摘要:

Acoustic waves generated by a point source in the ocean or atmosphere may be reflected due tototal internal reflectionby regions of increasing sound velocity‐ i.e., by the bending back of ray paths at turning points. This phenomenon is usually accompanied by the simultaneous formation ofcaustics. The relationship between caustics and turning point effects can be explored by the use of WKB and stationary phase approximations for the case of a harmonic point source in a homogeneous fluid overlying a stratified section. If one examines the behavior of reflected waves traveling along a given ray path, one sees that the conditions for formation of a caustic in the stratified section and for a −π/2 phase shift occurringwithinthis section are identical. If, on the other hand, the point source is removed to infinity, one recovers the plane‐wave case: The caustic becomes the horizontal turning‐point plane and the −π/2 phase shift becomes a turning‐point effect. These features are reviewed; and it is concluded that, in stratified media, the −π/2 phase shift due to a turning point is simply the plane‐wave limit of the caustic effect, a limit that is approached asymptotically when the source is removed to great distances.

ISSN:0001-4966    

DOI:10.1121/1.1911160

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Mathematical definitions of all tube cavity vibration modes are developed from experimental results. The crossover from one mode to another is defined in terms of pertinent design parameters. A simple, accurate, and first time‐presented solution is developed for the spring‐mass vibration mode. The higher harmonics associated with the sping‐mass resonators, commonly called Helmholtz resonators, are identified from experimental data. Tube cavity limit conditions of zero length as well as zero and infinite volumes are included in this study. The resonance modes observed are spring mass, half‐wave, and quarter‐wave. Using the new solution for the spring‐mass mode, natural resonances are found to be bracketed by the limit conditions of the adiabatic and isothermal processes. The classical works of Von Helmholtz and Rayleigh concerning Helmholtz resonators is extended to include boundary‐layer theory, which theory was not introduced by L. Prandtl until 1904, a considerable time after the work of Von Helmholtz and Rayleigh.

ISSN:0001-4966    

DOI:10.1121/1.1911161

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The method of studying speech perception by means of a formant synthesizer has been facilitated by the use of a small digital computer to control the synthesizer, record the listeners' responses, and analyze the results. This method has been applied to the perception of the semivowels /w, r, l, y/ in isolated CV syllables. It has been found that the frequency of the locus of the first formant greatly affects the quality of the perceived sounds, and that the shape of the first‐formant transition is rather critical in the case of /l/ before open‐front vowels.

ISSN:0001-4966    

DOI:10.1121/1.1911162

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

Previous research showed that the logarithm of the absolute voltage of a speech waveform is nearly uniformly distributed between same arbitrary threshold and a peak. In the present study, the equivalent peak level (epl) of a speech sample is defined as that peak of a log‐uniformly distributed random variable that would have produced the same rms above threshold as was measured from the speech sample. Measurements on 116 speech samples show that if threshold is varied over a range of at least 35 dB on repeated measurements of the same speech sample, theeplvaries ±1 dB. Equivalently, if the measuring threshold is held constant, theeplwill follow known speech level changes on a decibel‐for‐decibel basis to within ±1 dB over a 35‐dB range. Theeplmay be measured with any facility having means of obtaining rms‐above‐threshold.

ISSN:0001-4966    

DOI:10.1121/1.1911163

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

This study was conducted to examine the differential effects upon vowel intelligibility of various degrees of time compression (TC) and frequency division (FD), with and without time restoration. A male speaker and a female speaker were used. For a given percentage of distortion, FD degrades vowel intelligibility more severely than TC. Restoring time to normal for FD speech produced by slow playing via the Fairbanks speech compressor does not enhance intelligibility. Vowel confusions under TC are related to duration; those for FD conditions appear to be closely related to the perception of Vowel Formant 2 and to a lesser degree, Vowel Formant 1. Patterns of male and female vowel confusions are generally much alike for all conditions and types of distortion. Results tentatively indicate superior female vowel intelligibility under all conditions of distortion, the advantage being largest for FD, somewhat less for TC. These results suggest that over a limited range of frequency division (up to 40%), vowel phonemic quality is relatively unaffected by proportionate shifting of fundamental frequency and formant structure, indicating that a “relative‐vowel” hypothesis of vowel phonemic quality may hold for limited shifts in the frequency of vowel spectra.

ISSN:0001-4966    

DOI:10.1121/1.1911164

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The absorption and dispersion of ultrasonic waves in binary mixtures of inert, monatomic gases is studied using the two‐fluid theory of gas mixtures. Expressions are derived for the absorption and dispersion at all ultrasonic frequencies. At low frequencies, these expressions reduce to those verified experimentally by Holmes and Tempest [Proc. Phys. Soc. (London)75, 898–904 (1960)], while at higher frequencies new viscous effects are shown to be present.

ISSN:0001-4966    

DOI:10.1121/1.1911165

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

We have used Mason and Bateman's formulation of the Akhiezer mechanism to compute the ultrasonic attenuation in quartz. The results are compared with the experimental measurements of various workers. For the seven pure‐mode ultrasonic waves that propagate on thex, y, andzaxes in quartz the agreement between the calculated and measured attenuation coefficients is always to better than a factor of 2. This is the first extensive calculation of its kind on a trigonal crystal, and it is particularly gratifying that the theory explains the approximate equality of the attenuations of longitudinal and transverse waves. We have also performed similar calculations on fused silica, but in this case the measured attenuation exceeds the Akhiezer attenuation by more than an order of magnitude. It appears that some other relaxation mechanism dominates the attenuation in this material.

ISSN:0001-4966    

DOI:10.1121/1.1911166

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

The design parameters and operation of a new type of ultrasonic resonator are described. The resonator consists of a solid cylindrical body that contains a centrisimetric conic cavity and is driven by a piezoelectric disk bonded to the base of the body. The system thus generated vibrates in a radial mode, and the maximum amplitude is measured at the rim of the system. Optimum vibration conditions are obtained when the ratio of height to radius of the resonator is 1.24. An approximate equation is derived for the frequency of vibration as a function of the properties of the material and the physical dimensions. A number of models were constructed, and their capabilities as atomizers were studied. Aerosol conversion capabilities were found to be in excess of 1350 ml of waterlike liquid per minute with an aerosol‐generating area of approximately 24 cm2for the fundamental‐mode‐type resonator. The edsign of fourth enharmonic resonators whose cross sections are similar to the ones described is discussed briefly.

ISSN:0001-4966    

DOI:10.1121/1.1911167

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

A method is presented for measuring ultrasonic attenuation in thin solid specimens. The sample is bonded to one end of a buffer rod, and a transducer is bonded to the other end. All three elements must have plane parallel faces. The system is excited by an rf burst shorter than the round‐trip travel time of the elastic wave in the buffer but longer than three times the round‐trip travel time in the specimen. The specimen and its interface with the buffer act as a multiple‐path elastic wave interferometer. The envelope of the returning echo in the buffer rod shows a stepwise amplitude modulation, one step per round trip in the specimen. The amplitudes of the first three steps are measured at rf frequencies at which the phase delay in the specimen is an integer multiple of π. From the amplitudes, the attenuation in the specimen and the reflection coefficient at the buffer‐specimen interface are computed. Measurements can be made over a broad frequency band with a single transducer, so the frequency dependence of the attenuation can be determined. The method has been applied to thin brass samples. The results are presented and interpreted.

ISSN:0001-4966    

DOI:10.1121/1.1911168

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP

摘要:

By superimposing radiation spherical wave modes of the scalar‐wave equation, a beam is formed. The farfield of the chosen beam is the truncated delta‐function spectrum expanded on the basis of Legendre functions. By introducing an error criterion, the passage from the farfield to the nearfield is investigated. It is found that the nearfield along the direction of the main beam begins approximately at a radial distancernf, given byrnf= 2G/k(G=GAIN,k= 2π/λ, λ is the wavelength) and for other directions, on the average, byr̄nf= 2√G/k. For both radial distancesrnf, andr̄nf, the corresponding errors are bounded between 8% and 15%.

ISSN:0001-4966    

DOI:10.1121/1.1911169

出版商:Acoustical Society of America    

年代:1968

数据来源: AIP