摘要:

Measurements were made of the masking of pure tones by bands of noise, each band width corresponding to a pitch interval of 250 mels, with band level varied from 40 to 120 db (above 0.0002 microbar). A modified Békésy audiometer permitted recording as a function of frequency, so that masked audiograms could be compared with the noise spectra to ascertain how well the ear followed the frequency characteristics of the noise bands at different levels. It was found that values for the critical band according to Fletcher and to Hawkins and Stevens predict quite precisely the masking for tones whose frequencies lie within the band. More masking than would be predicted is found for frequencies above the band, the discrepancy increasing as the noise level increases. Below the band, the masked threshold is again predicted precisely by the critical‐band hypothesis, but only to a point, beyond which, for high noise levels, a “remote” masking is observed that is constant with frequency for any particular level and is much more than can be predicted from the effective levels (from either system noise or imperfect band rejection) obtaining at those low frequencies where this masking is observed. Whereas masking for tones within the band increases linearly with level, this “remote” masking rises in an accelerated manner, as a function of level, and then shows deceleration at the highest levels. Some speculative interpretation is attempted.

ISSN:0001-4966    

DOI:10.1121/1.1908426

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The sound‐exclusion properties (acoustic attenuation) of various earphones and ear defenders and combinations thereof were determined by the binaural, free‐field, absolute‐threshold‐shift technique with the psychophysical method of adjustment. The earphones included dynamic earphones in two different sockets and hearing‐aid‐type receivers with individually molded inserts. The ear defenders included certain conventional and newly developed earplugs and muffs, and a hard‐shelled noise‐exclusion helmet.In a preliminary evaluation of a limited number of devices, both tones and noise bands were used as stimuli, and comparative results are shown. In evaluating the effectiveness of some of the newer devices, noise bands alone were used. For the exclusion of external sound, particularly that below 1200 cps, the order of merit for earphone combinations was: (l) large volume muffs with insert phones; (2) helmet with insert phones; (3) semiplastic sockets with dynamic phones; and (4) helmet with dynamic phones. The best combination provided the following amounts of attenuation in the seven octave bands from 75 to 9600 cps: 14, 22, 28, 33, 38, 44, and 40.

ISSN:0001-4966    

DOI:10.1121/1.1908428

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The maximal interaural time difference that could be introduced into one channel without loss of sidedness was determined for various noise bands. Through the presentation of stimuli by several methods it was found that time values vary with frequency of the noise bands, being greater for lower bands. Marked intersubject differences were found. For the lowest and most effective band used (106–212∼), times ranged from 7.5 to 20.6 msec.

ISSN:0001-4966    

DOI:10.1121/1.1908430

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The speed and attenuation of sound at 11 Mc were measured in He, Ne, A, Kr, and Xe at various pressures between atmospheric and a few mm Hg, and the results were compared with existing theories.

ISSN:0001-4966    

DOI:10.1121/1.1908432

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

An ultrasonic pulse method is used to measure the velocity dispersion due to changes in sound intensity in three fatty acids. With constant frequency the velocity increases with increasing sound amplitude in the ranges from the melting points to approximately 15°C above.

ISSN:0001-4966    

DOI:10.1121/1.1908434

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

In this paper a new method for measurement of absorption of ultrasonic waves in liquids is developed. Its main feature is a low frequency (L.F.) modulated ultrasonic transducer actuating a thin circular disk which is placed at some distance above it within the ultrasonic cell. The disk is suspended by a glass rod from a metallic reed which forms one plate of a small air condenser. The vibrations of the suspended system, which are resonant with the L.F. modulating component, are detected by a crystal controlled oscillator, working at the steep portion of its crevasse. The output of the oscillator is applied to a wave analyzer whose output gives directly a measure of the ultrasonic pressure on the disk.In a further development in the detecting end, a null method is used in which the reed is brought to rest by a L.F. balancing voltage supplied by the same modulator and the zero reading is adjusted by the aid of a cathode‐ray oscillograph.Two liquids are chosen for testing the apparatus, namely, benzene and water representing both high and low absorptive liquids, respectively. The values obtained are,2α/v2 = 16.2×10−16 sec2 cm−1for benzene, and2α/v2 = 52×10−17 sec2 cm−1for water, which compare favorably with the values obtained by the more elaborate pulse method. Moreover, the high sensitivity of the method eliminates all the complications which usually accompany high‐power ultrasonic beams.

ISSN:0001-4966    

DOI:10.1121/1.1908436

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

An account is given of experiments carried out in the lower troposphere to compare observations of audible range and of angle of descent of sound from shell bursts at various heights up to 10 000 ft with calculations made from elaborately measured temperatures and winds. In stable conditions there is satisfactory agreement. Several cases were observed of anomalous propagation in which sound rays starting upward from the source are bent back to the earth. These showed the phenomena associated with anomalous propagation on the larger scale, namely, inner and outer audibility zones, “zone of silence,” and double or multiple reception of the single pulse from the source. From the experience gained in these experiments suggestions are made for (1) observation of the time interval between components of the usual double or multiple bangs from a single pulse source and (2) observation of the sound from explosions in the air as well as on the ground to obtain more information than has been obtained hitherto from sound propagation, about temperature and wind in the high atmosphere. Rocket explosions might be used for the latter purpose.

ISSN:0001-4966    

DOI:10.1121/1.1908438

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A theoretical analysis is made of the reflection of a plane wave of sound from a surface whose acoustic impedance is not uniform. The sound pressure received at a distant point consists of a specularly reflected wave plus scattered radiation. The magnitude of the specularly reflected wave is independent of wavelength and depends upon the impedance of the portion of the surface which may be viewed at the specular angle from the receiver. If the impedance differs only slightly from a constant value the directivity pattern of the scattered radiation is determined by the product of three factors.The condition for the scattered radiation to obey Lambert's cosine law is that the reflecting surface is one of approximately zero pressure and that there is no correlation in the impedance irregularities. The condition for perfectly diffuse scattered radiation is that the surface behave as approximately rigid with no correlation in the impedance irregularities. In the case of a surface whose impedance irregularities have a slow statistical variation with position the scattered radiation is confined to a beam whose axis lies along the direction of specular reflection but the energy within the beam is not necessarily closely confined to the specular angle.

ISSN:0001-4966    

DOI:10.1121/1.1908440

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A plane sound wave is assumed to be incident upon an irregular pressure release surfacez=ζ (x,y). The solution for the reflected field is regarded as a superposition of plane waves having an amplitude spectrumA(k1,k2). Next, the Fourier transformG(x,y) ofA(k1,k2) is introduced and subjected to the boundary condition on ζ This leads to an integral equation forG(x,y) that cannot be readily solved. However, if one causesG(x,y) to depend exponentially on a functionu(x,y), then a differential equation may be derived from this integral equation, the solution of which gives an approximate form ofu(x,y); the degree of approximation involved depends on the smallness of ζ.This method is applied to the problem of sound scattering from a one‐dimensional sinusoidally corrugated surface and the results compared with experimental measurements of LaCasce and Tamarkin and also with the results of a theory due to Rayleigh. This comparison shows the predictions of the theory presented here to be as good as the Rayleigh theory in all cases and closer in the majority of cases.

ISSN:0001-4966    

DOI:10.1121/1.1908442

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A proposed technique for measuring the noise of a ducted fan by means of a microphone placed inside the fan duct was evaluated by comparing results so obtained with those obtained by measuring the total sound energy radiated from the duct into a reverberation chamber. A suitable microphone windscreen was developed and was used to reduce the aerodynamic self‐noise generated by the microphone to a negligible minimum. Good correlation was obtained, and by studying transverse resonance conditions in a duct it was attempted to show how suitable microphone stations could be found to eliminate measuring errors caused by the existence of standing waves.

ISSN:0001-4966    

DOI:10.1121/1.1908444

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP