摘要:

The method of matched asymptotic expansions is applied to the problem of supersonic, nonequilibrium, axisymmetric flow about a slender, pointed body of revolution with thickness/length ratio ε. The form of the nonlinear equations which govern the first‐order perturbations in the far‐field depend on the relative orders of magnitude of ε, δ, and τ (δ is a measure of the nonequilibrium effects and τ is a characteristic time of the nonequilibrium process). A comparison is made between the axisymmetric and plane‐wave solutions under typical sonic boom conditions, and it is suggested that the one‐dimensional theory is limited in its applications.

ISSN:0001-4966    

DOI:10.1121/1.2002983

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Three sound recordings of tornadoes have been analyzed over the frequency interval between 100 and 2000 Hz. The low‐frequency analysis was limited by the spectral response of the microphones, and the upper limit was imposed both by the microphone response and the low signal level. Although the recordings are of low quality and the audio information required major spectral corrections, the data do indicate that the recorded audio emissions from the tornadoes decrease in intensity as a function of increasing frequency. An attempt has been made to correlate the spectra with real (and conjecture) physical properties of the tornadoes. It is clear to us from our objective analysis (and the more difficult subjective evaluation of the recordings) that audible identification of tornadoes is possible, and a study of tornadic sounds might provide a new tool for gaining insight into electrical and mechanical disturbances within a tornado's funnel system. Study of the change in intensity of the sounds emitted by the approaching Guin storm at both high and low frequencies suggests that noises generated by the high speed winds of the tornado cyclone as it scours the ground can be discerned from noises associated with the winds and/or electrical discharges aloft.

ISSN:0001-4966    

DOI:10.1121/1.2002991

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Noise certification under FAR 36, Appendix F, requires the measurement of maximumA‐weighted sound level during flyovers at maximum continuous power, 1000 ft above ground, at a sealevel airport. In order to investigate the effect of altitude and horsepower on sound‐level measurements, tests on a Cessna 172 and Beech 33 were made at three airports having elevations of 42, 2350, and 6750 ft. Engine power was varied from approximately 50% of raged maximum continuous to the maximum available at the test altitude. The results for these two representative general aviation aircraft show that above approximately 65% powerA‐weighted sound level is essentially constant and independent of altitude, depending primarily on propeller tip helical Mach number. Noise certification of propeller‐driven small aircraft can thus be reliably performed at a wide range of test sites irrespective of site elevation. [Work supported by FAA.]

ISSN:0001-4966    

DOI:10.1121/1.2002995

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

The concept is investigated of reducing jet noise from aircraft jet engines, flare stacks, and similar high velocity jets by reducing the turbulence in the turbulent mixing region by means of magnetohydrodynamic interaction in an electrically conducting medium. Electrical conductivity is to be obtained by ion seeding. The feasibility of the concept is investigated by means of a shallow‐surface‐wave analogy to the aerodynamic noise generation, propagation, and radiation using a 4 by 6‐ft mercury ripple tank. The mercury layer was 1/8 in. thick to assure nondispersive wave propagation. Jet velocities from Mach 0.4 to 1.1 were studied and good simulation of sound power, directivity, and spectrum characteristics of the jet were established relative to an axisymmetric air jet.

ISSN:0001-4966    

DOI:10.1121/1.2002996

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Practical use of speaker verification implies a preliminary task of user identification. The identification mode is selected to be aspokensequence of six digits. Verification may then be performed on the identifying speech data. This “total voice” system must recognizeconnecteddigitsindependentof speaker with very high reliability. Three sequence constraints aid recognition: (1) two parity checks must be satisfied, (2) all digits must be different, and (3) “difficult” digit pairs are disallowed. The speech processing strategy features highly reliable time registration and accommodates multiple concurrent hypotheses at various processing levels. Operational performance appears feasible from preliminary results: sequence recognition‐<1% rejection,<0.2% error; speaker verification‐<1% user rejection,<2% impostor acceptance. This system has been simulated in real time on a minicomputer. [Research was sponsored by the Air Force Systems Command, Rome Air Development Center.]

ISSN:0001-4966    

DOI:10.1121/1.2003004

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Continuous speech was treated as if produced by a finite‐state machine making a transition every centisecond. The observable output from state transitions was considered to be a power spectrum—a probabilistic function of the target state of each transition. Using this model, observed sequences of power spectra from real speech were decoded as sequences of acoustic states by means of the Viterbi trellis algorithm. The finite‐state machine used as a representation of the speech source was composed of machines representing words, combined according to a “language model.” When trained to the voice of a particular speaker, the decoder recognized seven‐digit telephone numbers correctly 96% of the time, with a better than 99% per‐digit accuracy. Results for other tests of the system, including syllable and phoneme recognition, will also be given.

ISSN:0001-4966    

DOI:10.1121/1.2003011

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

This paper describes a digital imprementation of a pitch‐extraction algorithm which is based on bit‐pattern comparison. The NLAF is a variation on autocorrelation analysis where, instead of correlating the input speech waveform, the autocorrelation is computed either on the sign of the signal or on the sign of the first derivative of the signal. After a review of the method, the pitch extractor implementation is described. Then, experimental results are presented when analysis is first performed on glottal waveform, and then on speech waveform. The comparison of the results leads to the necessity of preprocessing the speech waveform in order to eliminate the adverse effects of the frequency components generated by resonances within the vocal tract. This preprocessing is performed by use of a very simple digital fileteter which considerably decreases the probability of an erroneous determination of the pitch period.

ISSN:0001-4966    

DOI:10.1121/1.2003016

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Diffraction by parts of the body and nearby objects (e.g., parts of machines), the proximity of the source, and the directional properties of the external ear lead to differences between free‐field measures of sound pressure and the levels experienced by an individual. A miniature microphone located within the ear at the cavum of the concha is free of these effects and is therefore well suited to studies of an individual's exposure to noise. For operators of snowmobiles, sound pressures at the concha exceed levels measured 8 cm from the side of the head by ∼7 dB from 2–4 kHz, but are ∼3 dB less at frequencies below 500 Hz. For these and similar shaped source spectra,A‐weighted sound pressures at the concha (typically 100–105 dB) exceed levels near the head by ∼3 dB. Predictions of theA‐weighted levels experienced by an operator from measurements of sound level 15 m from these vehicles can introduce errors of ±5 dB.

ISSN:0001-4966    

DOI:10.1121/1.2003021

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP

摘要:

Previous results showed braking reaction times (Rt's) increased with higher intensity intravehicular noise (IN). Experiments were performed to relate electromyographic (EMG) activity to these RT changes. IN, recorded in a moving car, was reproduced at 77 and 85 dB, through a calibrated sound system, to subjects in a simulated vehicle. During 3‐h exposures, subjects were required to maintain a fixed speed on the speedometer and brake when a red light lit up at random intervals. Surface electrodes recorded EMG activity from right gastrocnemius and rectus femoris muscles. Digital counters provided the braking reflex Rt's. For the rectus, at 77 dB, following a latency of 0.1–O.3 sec, there was a burst of activity which tapered off. With time, the amplitude of the response diminished and its duration increased. At 85 dB, this latter pattern, generally associated with fatigue, appeared earlier in time than at 77 dB, with significantly longer duration. For the gastrocnemius triphasic response, there was a progressive decrease in activity of the 1st and 2nd components which occurred earlier at 85 rather than 77 dB. These findings suggest that higher intensity IN induces increasing motor fatigue.

ISSN:0001-4966    

DOI:10.1121/1.2002438

出版商:Acoustical Society of America    

年代:1976

数据来源: AIP