefficient of a standard sample appears to vary with the chamber absorption especially at the high frequencies.

ISSN:0001-4966    

DOI:10.1121/1.1907626

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

The lack of validity of the diffusion assumption makes it desirable to search for further means of assessing the acoustical properties of a room rather than a criterion based solely upon reverberation time. A pulse technique is devised to study transient sound in a room. An electroacoustic system was constructed for generating sound pulses in a room and for receiving and recording the pulses after transmission to various points in the room. Acoustical testing also included the determination of the absorption coefficient of a wall and the tracing of a sound path in a room. A pulse statistical analysis was made to give the root‐mean‐square deviations of the pulse transient response from the exponential steady state decay based on the known reverberation time. It is concluded that for the lecture room tested arms deviation less than 0.50 represents good acoustical conditions for speech.

ISSN:0001-4966    

DOI:10.1121/1.1907627

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

The improvement of message reception in noise with multiple observers was analyzed. Two sets of decision rules were employed to select among the responses of three observers as follows: one based upon the response agreement among observers; one based upon the listeners' confidence ratings of accuracy of message reception. Confidence ratings substantially improved the selection of the correct message reception in the absence of response agreement among observers.

ISSN:0001-4966    

DOI:10.1121/1.1907628

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

An experiment was designed to investigate whether irrelevant loud bursts of low‐frequency noise affected decision‐making unfavorably. Analysis of the results revealed a significant decrement in performance due to the noise distraction. Scores for the whole of a 4‐min test were unimpaired because only those responses immediately following the noise were affected. A second experiment demonstrated that effects were related to intensity.

ISSN:0001-4966    

DOI:10.1121/1.1907629

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP

摘要:

The aim of tim present experiments was to investigate the accuracy with which thenumberof simultaneously sounding tones can be perceived. Various one‐, two‐, and three‐tone stimuli were presented to subjects, who had to judge how many tones were present. Results showed that discrimination of number of different tones sounding was far from perfect (even though Ohm's acoustic law would lead us to expect high accuracy). The discrimination of the average person was not improved even when frequency components were widely spaced. Accuracy of perception was poorer for two‐ and three‐tone combinations than for one‐tone stimuli. Number of allowable response categories also affected percent correct responses. Individual differences in performance were found to be large.

ISSN:0001-4966    

DOI:10.1121/1.1907630

出版商:Acoustical Society of America    

年代:1959

数据来源: AIP