摘要:

A review of the history, design, performance, and application of electret transducers is presented. Particular emphasis is placed on foil‐electret transducers incorporating a thin‐film electret made of Teflon or related materials. Such transducers have excellent frequency response, low distortion, small vibration sensitivity, and have been used over a frequency range extending from 10−3to 2 × 108Hz. They can be made in a variety of shapes over a large range of sizes and are generally not affected by adverse environmental conditions. More than 10 million electret transducers are being manufactured annually as microphones with various directivity patterns for use in amateur and studio applications, tape recorders, sound‐measuring instruments, telephone‐operators' headsets, hearing aids, and acoustic‐graphic tablets, and as transducers in earphones and phonograph cartridges. Electret transducers are also used for experimental and research applications in such widely different fields as gas analysis, opto‐acoustic spectroscopy, aeronautics, atmospheric studies, telephony, ultrasonics, acoustic holography, data transmission, and leak detection in space stations.

ISSN:0001-4966    

DOI:10.1121/1.1913507

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The electret capacitive microphone element is ideally suited for miniature applications. However, because these miniature elements represent very high impedances at audio frequencies, a complete microphone will almost always incorporate a high input impedance preamplifier, which is a source of noise in the microphone. Proper design of both the preamplifier and electret element is required in order to avoid poor signal‐to‐noise ratio. The electret element and accompanying preamplifier are electrically modeled to indicate the important parameters for minimizing the noise. This model is then coupled with the performance equations for two simple electret element geometries to indicate the effects of a noisy preamplifier on the optimum element design. The results indicate that the best signal‐to‐noise level is not obtained at the design point which yields the maximum acoustic sensitivity, but at conditions which maximize the electret element capacitance. The characteristics of two typical wide‐band electret microphones are given to indicate the type of performance actually possible from very small microphones.

ISSN:0001-4966    

DOI:10.1121/1.1913508

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The design of a small (0.8×0.5×0.2 in.) and light (1g) noise‐cancelling microphone based on the electret transducer is described. The microphone, mounted at the end of a boom, is used for a new operator's headset. The basic electret transducer is used as a first‐order gradient microphone whose directional characteristic is adjusted so that, for a given bandwidth and background noise spectrum (speech from nearby operators), the noise‐cancelling property is optimized. Pratical results indicating a minimum of 8 dB noise rejection are presented.

ISSN:0001-4966    

DOI:10.1121/1.1913509

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The desired properties for most microphones are high sensitivity, flat frequency response, insensitivity of response to vibration and temperature fluctuations, and long life. The life of an electret is particularly important since the self‐polarization has a natural decay. It is possible, however, to design an electret microphone that has little variation in sensitivity even though the charge density of the trapped charges diminishes to 1/4 of its original value. This is possible by controlling every contributing parameter. The parameter that is hardest to control is the tension of the polymer diaphragm and the problem has been resolved by choosing a rectangular backplate with a diaphragm tensioned across parallel ridges. Stray capacitance and electrical load are among the parameters that affect the stability of the microphone, especially when the microphone becomes small as for a lightweight operator's headset. The results of computations and experiments are given.

ISSN:0001-4966    

DOI:10.1121/1.1913510

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

A study of the resonances of the violin is described. Magnetic pickups attached to the bridge of a violin (sans body) responded to the lateral motion of the strings to produce a signal used to excite a set of electrical resonances. The parameters, such as center frequency, bandwidth, and attenuation of the resonances were adjustable and by means of these variables a variety of tones were produced and presented to experienced subjects from the music world. Subjective evaluation of the various tones indicates that the preferred violin tone is produced when the resonance frequencies are irregularly spaced with respect to the harmonics of the string vibration and the bandwidths have values which achieve a 12‐dB peak‐to‐valley differential in the resonance response curve. A theory of enhancement of tone quality by resonant filters is presented and the construction of an electronic violin is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1913511

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The acoustic noise spectra from 0 to 700 kHz of a boiling water reactor (BWR) have been normalized and compared with the available noise data of a pressurized water reactor (PWR). At frequencies near 500 kHz, which is the interesting frequency range for acoustic emission technology, the hydraulic turbulent and cavitation noises of the PWR were about 0 dB for the selected normalization and the noises of the BWR varied from 0 to 15 dB for the various locations that were monitored. Since the intensity levels of acoustic emission signals are on the order of 0 to 20 dB, the PWR background noise should not completely mask them; however, the BWR noise would obliterate some weak acoustic signals in some locations but allow them to be observed in other locations.

ISSN:0001-4966    

DOI:10.1121/1.1913512

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

A method is described for deriving the volume velocity waveform at the glottis during voiced speech by inverse‐filtering the volume velocity waveform at the mouth. Unlike the previously used technique of inverse‐filtering radiated acoustic pressure, this method provides a signal that is accurate down to zero frequency, not susceptible to low‐frequency noise, and easily calibrated in amplitude by a constant air flow. The primary limitation is the need for a transducer that will measure volume velocity at the mouth with adequate fidelity. In this work, volume velocity was recorded from a specially designed circumferentially vented wire screen pneumotachograph mask which provided a time resolution of 1/2 msec, without serious speech distortion. Inverse‐filtered volume velocity was recorded with two adult male subjects for voicing in the modal register. Typical results are shown which indicate the way in which the glottal waveform varied with changes of fundamental frequency, subglottal pressure, and a dimension of voice quality related to the degree of compression of the vocal folds. Also considered are the effects of glottal‐supraglottal acoustic interaction, and the effect on the glottal waveform of air displaced by the movements of the vocal folds.

ISSN:0001-4966    

DOI:10.1121/1.1913513

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

A deaf person with recruitment perceives sound as though listening through a volume expander followed by an attenuator, the expansion ratio and attenuation being typically frequency dependent. (Other perceptive aberrations may also be present, of course.) The subject is often prevented from using enough hearing‐aid gain to bring weak consonants into the useful dynamic range of his hearing, because this amount of gain would make lower‐frequency, high‐amplitude vowels intolerably loud. Such subjects commonly find amplified speech to have poor intelligibility. In a preliminary experiment it is established that recruitment in normal subjects, induced by masking or simulated by expansion of the signal, reduces the intelligibility of amplified speech severely, and that this intelligibility can be largely restored by signal processing. The implication is that recruitment in deaf subjects is a sufficient cause for loss of intelligibility, whether or not there are other causes. In the present experiments, speech is processed by a two‐channel amplitude compressor whose frequency‐dependent compression ratio is adjusted to compensate the recruitment of the individual subject, and the compressed speech is subjected to frequency‐selective amplification similarly adapted to the subject. The aim is to amplify each acoustical element of speech, at each frequency‐amplitude coordinate of the speech band, to a relative loudness for the deaf subject corresponding to the relative loudness of that speech element perceived by normals. This processing improved speech recognition, both in quiet and in the presence of competing speech introduced before processing, for six perceptively deaf subjects. Subjects showed an improvement in either initial‐ or terminal‐consonant recognition of at least 22% and as much as 160% at optimum levels in quiet, and from 10% to 229% with speech interference 10 dB below the pre‐processed signal.

ISSN:0001-4966    

DOI:10.1121/1.1913514

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

An analysis is made of the distribution of intensity of the evanescent waves in a semi‐infinite elastic medium when a plane acoustic wave in a contiguous fluid medium is incident upon it. It is found that the maximum intensity and total energy in the evanescent wave occurs at the angle of incidence corresponding to the Rayleigh angle [θR = sin−1(C 0/CR), whereCRis the Rayleigh wave speed andC0is the sound speed in the fluid medium]. This result gives insight both mathematically and physically as to what should be expected for surfaces with curvature as well as for plates.

ISSN:0001-4966    

DOI:10.1121/1.1913515

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Electrostatic ultrasonic transducers are very attractive when considered from the point of view of simplicity. They are constituted by a condenser, the ultrasonic wave being directly excited on the electrodes. These transducers are currently used at low frequencies (microphones) and sometimes at higher frequencies (up to a few megahertz). At higher frequencies the bias voltage applied across the condenser has to be quite large and electrification of the central dielectric layer can appear. This paper describes such effects and presents the experimental conditions allowing the transducer to operate. The electrification of the dielectric layer is studied and the problems related to the conservation of the deposited charges are considered for Mylar and polypropylene foils of about 10‐μ thickness. In the present work the ultrasonic waves generated or received by these transducers have frequencies ranging from 10 to 200 MHz.

ISSN:0001-4966    

DOI:10.1121/1.1913516

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP