摘要:

Some experimental observations of the ocean surface under low wind speed conditions, carried out with the high gain acoustic distribution array, ADA, indicate that bubbles may play an important role in the noise generating mechanism in this wind speed regime. One of the mechanisms discussed in the theory is that of bubble collapse in the surface turbulence layer as first proposed by Furduev [Atmos. Ocean. Phys.2, 314 (1966)]. Under typical ocean conditions, low wind speeds, and the available bubble population data, the calculated noise level agrees well with experimental results, both in magnitude and in the shape of the spectrum. The spectrum has a peak in the frequency range of 100 to 1000 Hz and an ω−2behavior at high frequencies. Several geophysical parameters could influence the noise generation. Local wind speed probably controls the population of bubbles, and swell‐induced static pressure variations could play an important role in the critical turbulence pressure for bubble collapse. There seems to be further evidence that additional structure within the water, perhaps bubble density associated with different water masses, generates a patch type of distribution on the sea surface in the low wind speed situation.

ISSN:0001-4966    

DOI:10.1121/1.393694

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Observations are made which show that the underwater ambient noise spectrum generated by rain has a unique spectral shape which can be distinguished from other noise sources. Furthermore, the relationship between spectral level and rainfall is quantifiable. The spectral shape is dominated by a broad peak at 15 kHz, but also depends on the drop size distribution in the rain. A numerical study of the acoustic physics of a drop splash is used to explain the observed spectra. There are two contributions to underwater sound from the impact. The first contribution is from an initial acoustic water hammer pulse. The magnitude of this pulse depends on drop size, shape, and impact velocity. The contribution to the underwater sound spectrum is white and is very large for large drops. The second contribution occurs because at impact the incompressible continuity equation is not satisfied. Once this equation is satisfied, the splash is no longer an acoustic source. Numerically, the time required to closely satisfy this equation is roughly constant for all drop sizes at their terminal velocity. This time interval causes a low‐frequency rolloff at roughly 15 kHz in the sound spectrum.

ISSN:0001-4966    

DOI:10.1121/1.393695

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

An acoustic lens has been used to study the relative change in sound attenuation for frequencies in the range 90–300 MHz between the normal freezing and boiling points of liquid propane. These are the first results for the lower part of this temperature range and they confirm the predominance of the viscous contribution to the attenuation.

ISSN:0001-4966    

DOI:10.1121/1.393696

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A new ultrasonic power meter has been developed to measure the acoustic power flow of longitudinal vibration in a metallic rod as the product of particle velocity and vibration force. The technological difficulty that should be faced in measuring the vibration force by electrically obtaining the spatial differential coefficient of particle velocity has been overcome. The theory used here and the experimental verification of this method are also described.

ISSN:0001-4966    

DOI:10.1121/1.393697

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Determining when a worksite exposure should be considered impulsive is a problem which has complicated both research into mechanisms of auditory pathology and protection of the exposed worker. An ideal solution requires that a definition of impulsive noise be developed which is independent of specific characteristics such as duration and amplitude. As an alternative to duration independence, a fixed time window over which a statistic is calculated may also serve as a basis for classification. Additionally, selecting this window on the basis of TTS production in the ear provides a biological basis for the definition of impulsiveness. Classification of impulsiveness based on the sample kurtosis meets the requirements of a generally applicable impulse definition.

ISSN:0001-4966    

DOI:10.1121/1.393698

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

A super‐resolution imaging method is proposed which utilizes the finite extent of the object to estimate the hologram data outside the limited aperture from the hologram data detected at a small number of the points in the aperture by using the regularized singular value decomposition algorithm. The characteristics of this method are made clear by computer simulations, and the experiments show the usefulness of the method.

ISSN:0001-4966    

DOI:10.1121/1.393699

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Standing wave patterns were measured in the unoccluded ear canals of 13 human subjects, for applied pure tones of 3 to 13 kHz. Measurements were made, using a probe microphone technique, over a region which could be approximated as a duct of constant cross‐sectional area. Analysis of the patterns allowed the reflective properties of the middle ear to be determined in terms of an acoustic energy reflection coefficient, or reflectance, at the eardrum. Over all subjects the trend of the results was for the energy reflection coefficient to rise from about 0.3 at 4 kHz up to 0.8 at 8 kHz, and continue at this value to 13 kHz. There was, however, significant intersubject variation, especially at frequencies greater than 7 kHz.

ISSN:0001-4966    

DOI:10.1121/1.393372

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The physiological basis of auditory frequency selectivity was investigated by recording the temporal response patterns of single cochlear‐nerve fibers in the cat. The characteristic frequency and sharpness of tuning was determined for low‐frequency cochlear‐nerve fibers with two‐tone signals whose frequency components were of equal amplitude and starting phase. The measures were compared with those obtained with sinusoidal signals. The two‐tone characteristic frequency (2TCF) is defined as the arithmetic‐center frequency at which the fiber is synchronized to both signal frequencies in equal measure.The 2TCF closely corresponds to the characteristic frequency as determined by the frequency threshold curve. Moreover, the 2TCF changes relatively little (2%–12%) over a 60‐dB intensity range. The 2TCF generally shifts upward with increasing intensity for cochlear‐nerve fibers tuned to frequencies below 1 kHz and shifts downward as a function of intensity for units with characteristic frequencies (CF’s) above 1 kHz. The shifts in the 2TCF are considerably smaller than those observed with sinusoidal signals. Filter functions were derived from the synchronization pattern to the two‐tone signal by varying the frequency of one of the components over the fiber’s response area while maintaining the other component at the 2TCF. The frequency selectivity of the two‐tone filter function was determined by dividing the vector strength to the variable frequency signal by the vector strength to the CF tone. The filter function was measured 10 dB down from the peak (2TQ10 dB) and compared with theQ10 dBof the frequency threshold curve. The correlation between the two measures of frequency selectivity was 0.72. The 2TQ10 dBdoes change as a function of intensity. The magnitude and direction of the change is dependent on the sharpness of tuning at low and moderate sound‐pressure levels (SPL’s). The selectivity of the more sharply tuned fibers (2TQ10 dB>3) diminishes at intensities above 60 dB SPL. However, the broadening of selectivity is relatively small in comparison to discharge rate‐based measures of selectivity. The selectivity of the more broadly tuned units remains unchanged or improves slightly at similar intensity levels. The present data indicate that the frequency selectivity and tuning of low‐frequency cochlear‐nerve fibers are relatively stable over a 60‐dB range of SPL’s when measured in terms of their temporal discharge properties.

ISSN:0001-4966    

DOI:10.1121/1.393373

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

The shape of the auditory filter was estimated at three center frequencies, 0.5, 1.0, and 2.0 kHz, for five subjects with unilateral cochlear impairments. Additional measurements were made at 1.0 kHz using one subject with a unilateral impairment and six subjects with bilateral impairments. Subjects were chosen who had thresholds in the impaired ears which were relatively flat as a function of frequency and ranged from 15 to 70 dB HL. The filter shapes were estimated by measuring thresholds for sinusoidal signals (frequency  f ) in the presence of two bands of noise, 0.4 f  wide, one above and one below  f . The spectrum level of the noise was 50 dB (re: 20 μPa) and the noise bands were placed both symmetrically and asymmetrically about the signal frequency. The deviation of the nearer edge of each noise band from  f  varied from 0.0 to 0.8 f. For the normal ears, the filters were markedly asymmetric for center frequencies of 1.0 and 2.0 kHz, the high‐frequency branch being steeper. At 0.5 kHz, the filters were more symmetric. For the impaired ears, the filter shapes varied considerably from one subject to another. For most subjects, the lower branch of the filter was much less steep than normal. The upper branch was often less steep than normal, but a few subjects showed a near normal upper branch. For the subjects with unilateral impairments, the equivalent rectangular bandwidth of the filter was always greater for the impaired ear than for the normal ear at each center frequency. For three subjects at 0.5 kHz and one subject at 1.0 kHz, the filter had too little selectivity for its shape to be determined.

ISSN:0001-4966    

DOI:10.1121/1.393374

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP

摘要:

Frequency difference limens were determined as a function of stimulus duration in five normal‐hearing and seven hearing‐impaired subjects. The frequency DL duration functions obtained from normal‐hearing subjects were similar to those reported by Liang and Chistovich [Sov. Phys. Acoust.6, 75–80 (1961)]. As duration increased, the DL’s improved rapidly over a range of short durations, improved more gradually over a middle range of durations, and reached an asymptote around 200 ms. The functions obtained from the hearing‐impaired subjects were similar to those from normal subjects over the middle and longer durations, but did not display the rapid changes at short durations. The paper examines the ability of a variation of Zwicker’s excitation‐pattern model of frequency discrimination to explain these duration effects. Most, although not all, of the effects can be adequately explained by the model.

ISSN:0001-4966    

DOI:10.1121/1.393375

出版商:Acoustical Society of America    

年代:1986

数据来源: AIP