摘要:

Acoustic waves are dispersed by two distinct mechanisms as they propagate through a fiber‐reinforced viscoelastic material. The first mechanism, viscoelastic dispersion is characterized by an increase in phase velocity with an increase in frequency and an exponential attenuation of wave amplitude with propagation distance. The second, geometric dispersion may be characterized as a wave‐filtering phenomenon in which periodic waves are selectively transmitted and reflected. Regions of transmission are commonly called “passbands” and regions of reflections have been designated “forbidden bands.” When both mechanisms are present in one material, one or the other may be dominate. In this study, the interplay between these two mechanisms is examined experimentally by determining the dispersion characteristics of two fiber‐reinforced viscoelastic materials (viscoelastic dispersion dominating in one and geometric dispersion dominating in the other).

ISSN:0001-4966    

DOI:10.1121/1.3437190

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The properties and some potential applications of a high damping copper‐manganese‐based alloy are described. The applications illustrated include the use of the high damping property to suppress mechanical vibrations in engineering structures and machine parts and hence to reduce radiated sound levels. [This study is supported by The International Copper Research Association.]

ISSN:0001-4966    

DOI:10.1121/1.3437191

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Two aspects of coarticulation are examined in deaf and normally hearing adolescents: (1) the influence of the final consonant (/t,s,n,d,z/) on the duration of /i/ and /I/, and (2) the influence of initial consonants (/b,d,f/) on theF2frequency of /i/ and /u/. Whereas in normal speech the tense vowel /i/ is only relatively longer than the lax vowel /i/, in the speech of the deaf subjects these two vowels occupy more restricted durational ranges. The vowel does not appear to coarticulate with the final consonant in the normal manner, and thus does not aid in signaling the consonant. In the speech sample examined, initialF2transitions appear “reduced”—both the amount of frequency change and the rate of frequency change are less than for normal subjects. Transitions do not appear to result automatically from the mechanical characteristics of the vocal tract. Results are discussed with reference to the possible role of learning in coarticulation effects. [Supported by NINDS Grant NS03856.]

ISSN:0001-4966    

DOI:10.1121/1.3437202

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Most studies on the early detection of laryngeal pathology have been based upon perception. The few acoustical studies performed have been based upon pitch perturbation, amplitude perturbation, or inverse glottal wave filtering. Each of these techniques has rather severe limitations depending upon the degree of the pathology being studied. In this study a new measure is introduced for early detection of laryngeal pathology based upon a linear speech production model. The signal of interest is referred to as a residue signal and is obtained by extracting both the vocal tract characteristics (as with inverse glottal wave filtering) and the smoothed or trend characteristics of the glottal volume velocity and lip radiation characteristics. The analysis technique is easily automated and even though it does not resolve difficult problems such as source‐tract inseparability for severe cases, it has been demonstrated to be a fairly sensitive indicator of laryngeal disease in a study of ten normal and ten pathological speakers. Physical properties of this technique will be given along with examples. [This research has been supported by the National Institutes of Health under Grant No. RO‐NB‐08177.]

ISSN:0001-4966    

DOI:10.1121/1.3437208

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Lighthill's theory of jet noise employs a nonconvected wave equation forced by a certain source term. In its exact form this term accounts fully for the co‐moving fluid, including convection and refraction effects. In practice, holding the density factor constant suppresses the latter and throws some doubt on values of the former. Alternate formulations via convected wave equations are gaining interest. An attempt is made to bring some coherence into the subject.

ISSN:0001-4966    

DOI:10.1121/1.3437212

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Much confusion about jet noise has come about as a direct result of inadequate facilities and insufficient knowledge and control of test conditions, in many cases giving rise to completely erroneous conclusions. The facility described here has been carefully designed, accounting for the shortcomings of other facilities and being guided by the stringent demands of ongoing jet noise research at Lockheed. The design goal was the capability of testing model jets up to 2000°F at pressure ratios as high as 8, in a free‐field environment anechoic at all frequencies above 200 Hz. A comprehensive series of flow visualization and temperature mapping experiments in a one‐sixth scale‐model anechoic room was conducted. The results dictated the design of the exhaust collector/muffler to provide entrainment and room cooling air in the quantities demanded by the jet operating conditions. In order to optimize the choice of material and anechoic wedge design to achieve the 200‐Hz requirement, a special impedance tube was used extensively in performance evaluations. Some features of the facility are (1) an exhaust collector providing air in quantities dictated by the particular jet operating condition with no special forced‐air injection or fan installation and (2) a “cherry‐picker” crane used to gain access to instrumentation, etc., for maintenance, calibration, and setup, thus eliminating the need for access platforms, etc. The crane is stowed by remote control under an anechoic cover during all test operations. The superior performance of the room is described by intensity/distance plots obtained from a fixed loudspeaker source and moving microphone installation.

ISSN:0001-4966    

DOI:10.1121/1.3437223

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The need for detailed knowledge of conditions at the interface of the free stream at the mouth of a cavity resonator has led to the development of a computer‐assisted technique for velocity profile measurement. Basically, hot wire data are recorded simultaneously with sound pressure microphone data so as to permit later sorting out: of local velocity readings as a function of phase in the acoustic cycle. From the ensemble of stored surveys, turbulent velocity fluctuations can be averaged out and the time sequence of oscillating profile configurations can be reconstructed. By subtracting the mean profile from each element of the sequence, the profile of the underlying instability wave is exposed. Another use of the results is the determination of the fluctuating component of volume flow into the cavity by numerical integration. [Sponsored by Naval Ship Systems Command under a GHR contract administered by NSRDC]

ISSN:0001-4966    

DOI:10.1121/1.3437226

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

In an effort to further describe the monaural and binaural processes involved in the detection of a gated tonal signal in a background of gated masking noise, a series of experiments has been initiated utilizing computer‐generated, reproducible waveforms. The signal used has been a gated 500‐Hz tone and the masker is a gated 400–600‐Hz bandpass noise. Our initial effort has been concerned with the influence of the magnitude of the phase angle alpha, the angle at which the tonal signal is added to the noise masker, on the detect‐ability of the signal in both homophasic and antiphasic masking conditions. Our results suggest that the magnitude of alpha is extremely important in accounting for performance in the homophasic masking conditions but that the magnitude of alpha does not strongly influence performance in the anti‐phasic masking conditions. These results are compared to predictions based on an electrical model described by Jeffress [J. Acoust. Soc. Am.41, 480–488 (1967)], which consists of a leaky integrator preceded by a narrow bandpass filter and half‐wave rectifier.

ISSN:0001-4966    

DOI:10.1121/1.3437229

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

Masked thresholds were measured for signal frequencies below, within, and above two 190‐Hz noise bands centered arithmetically at 1125 Hz. One noise band was computer synthesized and had amplitude fluctuations characteristic of analog noise bands typically used in masking experiments. The other noise band, matched in spectrum and level to the synthesized noise band, was generated by modulating the frequency of a sinusoid with a low‐pass filtered, clipped, Gaussian noise. The resulting waveform was free of envelope fluctuations. Masked thresholds in the FM noise were 5–10 dB lower than in the synthetic noise in the flat portions of the spectra. Synthesized noise resulted in trapezoidal appearing audiograms. The FM noise audiogram tended to be unimodal. These results are interpreted as evidence in support of envelope detection hypotheses which predict substantially improved performance in the detection of signals in noise when the noise has a constant envelope.

ISSN:0001-4966    

DOI:10.1121/1.3437237

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP

摘要:

The scattering of a plane wave by a thin spherical shell surrounding a solid sphere is determined. The results are given for shells of uniform thickness and axially symmetric cases for shells of nonuniform thickness. For a shell of uniform thickness the method of separation of variables can be applied, but for shells of nonuniform thickness the more general method described must be used. Numerical results are given for the scattering coefficients, target strength and farfield pressure distribution. It is shown that replacing a solid sphere by a thin spherical shell of the same diameter which surrounds a smaller internal solid sphere can lead to variations in the target strength of the order of ±10 dB. The results depend on the interaction between waves reflected from the inner body and the shell. Thus, important parameters are the spacing between body and shell and the wavelength. As body spacing and frequency are increased, the effect is to increase the average target strength.

ISSN:0001-4966    

DOI:10.1121/1.3437244

出版商:Acoustical Society of America    

年代:1974

数据来源: AIP