摘要:

A new optical method has been developed for determining the shape of ultrasonic waves. Because of the varying index of refraction, collimated light incident on an ultrasonic wave emerges with a modulated phase. By interfering light of fixed phase with the emergent front, information about the waveform can be obtained. The theory provides for determination of ultrasonic waveform including relative phase between harmonics. The process of finding the waveform consists of simply taking the arccosine of experimental results. Comparison of experimental results with theory will be discussed. (This work was supported by the Office of Naval Research.)

ISSN:0001-4966    

DOI:10.1121/1.1936742

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

Basic physical limitations of the quality and complexity of decisions, imposed by the volume, bandwidth, and time available for processing the acoustic field, are developed. Markov‐Gauss space‐time noise‐probability density functions, together with several states of knowledge concerning signal waveform and wave front, are used to illustrate decibel departures of optimum minimum‐average‐risk space‐time processors, from pattern formation and correlation processing. Volume, bandwidth, and time are related to each other and to possible processing gain. Sensitivity of both the decibel differences and these relationships to signal and noise states of knowledge is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1936746

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The results of vibration and acoustic tests of a missile structure are presented. Sinusoidal vibration surveys, low‐level acoustic tests using a random siren as the noise source, and high‐intensity random‐noise tests to determine the probability of acoustically induced structural failures during underground launch operations were conducted. Resonance frequencies, mode shapes, and damping ratios were determined, and the absolute value of the driving point impedance as a function of frequency was calculated. The correlation between localized excitation (random siren) and excitation of the entire structure during an in‐silo launch was investigated. The linearity of acceleration response of skin panels, stringers, and ring frames was determined from13‐octaveband analysis of the output signals of accelerometers and of microphones over a range of over‐all sound‐pressure levels from 140 to 165 db.

ISSN:0001-4966    

DOI:10.1121/1.1936758

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

A cantilever beam of 2024‐T3 aluminum sheet (3.25 in.×1.00 in. × 0.063 in.) was shaken mechanically at its clamped end, the force being applied parallel to the thickness dimension. A 0.125‐in.‐diam hole near the clamp afforded points of strain concentration. Both pure tone and broadly filtered random forces were used to excite resonant response in the fundamental mode, at about 185 cps. The rms surface strain in a cross section containing the hole ε was monitored and held constant. LifeNto first visible appearance of a fatigue crack was measured; lifetimes ranged from about 1×105to 6×105cycles. The pure‐tone data agree very closely with previous measurements on a riveted cantilever excited by sound. In comparison with available fatigue data for reversed bending in a specimen with small concentration factor, these data yield a fatigue strain concentration factor of 1.6, in good agreement with theory. The measured ratio of rms pure‐tone strain to rms random strain yielding the same life‐time ranged from 1.5 to 1.6 (3.6 to 4.3 db). The ratio calculated from assumptions of (a) Rayleigh distribution of peak strains, (b) noninteracting accumulation of damage, and (c)Nε8.25= const, is 1.4 (3.1 db). (Research sponsored by Aeronautical Systems Division, Air Force Systems Command, U. S. Air Force.)

ISSN:0001-4966    

DOI:10.1121/1.1936762

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

In space communication, every means for transmitting maximum information should be provided. The new proposed system transmits the sense of direction as well as voice information between any number of suitably equipped persons. The transmission takes place with omnidirectional radiators. The reception is achieved by means of two limacon receptors, using “stereophonic‐binaural converters” to provide the desired space perspective.

ISSN:0001-4966    

DOI:10.1121/1.1936768

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP

摘要:

The principle of voice‐excitation for a vocoder synthesizer involves generation of a flat spectrum of pitch harmonics or noise from an uncoded subband (“baseband”) of the original speech [M. R. Schroeder and E. E. David, Jr., “A vocoder for transmitting 10 kc/sec speech over a 3.5 kc/sec channel,” Acustica10, No. 1, 35–43 (1960)]. The necessity for a decision‐making pitch detector is thereby removed. The principal advantage of the voice‐excited vocoder is its insensitivity to input signal‐to‐noise ratio and equalization. A voice‐excited vocoder with a 720‐cps (250–970 cps) baseband and 17 spectrum channels low‐passed to 25 cps each, covering the band 970–3700 cps, has been built and evaluated. The test shows an average PB‐word intelligibility of 86%, compared to 92% for input speech of the same bandwidth, both with and 18‐db signal‐to‐noise ratio. Quality tests indicate that about 85% of all listeners rate the speech “as good as” the input while only 20% so consider a vocoder using conventional pitch control. The vocoder performed about equally well for each of the 12 speakers in the quality test. Voice‐discrimination tests indicate that voice identity is well preserved. Crucial factors influencing the remade speech quality are the accuracy of spectral flattening and the impulse response of the analyzer low‐pass filters. Studies of voice‐excitation indicate that the baseband, if used for excitation only, can be coded into about 2000 binary pulses per second. Accordingly, the vocoder was modified with the addition of four spectrum channels, and operated with a digitally coded baseband. In total, it is estimated that about 4000 binary pulses per second are required for all‐digital transmission. A tape demonstrating these results will be played.

ISSN:0001-4966    

DOI:10.1121/1.1936772

出版商:Acoustical Society of America    

年代:1961

数据来源: AIP