ISSN:0001-4966    

DOI:10.1121/1.1906915

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

The conventional methods of design of axial flow compressors usually consider the losses introduced by the generation of noise to be of negligible magnitudes and therefore ignore them entirely in a study of compressors. As a result, there is scant information on the relation between the noise level and the performance properties of compressors. In the present paper an attempt is made at working out the basis of a method of evaluation of the noise characteristics from compressors. The method makes considerable use of the experimental findings of Beranek and Rudmose on noise due to airplane propellers. The analysis presented here considers the noise to be generated by a number of point sources of equal magnitudes but of random phase. The resultant intensity level of these point sources (presumed to be located at the tip of the compressor blades) is calculated on a statistical basis. The expected noise level is given as a function of blade tip speed, clearance between blades and stator case, number of blades and the power expended in driving the compressor.

ISSN:0001-4966    

DOI:10.1121/1.1917467

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

Two or three pairs of nichrome steel electrodes (20μ) are inserted into each of two or three turns of the cochlea of guinea pig. One electrode of each pair is in scala tympani, one is scala vestibuli. Such “differential” leads record nearly pure cochlear microphonics from segments of the cochlear partition only about 1 mm long. Low tones evoke microphonics from all four turns; but as the frequency is increased (holding the response from the basal turn constant by adjusting the intensity of sound), the microphonic ceases, first at the apical turn, then at the third turn, and finally at the second turn. The normalspace patternfor each frequency as revealed by its cochlear microphonic is a long plateau rising slowly from the basal end but with a much more abrupt fall toward the apex. The locus of this abrupt fall is characteristic for each frequency. Thespace-timepattern of an acoustic signal is a decelerating traveling wave moving from base toward apex. The velocity of a 500-cycle wave falls from about 70 m/sec in the basal turn to about 2 m/sec near the helicotrema. These electrical patterns are in general agreement with the mechanical patterns described by Békésy.The space and space-time patterns remain normal even when large sections have been removed from the bony shell of the second turn, over both scala vestibuli and scala tympani. The traveling wave still originates in the basal turn even when the cochlea is stimulated by sound transmitted through the fluid in a small pipette that enters scala vestibuli of the third turn through a hole in the bony shell.

ISSN:0001-4966    

DOI:10.1121/1.1917474

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

Audiometric tests (pure tones and speech) were conducted on deafened ears in quiet and at several levels of calibrated white noise. The cases were classed into six groups based upon results of their threshold audiograms. “Recession” describes the gradual approach toward normal levels with increasing noise of masking curves obtained from such ears. In cases displaying recruitment recession was relatively rapid, although not complete in some cases. A pure tone‐to‐speech perception ratio was established between the three audiometer frequencies within the speech range and the level of the 50 percent spondee score. Corrections were applied for (1) the type of loss and (2) the subject's unfamiliarity with the quietness of audiometric test rooms. This ratio is expected to differentiate speech losses as to their auditory or extra‐auditory origin. From the results of speech tests, the “Social Adequacy Index for Hearing” in noise was evaluated in the same manner customarily used under quiet conditions. Adjustment of the test results was necessitated because of the narrowing of the “Useful Auditory Area” induced by noise. The SAI frequently increased but in some cases decreased with noise, indicating the relative hearing efficiency in noise of deafened subjects.

ISSN:0001-4966    

DOI:10.1121/1.1917479

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

Because of the statistical nature of speech sounds, time-exposure photographs of the display on a Panoramic Analyzer can be used as high speed acoustic spectrograms of impulsive sounds. For speech systems, the analyzer is modified to sweep at the syllable articulation rate. The photographs can be interpreted to show quantitatively a number of characteristics of speech communication systems. So far, this process has been applied to the study of the characteristics of speech recordings and in the calibration of speech audiometers.

ISSN:0001-4966    

DOI:10.1121/1.1917485

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A screening audiometer has been developed, which functions automatically. It has the following salient features: The subjects are required to make simple numerical choices as indicated by visual instructions. A single numerical choice for each frequency serves to evaluate the two ears separately. The stimuli consist of interrupted tones of pre-set levels. The operation of this instrument and the results of the field tests are presented.

ISSN:0001-4966    

DOI:10.1121/1.1917491

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A corner loudspeaker‐enclosure combination is described which embodies a modification of the acoustic phase inverter. The design incorporates an array of perforations to provide the inductance. Control of theQis achieved by varying the size and shape of the array. An electrical analog has been built and tests indicate good agreement between the analog response and the measured response of the loudspeaker. The loudspeaker to be demonstrated has a response flat to within ±4 db from 45 cps to 13,000 cps and occupies a volume slightly larger than one‐half cubic foot.

ISSN:0001-4966    

DOI:10.1121/1.1917498

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A semicircular array, 30 ft in diameter, of 41 five‐inch loudspeakers has been built in the Harvard anechoic chamber to provide a two‐dimensional isotropic noise background. A separate gas‐tube noise generator powers each speaker. Signals can be superposed on the noise applied to any speaker. With this array it is possible to study methods of detecting and locating signals in such a background. Included in the instrumentation is circuitry for the measurement of the signal‐to‐fluctuation‐noise ratio at the detector output. Preliminary experiments using a two‐element array and a square‐law or correlation detector have been carried out. The measured patterns and signal‐to‐noise ratios are in good agreement with theory.

ISSN:0001-4966    

DOI:10.1121/1.1917503

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

A technique reported by Barone and Nuovo has been adapted to the measurement of ultrasonic absorption in liquids. The output of an rf transmitter is 100 percent modulated at an audiofrequency and applied to a crystal. This results in a periodic variation of the radiation pressure in the acoustic beam. A condenser microphone has been constructed, employing a front face consisting of a thin sheet of plastic, coated with silver paint. The microphone responds to the audiofrequency variation radiation pressure, and its output is preamplified and then detected on a harmonic analyzer. The apparatus has been tested in water in the frequency range 9 to 30 mc, and the measured values of the absorption coefficient agree with the accepted values within a few percent. Measurements of the absorption coefficient have been carried out in a number of uni‐univalent electrolytic solutions.

ISSN:0001-4966    

DOI:10.1121/1.1917510

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP

摘要:

Using a 2.5‐megacycle spherical focusing radiator, cavitation is produced in water at a location remote from any liquid‐solid interface. The cavitating region is localized in the high intensity core of the focal region, about one millimeter diameter by 10 millimeters long. A feather‐like cavitation burst is sporadically produced within the focal region. High speed camera studies show that the total duration of the burst is less than 3 milliseconds, and that the cavitation at any one point of the burst is completed in a matter of microseconds. The cavitating region propagates along the feather shaped burst from the quill end (nearest the radiator) to the tip end, at a velocity of 5 to 10 meters per second. It is believed that this velocity corresponds to that of the radiation pressure induced streaming velocity of the liquid along the axis, of the focal spot, and that a discrete, traveling, oscillating cavity initiates the surrounding cavitation. These sporadically produced cavitation bursts appear without other effects in degassed water. On the other hand, in aerated water identical appearing bursts produce noncollapsing air bubbles which blow off down stream (because of liquid streaming) providing there is no standing wave pattern present. In the presence of standing waves many of the bubbles are trapped in the standing wave pattern until they grow too large (by accumulation of other bubbles) when they again blow down stream. Slides showing the nature of these phenomena will be exhibited.

ISSN:0001-4966    

DOI:10.1121/1.1917515

出版商:Acoustical Society of America    

年代:1952

数据来源: AIP