摘要:

It is well known in sonar work that the pulse form of a direct echo from a target bears little relation to the form of the original signal. This is true even for regularly shaped bodies, such as a sphere. In this paper, the case of a homogeneous elastic sphere in water is examined theoretically and it is shown in comparison with experimental results, that the observed effects originate from vibrations induced in the sphere by the incident sound. Calculated results are presented for a variety of solid materials and it seems that echo forms could possibly provide information about the size and constitution of a sonar target.

ISSN:0001-4966    

DOI:10.1121/1.1909055

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

The scattering strength of the sea surface was measured for a range of wind velocities, grazing angles, and frequencies, in octave bands in the frequency range from 400 to 6400 cps. An empirical equation was obtained relating the scattering strength of the sea surface to the above variables, for grazing angles below 40°. At low grazing angles, scattering of sound from a subsurface layer of isotropic scatterers, probably of biological origin, frequently masked the reverberation due to scattering from surface roughness. For a given wind speed, the scattering strengths measured in this study at grazing angles below 20° were appreciably less than those obtained by other observers at higher frequencies. At higher grazing angles, of the order of 40°, there was little systematic difference between the measurements made at high and low frequencies.

ISSN:0001-4966    

DOI:10.1121/1.1909057

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

The detection of underwater acoustic signals by simple auto‐ and cross‐correlation receivers in the presence of nonnormal, as well as normal background noise is examined on the basis of signal‐to‐noise ratios calculated from a generalized deflection criterion. Particular attention is devoted to the effects of impulse noise and mixtures of impulse and normal noise on system performance. Comparisons between system behavior vis‐à‐vis the two types of interference are made. For impulse noise equivalent in spectral distribution and average intensity to a Gaussian noise background it is found that the output signal‐to‐noise (power) ratios are related by the canonical expression(SN)I2 = (S/N)G21 + (1 − μ)Λ(S/N)G2, 0⩽μ⩽1, where Λ(⩾0) is the “impulse factor” and μ is the fraction (in average intensity) of the total noise background that is attributable to normal noise. Impulse noise always degrades system performance vis‐à‐vis normal noise in the autocorrelation reception of stochastic signals, characteristic of applications where passive receiving methods must be used. This degradation can be considerable [O(10 dB or more)] if the noise is highly impulsive (large Λ) and if large values of(S/N)out2(>0 dB)are required (for high accuracy of decision). On the other hand, when coherent (i.e., deterministic) signals are employed, so that cross‐correlation reception is possible, the degradation may be reduced essentially to zero (i.e., Λ → 0) under realizable conditions of operation. It is observed for impulsive, as well as normal noise backgrounds, that cross‐correlation receivers are linear in their dependence on signal‐to‐noise ratio, i.e.,(S/N)out2∼(S/N)in2if sufficiently strong injected signals are employed. The analysis is carried out largely in canonical form, so that the general results for(S/N)out2can be applied to other, special types of nonnormal noise backgrounds. Specific relations are included, along with a detailed summary of the principal results, showing the dependence of(S/N)out2on(S/N)in2, filtering, delay, noise and signal spectra, etc., for weak and strong inputs, little or heavy postcorrelation smoothing and for Gaussian as well as for impulse noise.

ISSN:0001-4966    

DOI:10.1121/1.1909059

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

A series of experiments designed to elucidate the differences between pure tones and octave bands of noise in terms of their ability to arouse temporary threshold shift (TTS) is described. It is shown that the critical‐band concept is not relevant at high intensities, and that therefore the 10 dB correction factor currently employed, being based on the critical‐band concept, must usually be incorrect. Instead, it is demonstrated that the differences in TTS producing ability of tones and noises can be attributed mainly to their differences in ability to arouse the acoustic reflex; since noises cause more sustained contraction of the middle ear muscles than do pure tones, less TTS will usually be produced by noise. Because this difference in contraction, and hence in effective attenuation, is a variable function of frequency, intensity, and time, no one single decibel correction factor is appropriate. It is suggested that damage‐risk criteria for pure tones should be developed empirically by measuring TTS directly rather than by attempting to modify existing criteria for octave‐band noise.

ISSN:0001-4966    

DOI:10.1121/1.1909061

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

The effects of monaural masking noise on loudness and on lateralization of clicks were compared for noise levels ranging between 35 and 65 dB SPL. For different fixed levels of unmasked clicks, sound‐pressure levels of the masked clicks were adjusted by the subjects to produce centered sound images. In a second experiment, similar procedures were used to generate equal‐loudness functions. For both loudness and lateralization, the effect of masking noise was greatest with high levels of noise and low intensities of click. In every case, the masking effect was greater for loudness than for lateralization.

ISSN:0001-4966    

DOI:10.1121/1.1909063

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

Responses to glissando at 1500 cps at a 50‐phon level using trained and untrained listeners are analyzed. The sensitivity to this type of frequency modulation is surprisingly good. A constant of 5 cps change emerges indicating that sensitivity depends on a linear relationship between rate of change and stimulus duration. This constant is of the same order of magnitude as the traditional DL for pitch memory. The low observed relationship of glissando to either pitch discrimination by the constants method or with sensitivity to the presence or absence of sinusoidal frequency modulation indicates that probably a third distinct pitch factor is represented.

ISSN:0001-4966    

DOI:10.1121/1.1909065

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

The attenuation of two different earplugs and a helmet was measured by a procedure combining the free‐field threshold shift and the Békésy threshold tracking methods. Subjects were then exposed to impulse noise which would produce between 12 and 64 dB TTS at 6000 cps unprotected and their TTS was subsequently measured after identical exposure during which they wore the protective devices. Following this the subjects were exposed unprotected to continuous noise which would produce a TTS at 6000 cps within ±6 dB of the TTS they experienced after their unprotected exposure to the continuous noise. Finally, they were exposed to the same continuous noise while wearing the protective devices, and their TTS following exposure was determined.It was found that the helmet afforded significantly less attenuation of low‐frequency sound than the ear plugs and roughly the same amount at higher frequencies. The three devices were found to be equally effective in eliminating the TTS following exposure to the continuous and impulsive noise. While the data did not lend themselves to definite conclusions regarding susceptibility to impulse and continuous noise, it appeared that susceptibility to one type of exposure was not necessarily significantly correlated with susceptibility to the other.

ISSN:0001-4966    

DOI:10.1121/1.1909067

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

The application to organ tones or other sustained musical sounds of the Stevens and the Zwicker methods for loudness‐level calculation from observed sound spectra is examined. It is found that both methods, although developed for dealing with noise of the continuous spectrum type, are applicable to organ tones, a particular class of discrete component sound, subject to the use in both methods of accepted equal‐loudness contours. Examples of the loudness levels of some typical organ tones are given. Questions needing further investigation are noted.

ISSN:0001-4966    

DOI:10.1121/1.1909069

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

Relative differential intensive limens were obtained at three frequencies and two intensities for three stimulus patterns involving fluctuation of a steady tone and three patterns involving separate stimuli. The same observers were utilized in all cases; in Experiment I, signals were presented through earphones. Somewhat lower thresholds were obtained for fluctuating intensity patterns than for separate stimuli patterns, thresholds tended to be lower at the higher intensity level, and thresholds were lower for patterns allowing greater opportunity for comparison within the patterns. When similar thresholds were obtained for patterns presented through speakers (Experiment II), thresholds were smaller, the influence of intensity was smaller, and the influence of (presentation) pattern type was less consistent. Significance of the findings is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1909071

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

The most common device for analyzing sounds is the bandpass litter, usually with a bandwidth of one octave or a fraction there of, e.g., (one‐half or one‐third. Recently 1/3‐octave filters are also being used for vibration analyses. The effective bandwidth of these filters is affected not only by the obvious design parameters of filter cutoff/frequency and the number of resonant elements employed which determines the attenuation rate outside the passband but also on the slope of the noise spectrum which is being analyzed. This paper investigates the effect of all these variables on the effective bandwidth of a symmetrical bandpass filter by means of a normalized mathematical model and expresses the result in terms of bandwidth error. It is found (1) that the bandwidth error increases very rapidly for large values of spectrum slope; (2) that the bandwidth error is symmetrical about an equal energy per octave spectrum slope and that an optimum design point for attenuation at the filter band edges exists for minimizing bandwidth error as a function of spectrum slope.

ISSN:0001-4966    

DOI:10.1121/1.1909073

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP