摘要:

Travel to distant conferences consumes time, energy, and human resources. An alternative, should it prove an adequate replacement for face‐to‐face communication, is ’’teleconferencing.’’ While the necessary and desirable elements of conferencing by telephone are yet to be quantified, we describe a rudimentary experimental system which provides a communication versatility that belies its simplicity. We establish a dedicated 4‐wire connection between distant conference rooms. We modify conventional speakerphones (Model 4A) by disconnecting the electronic hybrid to give separate transmit (microphone) and receive (loudspeaker) leads. We give each conferee an individual instrument. In a given conference room, we sum the signals from individual transmit leads and connect the results to the transmit path of the 4‐wire circuit. Similarly, we distribute the receive path of the 4‐wire connection to the individual receive leads. The voice operated attenuator (variolosser) and the noise‐operated gain‐adjusting device (NOGAD) in each individual speakerphone are left completely intact. Complementary connections are made in the other conference room. As a result of these features, the teleconferencing system is able to provide effective, interactive conferencing among large groups of users. Because of the distributed configuration of transmitters and receivers, it copes well with room reverberation and background noise.

ISSN:0001-4966    

DOI:10.1121/1.382138

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Many musical instruments consist essentially of resonant systems having only approximately harmonic normal modes and excited by a force that depends nonlinearly on the velocities associated with those modes. An approximate condition is derived for the resulting sound spectrum to consist of components rigorously locked into harmonic relationship. Such mode locking is favored by nearly harmonic normal mode frequencies, by large mode amplitudes, and by large nonlinearity in the driving force.

ISSN:0001-4966    

DOI:10.1121/1.382139

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Measurements were made of bass drum tones to determine the characteristic properties of the sound. The analysis was performed with computer‐modeled band‐pass filters with 3‐Hz bandwidth. Frequency, peak sound pressure level, and decay rate were determined for each of the major components from 40 Hz up to 1000 Hz. The frequencies of the components were found to change with time, decreasing as amplitude of vibration decreases. Observed frequencies agreed with theoretical values to within 5% in some cases and less than 1% in most. It was found that a blow near the center of the drum puts more energy into the lower‐frequency components, while a blow near the edge emphasizes those of higher‐frequencies. Drum tones were synthesized by summing the contributions of each of the important components. Recordings of synthesized and real tones were presented randomly in listening tests. Synthesis was viewed as successful when the synthetic tones were judged to be real as often as they were judged to be synthetic. In the listening tests, a jury of 31 people correctly identified the synthetic tones only 51% of the times those tones were presented.

ISSN:0001-4966    

DOI:10.1121/1.382140

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

This paper presents an excitation source model for speech compression and synthesis that allows thedegreeof voicing to be varied continuously by mixing voiced (pulse) and unvoiced (noise) excitations in a frequency‐selective manner. The mix is achieved by dividing the speech spectrum into two regions, with the pulse source exciting the low‐frequency region and the noise source exciting the high‐frequency region. The degree of voicing is specified by a parameterFc, which corresponds to the cut‐off frequency between the voiced and unvoiced regions. For speech compression applications,Fccan be extracted automatically from the speech spectrum and transmitted. Experiments performed with the new model indicate its power in synthesizing natural sounding voiced fricatives and in largely eliminating the ’’buzzy’’ quality of vocoded speech. A functional definition of buzziness and naturalness is given in terms of the model.

ISSN:0001-4966    

DOI:10.1121/1.382141

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

How three supersegmental variables (amplitude, pitch contour, and duration pattern) influence phrase boundary perception was investigated in two studies. Listeners located the phrase boundary in ambiguous algebraic expressions, such as ’’(A plus E) times O’’ and ’’A plus (E times O).’’ In one experiment, two values of each of three variables (appropriate or neutral) were orthogonally varied, using linear predictive coding analysis–synthesis procedures. There was a total of eight manipulations for each expression. In the other, the three suprasegmental variables were exchanged between the two alternative meanings of an expression, yielding a total of eight manipulations for each expression. Results from the two studies were consistent in showing that listeners use all three cues, and just these three to parse such utterances. That is, it was possible to completely shift the meaning of an expression uttered with one meaning into its alternate meaning by exchanging all three variables. In both studies, the effects of duration pattern and pitch contour were additive in total proportion correct. Possible models of how listeners process pitch and duration information independently in making a parsing decision are discussed.

ISSN:0001-4966    

DOI:10.1121/1.382142

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

Serial auditory thresholds are being recorded at six‐month intervals from 224 children and youths aged 4–18 years. The data from the first year show girls have lower threshold levels than boys, especially from 12–17 years, and, in both sexes, those aged 12–17 years have lower thresholds than those aged 6–11 years. Median total noise exposure scores tend to be higher in boys than girls, especially after 10 years of age, but these scores are not significantly correlated with auditory threshold levels. The incidence of exposure to events associated with noise increases markedly in teenage boys and is related to auditory threshold levels. There are no significant associations between auditory threshold levels and responses to general health questions or attained stature, but there are associations with meatal abnormalities. There is suggestive evidence, particularly in girls, that rapid maturation is associated with lower threshold levels, especially at lower frequencies.

ISSN:0001-4966    

DOI:10.1121/1.382143

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The purpose of this paper is to derive optimum estimators for the estimation of signal parameters when the signal is emersed in an additive, not necessarily Gaussian noise field. Also, expressions for evaluating their performance are derived and, for the nonrandom signal case, the results compared with the familiar Cramer–Rao equation which provides a lower bound for the variance of an unbiased estimate. A new lower bound is derived for the variance of an unbiased estimate of the signal power for the case where the signal is a zero mean, random process and again the performance of the estimator, which is optimum for this type of signal, is compared with this new bound. In general it is found that the performance of the optimum estimator when operated in a non‐Gaussian noise field is superior to that which can be obtained when the noise is Gaussian.

ISSN:0001-4966    

DOI:10.1121/1.382144

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

The theory of a new method of measuring acoustic intensity is presented. The method uses measurements of the cross spectrum of the pressures at two closely spaced microphones. To eliminate the measurement error due to instrument phase mismatch, a circuit switching procedure is used. The method is applicable to any acoustic field subject to the condition that the product of the wave number and the microphone spacing remains small. The method has been verified by laboratory experiment.

ISSN:0001-4966    

DOI:10.1121/1.382145

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

A loudspeaker‐driven simulation booth with extended rise‐time capability (down to 0.22 ms) has been used for subjective loudness tests ofN‐wave sonic booms. The test series compared signatures over a range of 0.22–10 ms in rise time, 100–250 ms in duration and 0.5–2.5 psf (24–120 Pa) in peak overpressure. In one sequence, the tradeoff between rise time and overpressure was measured for equal loudness; in another, the tradeoff between duration and overpressure. For equal loudness 10‐ms rise time required 8‐dB higher overpressure than for 1‐ms rise time. Duration had little effect in the range 100–200 ms, but at 250 ms noticeably enhanced the loudness. These results confirm those measured by Shepherd and Sutherland made at 1‐ms rise time and above (except for the anomalous enhancement at 250‐ms duration), and extend the measurements down to 0.22 ms. There is also good agreement with theoretical predictions (Johnson–Robinson, Zepler–Harel methods) except for the 10‐ms rise time and 250‐ms duration cases.

ISSN:0001-4966    

DOI:10.1121/1.382146

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP

摘要:

For very long supersonic aircraft the ’’midfield’’ sonic boom signature may not have evolved fully into anNwave at ground level. Thus in current boom minimization techniques the shape of the aircraft may be tailored to optimize this midfield wave form for reduced subjective loudness. The present investigation tests a family of ’’flat‐top’’ waveforms cited by Darden: all but one have a front shock height (‐pSH) less than the peak amplitude (‐pMAX). For equal subjectiveloudness, ’’flat top’’ vsNwave (peak overpressure ‐pN), the peak amplitude of the ’’flat top’’ signature was found to be substantially higher than that of theNwave; thus for equal peakamplitudethe ’’flat‐top’’ signature was quieter. The results for equal loudness were well fitted by an empirical law ‐pSH+0.11‐pMAX=‐pN; the equivalence shows how the front shock amplitude (‐pSH) dominates the loudness. All this was found compatible with predictions by the method of Johnson and Robinson.

ISSN:0001-4966    

DOI:10.1121/1.382127

出版商:Acoustical Society of America    

年代:1978

数据来源: AIP