摘要:

This instrument was designed to make a graphical record of small speed variations in any sound recorder capable of producing a millivolt output at about 1000 c.p.s. It consists of the following sequence of components: amplifier→clipper→bandpass filter→frequency discriminator→demodulator→oscillograph. The clipper eliminates amplitude variations and the filter passes only the fundamental frequency of the clipped wave. The discriminator is a parallelTequivalent of a Wien bridge balanced near the upper cut‐off frequency of the filter. The oscillograph is a model BL‐201 direct‐inking instrument made by the Brush Development Co. Over‐all calibration of the wow meter is accomplished by an RC oscillator and calibrating switch which changes the oscillator frequency 1 percent. Preliminary estimates and tests indicate that speed variations of less than 0.1 percent can be measured reliably if these variations occur between12and 100 per second.

ISSN:0001-4966    

DOI:10.1121/1.1902510

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

Fundamental to Acoustical Research is the measurement of sound pressure level. The W.E. 640‐AA Microphone was used at the Electro‐Acoustic Laboratory not only as a reference standard for the calibration of other types of microphone, but also for routine laboratory measurements. Given the pressure calibration of the microphone certain instrumentation and complementary calibrations are needed for the convenient use of this microphone under the various conditions encountered during research work. The following aspects of this subject are discussed: (1) Considerations regarding the effect of preamplifier input impedance on the frequency response and noise threshold of a condenser microphone. (2) An a.c. operated, cathode‐follower type preamplifier and associated circuits. (3) A flexible extension for use with the preamplifier. (4) The use of small probe tubes. (5) Random incidence calibration and collected free field calibrations. (6) A “supercharged” cathode follower circuit and its application as a preamplifier.

ISSN:0001-4966    

DOI:10.1121/1.1902518

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

In 1921 a method was formulated for calculating the articulation—the percent of speech sounds interpreted correctly during a telephone conversation—from the physical characteristics of the telephone system and the noise environment of the listener. This paper describes this empirical theory as recently revised to correspond to modern techniques. A quantityAis calculated which is called the articulation index and which is related to the various kinds of measured values of articulation. In particular, ifsis the percent of the fundamental sounds correctly perceived by the listenerA= −plog (l‐s), wherepis a factor depending upon the skill of the crew making the articulation tests. For a well‐practiced crew the value ofpis between 0.5 and 0.6. The value ofAis calculated by the equationA = F⋅V⋅E, whereFis the maximum value ofA, andVa factor which is zero at threshold intensity of the speech received by the listener and grows as the received speech intensity increases reaching unity for intensities between 60 and 70 db above threshold. The factorEis unity below this intensity but decreases from unity to 0.85 as the intensity levels increase to 60 db above these levels. The factorFis calculated by the equationF = ∫0∞D⋅Wdf, whereDis a function of the frequencyf, andWis a function of the relative response of the system. The functionsDandWhave been determined directly from articulation data. The factorsV(x) andE(x) are determined directly from articulation data. The value ofxfor systems in a quiet place is equal to a weighted average of the response of the system. When a noise is present which produces a maskingMat each frequency at the listener's ear, the same functions are used for determiningFandV, but the responseRat each frequency is replaced byR − M. However, for this case thexinE(x) is increased by an amount Δ, a quantity calculated directly from the maskingMof the noise and the intensity of the received speech.

ISSN:0001-4966    

DOI:10.1121/1.1902525

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

Exposure to intense airplane noise (115 db) for periods as long as 7 hours did not affect the performance of 5 subjects on a wide variety of psychological, physiological, and psychomotor tests. The noise was sufficiently intense to cause marked temporary hearing losses and tinnitus. It interfered seriously with communication and was reported as “annoying” by the subjects, but its effects on general efficiency were apparently nil. Following an extensive training period designed to minimize subsequent change in performance due to learning, the subjects were tested throughout 16 seven‐hour sessions, half of which were “noisy.” Eighteen different measures were applied to sample such functions as: motor coordination, reaction time, muscular tension, metabolism, breathing, speed of eye movement, speed of accommodation, dark adaptation, coding, reversible perspective, steadiness, etc. For many of the tests the data were so extensive that differences in performance of the order of 1 percent could have been detected with high reliability. Other functions, particularly those of a physiological nature, depended so much on the individual tested that generalizations were impossible. In general, the results of these experiments agree with common sense: although as a subjective experience noise is disagreeable and tiring, most types of mental, motor, and physiological activity are affected very little by noise as such. This research, begun under an OSRD contract, is continuing under contract with the U. S. Navy, Office of Naval Research. A full report of the present study is available from the Publications Board, Dept. of Commerce, Washington, D. C. (PB No. 8334, OSRD No. 274).

ISSN:0001-4966    

DOI:10.1121/1.1902527

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

This paper concerns itself with the acoustical specifications necessary for intelligible speech transmission through radiating enclosures, such as gas masks, placed on the front of the head. The results apply to enclosures whose internal dimensions are small by comparison with the wave‐lengths significant for speech. Transmission through larger enclosures is in general poor. A speech transmitting device for a gas mask will ordinarily consist of a diaphragm, a perforated protective cover, and an exhaust valve. The acoustical system, however, must include the cavity between the mask and the face. The enclosures considered here radiate through a circular constriction, a set of perforations, or a diaphragm; specifications for the more complicated case of a diaphragm and cover have not been obtained, but may be inferred from the data presented here. The enclosures all behave as Helmholtz resonators when tested on an artificial voice. They may be specified acoustically by the resonant frequency, and a quantity related to the inertance of the acoustical system. It has been found convenient to express this property of a set of perforations or of a diaphragm in terms of aneffective diameter: the diameter of the circular constriction that results in the same resonant frequency. The intelligibility obtainable through an enclosure increases when either the resonant frequency or the effective diameter is increased. In general, the intelligibility will be satisfactory if the resonant frequency is at least as high as 1500 cycles per second, provided that the effective diameter is at least 1.25 inches.

ISSN:0001-4966    

DOI:10.1121/1.1916431

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP