摘要:

The excessive heat development on the exterior structure of hypersonic re‐entry vehicles necessitates the use of heat‐resistant pressure transducers for the measurement of boundary‐layer pressure fluctuations on the vehicle's surface. An appropriate acoustic‐probe configuration was developed and tested in wind‐tunnel experiments. The frequency response of the probe configuration to flow excitation was studied under subsonic (M= 0.055) and supersonic (M= 2.92) flow conditions and was found to depend both on the probe interior geometry and on the physical characteristics of the flow. In addition, the influence of heat‐shield ablation on the frequency response was studied under these same conditions.

ISSN:0001-4966    

DOI:10.1121/1.1911226

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

This paper investigates the spatially dependent static pressure field induced by the steady motion of a compressible inviscid fluid in the interior of a two‐dimensional duct with small wall roughness. The statistically described wall‐roughness height and slope are assumed to be uncorrelated; this is equivalent to considering the wall‐roughness height and slope correlation functions as Dirac delta functions. The paper treats the case in which the duct‐flow velocity is less than the fluid sound speed. It is found that, for subsonic‐flow conditions, a wall possessing a random‐roughness distribution induces a static mean‐square pressure which is at a maximum at the duct walls. The static pressure field induced in the fluid by the passage through the roughened‐wall duct is derived and found to decay with increasing distance from the duct walls. An increase in the free‐stream Mach number results in an increase in the mean‐square pressure level in the duct; minimum values of mean‐square pressure result when the flow is assumed to be incompressible.

ISSN:0001-4966    

DOI:10.1121/1.1911227

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Excitation of an ultrasonic light modulator by random electrical signals will cause a random modulation to be imposed on the emerging light wavefront. A general solution for the resulting light diffraction patterns is obtained, from which the conditions are determined for which the diffracted light‐intensity distribution will be a valid representation of the power spectral density of the input electrical noise signal. The necessary experimental conditions for which the variance of the spectral measurements will be small are determined. The percentage of the light‐modulator bandwidth over which measurements will be valid is found to be dependent on the product of the spatial‐frequency bandwidth of the light modulator and the length of the optical aperture; this effect is evaluated and numerical results are presented. The case in which the input to the ultrasonic light modulator consists of a deterministic signal and noise is also considered. Using these results, the diffracted light levels due to signal and noise are obtained as a function of the input signal and noise power levels.

ISSN:0001-4966    

DOI:10.1121/1.1911228

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

This paper considers the detection of signal consisting of broad‐band Gaussian noise whose level varies in some predictable manner with time—e.g., the signal received from a passing object. This study differs from previous ones concerned with this problem in that the background noise is explicitly assumed to be nonstationary. The assumed noise model consists, in fact, of a stationary Gaussian noise amplitude modulated by another random process having a relatively very long correlation time. It is shown that the detectability with this noise model is considerably below that obtained previously with a stationary noise model, and the exact dependence on correlation time, fluctuation of the noise level, etc., is obtained.

ISSN:0001-4966    

DOI:10.1121/1.1911229

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

A method of electrically stimulating neural elements confined within small volumes of brain at predetermined deep locations without employing probes is considered theoretically. The basic principle of the method is the partial rectification in the focal region of an ultrasonic field of the alternating current that flows in the entire brain, or a major portion thereof, in response to an externally applied electric field of the same frequency. Since the magnitude of the electrical conductivity of the tissue varies with the temperature the adiabatic temperature, changes produced by the acoustic disturbance cause a periodic variation in the conductivity that results in a net unidirectional transfer of charge when the frequenices of the two fields are equal. The condition for stimulation is expressed quantitatively by a relation involving the amplitudes of the electric and acoustic field parameters, the thermal coefficient of electric conductivity of the tissue, the threshold quantity of charge that must be transferred unidirectionally per unit area to result in stimulation, the pulse duration, and a parameter that is determined by the geometric orientation of the electric and ultrasonic field vectors at the acoustic focus. Numerical calculations suggest that stimulation can be produced in the absence of thermal or other mechanisms of damage to the tissue. The values of the charge‐stimulation and electrical‐conductivity parameters employed in the calculations are derived from available experimental data.

ISSN:0001-4966    

DOI:10.1121/1.1911230

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The denaturation of enzymes in solution by cavitating ultrasound has been reported previously. This report presents the results of an extensive study of the effects of noncavitating ultrasound on solutions of α‐chymotrypsin, trypsin, aldolase, lactate dehydrogenase, and ribonuclease. In one set of experiments, the solutions were irradiated and then analyzed to determine the effects on the physical and chemical properties of the enzyme molecules. Irradiations were carried out at differentpH values and temperatures using 1‐MHz ultrasound at an intensity of 75 W/cm2, 10‐min continuous exposure, and 11‐MHz ultrasound at an intensity of 1000 W/cm2, 2000 0.1‐sec pulses. Analytical procedures employed included measurements of enzyme activity, specific optical rotation, uv absorption spectrum, and sedimentation coefficient. In a second set of experiments, enzyme‐catalyzed reactions were irradiated with ultrasound and simultaneously monitored spectrophotometrically. Ultrasound in the intensity range 0.5–35 W/cm2at the frequencies 1, 9, and 27 MHz were employed with the temperature andpH held constant. Comparison of the results of this study with those from studies employing cavitation shows that cavitation is a necessary condition for ultrasonic denaturation of the five enzymes of this study.

ISSN:0001-4966    

DOI:10.1121/1.1911231

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The time course of masking of a 500‐Hz signal was determined for remote masking (noise from 2000 to 4000 Hz) and for direct masking (noise from 200 to 4000 Hz) under conditions of simultaneous and delayed onset of the tracking task. For half the conditions, listeners varied level of the tone against a constant background noise, and in the other half, they varied the noise level to mask a constant level of pure tone. The results showed a downward shift occurred during the first two minutes of tracking for both direct masking and remote masking. The shift in direct masking was influenced by the onset of tracking and the method of control, while the shift in remote masking was independent of these factors.

ISSN:0001-4966    

DOI:10.1121/1.1911232

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Interposed between periodic presentations of a 1000‐Hz tone were stimuli of different frequencies and intensities. The amplitude of the vertex (V) potential to the 1000‐Hz tone became larger as the frequency of the intervening stimuli departed progressively from 1000 Hz. It also became larger when the sensation level of the 1000‐Hz tone was raised progressively above that of the intervening stimuli. The notion that changes in stimulus frequency and increases in stimulus intensity activate new neural units was adopted to explain the data. It was asserted that habituation of theVpotential is greatest when the same neural units are repeatedly activated and least when new neural units are brought into play.

ISSN:0001-4966    

DOI:10.1121/1.1911233

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Cochlear microphonics (CM) were recorded with glass micropipet electrodes inserted in the scala media in the four turns of the guinea pig's cochlea. From the data acquired, the longitudinal distribution of the cochlear microphonics were determined. The points of maximum sensitivity for different frequencies along the cochlear partition were 2 mm apart for a change of 1 oct. By increasing the intensity of the stimuli, there was a shift of 4 mm of the point of maximum voltage of the CM toward the base of the cochlea. The slope of the proximal side of the envelope was steeper than that of the distal side. Both proximal and distal slopes doubled their magnitude for a change from 300 to 3000 Hz. At moderate sound‐pressure levels (i.e., 70 dB), the voltages of the CM were the same for all frequencies at the point of maximum stimulation.

ISSN:0001-4966    

DOI:10.1121/1.1911234

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Studies carried out upon subjects with normal hearing have shown that (a) the mean sound‐pressure level at which a tone of 1000 Hz is judged to be unpleasantly loud is 100 dB and (b) following sustained stimulation with intensities above 100 dB, there occurs a dramatic increase in the resultant temporary threshold shift. The intensity at which both these events occur is independent of sensation level. It follows that variations in normal threshold of the order of ±10 dB are largely sensorineural in orgin. In addition, the fact that both these unrelated events occur at the same sound‐pressure level suggests that it marks a physiological limit of loudness perception. The results lead to the conclusion that the loudness function shows considerable intersubject variability and is dependent upon hearing acuity. Their application to loudness computations and to hearing conservation is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1911235

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP