摘要:

The attenuation of several ear protectors has been measured by a loudness balance technique in which the subject, upon removal of the device, raised the ambient sound level to match loudness to that which existed while wearing the ear protectors. Results were obtained for half‐octave bands of thermal noise in a diffuse sound field and for single frequency tones in free field. Attenuation measured by the former method was less at both low and high frequencies than for the latter. Both loudness balance methods gave lower values than had been anticipated from published data based on the absolute threshold method.

ISSN:0001-4966    

DOI:10.1121/1.1918080

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Corresponding problems of the radiation forces acting on a rigid spherical obstacle in a progressive plane or spherical sound field have been examined previously [L. V. King, Proc. Roy. Soc. (London)A147, 212–240 (1934); T. F. W. Embleton, J. Acoust. Soc. Am.26, 40–46 (1954)]. In these cases the sound field and scattering obstacle both have symmetry about the line joining the center of the obstacle to the source of the sound field, but for a cylindrical field there exists only a lower degree of symmetry. A more general expression has now been obtained for the radiation force in terms of the complex amplitudes of spherical harmonics required to synthesize the incident sound field—for the cases of greater symmetry this reduces to the simpler expressions previously obtained. The first 20 nonzero amplitudes have been evaluated for a cylindrical sound field, and it is shown that the force is one of attraction near the source, becomes zero at a certain distance, and at a greater distance is a force of repulsion. This is qualitatively the same as for spherical waves, but for any given size of obstacle or frequency of the sound field the point of zero force is always nearer to the source in a cylindrical field. Experimental results will be reported.

ISSN:0001-4966    

DOI:10.1121/1.1918087

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A report of the investigation of pulsed transducers being progressed at the Sperry Products Engineering Laboratory; use of new mathematical and experimental techniques to analyze transient performance of thickness mode x‐cut quartz plates; acoustic output for electrical inputs of step‐function, sharp unidirectional pulse, and sine wave packet; a low‐pass filter technique for short‐pulse generation.

ISSN:0001-4966    

DOI:10.1121/1.1918092

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The use of a high impedance acoustical load for the typeBdiaphone is capable of raising the efficiency of the fog horn system from an often encountered 0.2% to values of 10 to 16% for normal operating pressures. The high impedance is provided by a resonant exponential or catenoidal horn. To provide the high degree of frequency stability required by the highQresonant load the mechanical impedance of the diaphone piston is reduced to permit it to lock in with the horn frequency. The piston impedance is reduced by reducing its mass through the use of an aluminum alloy and by supporting it on a central shaft thus reducing (and stabilizing) the frictional forces. The stability thus achieved is such that the frequency varies only 3% for an air supply pressure change from 5 to 35 psi.

ISSN:0001-4966    

DOI:10.1121/1.1918097

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

An instrumentation for sound propagation studies over ground is described. The sound field distribution over ground as well as vertical wind and temperature distribution is obtained. Four model pulsed jets are used as a sound generator, and the sound field is sampled by ten microphones placed over ground up to a distance of about one mile. Temperature and wind distribution up to a height of about 25 feet are measured by five thermistor anemometers and thermometers. The microphones, the anemometers, and the thermometers are sampled by means of a rotating switch, and the corresponding signals are displayed on an oscilloscope. [Supported by Contract NAw‐6405 with National Advisory Committee for Aeronautics.]

ISSN:0001-4966    

DOI:10.1121/1.1918102

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Previous investigators have shown that speech waves can undergo any one of a number of severe forms of distortion in low ambient noise levels without serious reduction of word articulation. There are well‐known notable exceptions (e.g., center clipping). However, it is not enough to avoid these exceptional forms of distortion. In the study reported here it has been demonstrated that combinations of speech wave distortions, which individually are quite innocuous with regard to word articulation, can be devastating in their combined effect, even in the absence of serious noise. Four types of speech‐wave distortion were tried, individually and in combination, as follows: gross attenuation of high‐frequency components, multiple echo, random amplitude modulation, and gross irregularity of response‐frequency characteristic. Ambient noise was also a controlled environmental condition in some phases of the investigation. [This work was sponsored by the U. S. Air Force under Contract AF33 (616)‐2320.]

ISSN:0001-4966    

DOI:10.1121/1.1918111

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

One can distinguish between physical compression when only the physical characteristics of the speech oscillations are used, and linguistic compression if, beyond that, rules are used which are found by linguistic investigation. We shall consider here only the physical methods. They can be divided into two groups. In the first group, which may include several variations, every frequency of the original band is divided by a factornby means of frequency division, or time element skipping. On the receiver side, the original frequencies are then restored by means of frequency multiplication, or time element repetition. By the second group, on the transmitter side, the so‐called analyzer builds from specific characteristics of the speech oscillations certain code‐signals. These are transmitted in a narrow frequency band channel and control a speech synthesizer on the receiver side. The code signals contain statements about the excitation and the system functions. The excitation function indicates whether the air stream propelled by the lungs either excites the vocal cords producing a pitch frequency modulation when vowels are being uttered, or if unmodulated, produces a kind of noise, for example, if consonants are spoken. The system function indicates the varying positions of the mouth organs in the form of “envelope curve” of the speech spectrum. Four variations of the code‐signal system differ by the method of transmission of the system function “envelope curve.” (1) In the earliest system, the Bell Vocoder, the mean values of 10 or 16 frequency band sections of the envelope curve are measured and transmitted through 10 or 16 frequency channels. (2) In the Scan Vocoder, we scan the envelope of the short time‐frequency spectrum, for example, 30 times per second. The scan voltage is then transmitted through only one narrow band frequency channel as an envelope curve. (3) In the Pulse Vocoder, the envelope curve is transmitted after a transference to a succession of pulse groups. (4) By the Formant Vocoder, instead of the whole envelope curve, only three signals corresponding to the three envelope resonance frequencies (Formant frequencies) are transmitted. The system developed by the Air Force Cambridge Research Center is a Scan Vocoder which can be converted into a Pulse or Formant Vocoder by the exchange of a controlling unit. Also we obtain a unit system which makes it possible to combine the systems with others; for example, with the synthesizer of Lawrence, the Formant extractor of Flanagan, or the Formant extractor of Northeastern University. By means of block diagrams, the equipment on hand will be discussed. How it works will be demonstrated by the playing of a record.

ISSN:0001-4966    

DOI:10.1121/1.1918113

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A study has been conducted of the two major cues for stop consonants: the burst of the stop release and the transition of the formants in the adjacent vowels. Detailed frequencyvsintensity spectra of the bursts were prepared, while the transitions were studied by means of Sonagraph records. Possible criteria for identification were developed and tested. In order to assess the efficacy of the two major types of cue, perceptual tests were conducted with isolated segments containing either stop bursts or vowel transitions alone. [This work was supported in part by the Signal Corps, the Office of Scientific Research (Air Research and Development Command), and the Office of Naval Research.]

ISSN:0001-4966    

DOI:10.1121/1.1918116

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A study has been made of the diffraction of a spherical wave by a partition in order to investigate the shielding properties of a wall. Measurements carried out in an anechoic chamber will be compared with a theoretical analysis based on the Kirchoff‐Kottler and Sommerfeld‐Carslaw black‐screen treatments. [Supported in part by Contract NAw‐6405 with National Advisory Committee for Aeronautics.]

ISSN:0001-4966    

DOI:10.1121/1.1918124

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Questionnaires were submitted to 200 Air Force base personnel located in executive, engineering, and shop foremen offices. The questionnaires asked for ratings of the noise conditions, effects of noise on telephone usage and conversation, sources of interfering noises, nature of the job and the activity most affected by the noise. Octave band sound levels and time variation patterns were measured over a period of two weeks for correlation with the subjective information. The results are compared to previously published data [Leo Beranek,Acoustics(McGraw‐Hill Book Company, Inc., New York, 1954), pp. 423–425]. [This work was supported by the U. S. Air Force.]

ISSN:0001-4966    

DOI:10.1121/1.1918133

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP