摘要:

A solution is given to an approximate form of the wave equation corresponding to a sound source located above ground in a wind gradient. Owing to the effective refraction of sound rays in the presence of a wind gradient, the sound field on the upwind side of such a source will exhibit a shadow provided the wind velocity increases with height. The behavior of the sound field diffracted into this region is found to be similar to that in a temperature‐created shadow. Within the normal region the solution reduces to ray acoustics in the high frequency limit. [Supported by Contract NAw‐6463 with the National Advisory Committee for Aeronautics.]

ISSN:0001-4966    

DOI:10.1121/1.1905021

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The instrumentation described in this paper was designed to facilitate a study of the propagation of speech over ground for distances of the order of one mile. The state of the atmosphere near the ground plays an important part in determining the propagation of the speech signal. Therefore, attention was directed to specialized micrometeorological instrumentation for determining the state of the atmosphere and relating it to the measured sound attenuation. An array of precision cup anemometers and thermocouples on a 30 foot tower are scanned automatically to provide a measure of the vertical wind velocity and temperature gradients. A high‐speed vane and thermistor are used to give a measure of atmospheric turbulence. The cup anemometers were calibrated in a wind tunnel and a gust calibration was made for the turbulence measuring devices. The electrical signals obtained from these systems are fed into a mobile laboratory for analysis and magnetic recording. The recording maintains the original time relationship with the acoustic signals picked up along two test courses. The mobile laboratory also contains a power amplifier with tape recorder input for driving the loudspeakers and equipment for automatically scanning the seven microphones on each course. This system was successfully used during a four‐week measurement program near DeBlois, Maine, during the fall of 1955. [This work was supported by the U. S. Signal Corps under Contract No. DA‐36‐039 SC‐64503.]

ISSN:0001-4966    

DOI:10.1121/1.1905029

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The loudness of bands of noise, equal in masking and of variable band width, is compared with the loudness of a sinusoidal tone having the midfrequency of the band of noise. When the band width is made very narrow the noise is as loud as the tone of equal sound‐pressure level. When the band width is increased above a certain value, the loudness increases while the sound power of the noise is held constant. The critical value at which this increase begins is measured as a function of the midfrequency of the noise. The results are consistent with known “critical bands” having a band width of 80 cps below 500 cps and a relative band width of 17% above 500 cps. For low and medium levels of noise, total loudness can be computed by determining the energy falling into the different critical bands and adding the loudness corresponding to this energy in each band. For high noise levels a substantial difference is found between computed and measured loudness. This difference is caused by mutual masking. A graphical procedure, using curves similar to the familiar masked‐threshold curves, can be used to make allowance for this masking, and the procedure makes possible the computation of the loudness of high levels of continuous noise if its power spectrum is known. The loudness of white noise is computed as a function of its intensity level with results in good agreement with experimental determinations of the present and other authors.

ISSN:0001-4966    

DOI:10.1121/1.1905031

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Response curves (in terms of percent of positive responses) to intensity variations at levels of 10, 20, or 40 db above threshold while stimulating the opposite ear with a continuous tone of the same frequency (1000 cps) at a level of 10, 20, 40, or 80 db above threshold, were studied on three subjects. In all cases, a contralateral continuous tone, although of same frequency, provokes always an increase of the differential threshold in the tested ear. The differential threshold is directly related to the intensity applied on the opposite ear, but inversely related to the intensity applied on the tested ear. By combining, for the same subjects, the values of undisturbed thresholds and of those disturbed by a contralateral tone, we can translate results into equivalent levels and to equivalent values of attenuation. The equivalent level decreases rapidly, and the decrease is steeper as the sound intensity on the tested ear is increased. The experiments suggest that the effect described above is not due to masking. The author proposes the idea that a sufficient contrast relative to the constant background (resulting from the tone applied on the contralateral ear) is necessary for perceiving differences of intensity.

ISSN:0001-4966    

DOI:10.1121/1.1905036

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

As absorbing material, a thin sheet of aluminum or plastic impregnated cardboard perforated with many very small holes is used. The flow resistance is between 20 and 80 g cm−2sec−1. By forming a cone of the material a sufficient air volume behind the perforated sheet is achieved and high absorption at low frequency is obtained. The construction of some mechanical fixtures facilitating the assembly of the cone and fixing the assembled cone in the ceiling will be shown. Some practical use of the new type of absorbers, together with measured results of its absorbing characteristics will be shown.

ISSN:0001-4966    

DOI:10.1121/1.1905048

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The important factors involved in the development of a phonetic typewriter are as follows: the particular form in which the words are typed, the means for analyzing the sounds of speech, the identification of the analyzed sounds, the encoding and decoding of the sounds for the operation of the actuating mechanism, and the design of the mechanism for actuating the typewriter. A study has been made of these problems. As a result of this study a simplified model of a phonetic typewriter has been developed incorporating all of these aspects. This model serves to illustrate the principles involved and provides means for further study towards a complete system.

ISSN:0001-4966    

DOI:10.1121/1.1905052

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

An evaluation of the intelligibility of speech transmitted by a formant‐coding compression system is described. The compression system codes the input speech in terms of six electrical signals, representing the frequencies of the first three formants, the intensity of voicing and of friction, and the fundamental vocal frequency. These signals occupy a total band width of approximately 50 cps and require signal‐to‐noise ratios of approximately 30 db for their transmission. Natural monosyllabic utterances are transmitted through the compression system and are presented to listeners for identification. Vowel and consonant articulation scores are computed and sound‐confusion matrices are constructed. The contribution to intelligibility of various components of the system and the inherent limitations of the system are examined and discussed. [This work was supported in part by the Air Force Cambridge Research Center under Contract No. AF 19(604)‐626.]

ISSN:0001-4966    

DOI:10.1121/1.1905057

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

In the revolution of musical doctrine under the impact of electroacoustical techniques, informational theory brings some concepts for building a new theoretical system which is now conspicuously lacking. Music is one of the messages of our environment, and the human channel, as a consequence of psychophysical uncertainty principles, determines basically the repertories of sound symbols. According to the point of view of the receiver, various repertories appear as “semantic” (intelligible) and “aesthetic” (for a large part sensorial) messages intricately involved in the sound sequences. It is possible to separate experimentally informational contents related to the various repertories, which can be distinguished according to the time scale of perception. The system of values thus obtained gives a metrical pattern of the musical signal. Taking into account the variations of informational rates which a particular individual can grasp, it is possible to reach some general rules governing the musical signal, especially in the simple cases of the work of arrangers of light music, which is more clearly defined after the capacity of the listener. These rules appear particularly useful in the new field of experimental music which makes a wide use of electronic and acoustical means.

ISSN:0001-4966    

DOI:10.1121/1.1905063

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

We give a new method of measuring the difference of ultrasonic absorption of two liquids. Two pulse generators excite two ultrasonic quartz transducers in two vessels. Having passed through the two vessels, the pulses are reflected from the opposite walls of the vessels and are received by the same quartz transducers. The received signals are amplified, detected, and then amplified again by a differential dc amplifier at the output of which there is a milliammeter. The milliammeter readings are proportional to the difference of two liquids absorption coefficients. The sensibility of the apparatus is evaluated by the valuexΔα giving a current 1 ma (x—the double width of the vessel, Δα—the difference between absorption coefficients). For example, atx= 9 cm the sensibility equals Δα = 0.01 cm−1per 1 ma. The accuracy of Δα measurement is nearly 3–5%. By means of the apparatus described in the foregoing, one can measure the absorption of ultrasonic diluted solutions, the temperature absorption coefficients of ultrasonic waves in liquids, coefficients of reflections from solid‐liquid, liquid‐liquid boundaries, and other applications.

ISSN:0001-4966    

DOI:10.1121/1.1905069

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A new method for measurements of the dispersion of waves is described. The experiment gives directly the value ofD=d2k/dω2(k, wave number; ω, angular frequency). The measurements ofDis based on the transformation of phase modulation in amplitude modulation and vice versa, when a modulated sine wave travels through a dispersive medium. The sensitivity of the method depends only on the noise level in the receiver and it is independent of the unavoidable small random variations of the sound velocity in the medium under test. The results of experiments on ultrasonic waves in thin wires (longitudinal waves where used) and in water at 1 Me/sec (modulation frequency: 100 kc/sec) are given. The experiments were undertaken for determining the possibilities of the method in acoustics. In the case of longitudinal waves in a thin wire the dispersion can be calculated from the theory of elasticity. The value ofDobtained experimentally is in a good agreement with the calculated one. The relative change of the phase velocity corresponding to the frequency interval of 100 kc/sec was about 10−4In water, no dispersion was observed that agrees with the theory. A variation of velocity of 1 cm/sec in a frequency interval of 100 kc/sec was detectable when the receiver band was about 1 kc/sec. In order to prevent reflected waves an optical indicator of ultrasonic waves was used.

ISSN:0001-4966    

DOI:10.1121/1.1905070

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP