摘要:

Sound‐reflecting characteristics of the sea floor have been studied by means of a semiautomatic system for recording the peak pressure and the time integral of the square of the pressure of the bottom echo received by an Edo UQN‐1b transducer operated at 12 kc. This system was used to record a detailed reflection profile across the Madeira Abyssal Plain during the fall of 1961. In addition, a self‐contained pinger, employing a somewhat similar transducer, was lowered to the bottom in this area to measure bottom‐echo levels and propagation loss. The nature of the sea floor as an acoustic reflector at 12 kc is discussed, based on the echo levels measured during the pinger lowering. The variation in the reflecting properties of the sea floor across the Madeira Abyssal Plain is discussed.

ISSN:0001-4966    

DOI:10.1121/1.1937308

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

A ripple structure on a sandy ocean bottom produces important changes in the reflection characteristics of the boundary. This effect is analogous to the focusing produced by an optical‐reflection grating. The acoustics problem has been studied since the time of Lord Rayleigh, with successes being limited to the case of extremely small corrugation amplitudes. This paper presents some results obtained from a model‐tank experiment wherein a scale factor of about 20:1 is applied to the acoustic frequency and linear dimensions. The model boundaries were constructed of a plaster‐sand mixture and of very fine sand in both flat and corrugated forms. In the experiments, the incidence angle was set while a complete semicircular reflection pattern was recorded for the whole region above the sonified boundary section. The incidence angle was varied from grazing to normal, so that families of reflection patterns were produced for the flat and corrugated‐plaster and for the flat and corrugated‐sand boundaries. These patterns are shown in the paper, as well as some curves showing the specular and backscatter [toward the sound source] amplitude dependence upon incidence angle and boundary contour. The experimental work is still in progress.

ISSN:0001-4966    

DOI:10.1121/1.1937309

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

A previous study [G. F. Lysaught, R. J. Rosov, and K. S. Harris, J. Acoust. Soc. Am.33, 842 (1961)] showed that although the glottis behaves differently for the oral labial stops /p/ and /b/, there is no significant difference between them in the pattern of electromyographic activity at the lips. In the present study, we found that /m/ has the same pattern of lip action. Furthermore, we were able to analyze the action of the musculature of the soft palate in the articulation of these three sounds. For the nasal /m/, no activity was observed anywhere in the palatal region. For /p/ or /b/, however, there is an identical burst of activity which can be measured on either superior or inferior palatal surface. The shape of the electromyographic pattern changes depending on whether the /p/ or /b/ follows a nasal or another oral sound, but changes in the same way for /p/ and /b/. The findings of these studies suggest a simple picture of the independent action of the various parts of the articulatory apparatus, which parallels the results of perceptual studies.

ISSN:0001-4966    

DOI:10.1121/1.1937318

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

The audible broad‐band noise generated by the discharge of high‐velocity gases into a relatively static medium can be reduced by effectively changing the discharge area to a multiplicity of smaller jets. Therefore, the normally audible noise is partially transformed into the inaudible range. The MULTI‐JET, essentially a perforated colander, utilizes this principle. To obtain satisfactory performance with this type of diffuser, it has been found necessary to match accurately the diameter of the colander to that of the jet‐exhaust nozzle. This matching involves a careful correlation between the diameter of the engine nozzle and the diameter of the colander together with the proper spacing between the nozzle and the colander entrance so that a close coupling is realized. Field tests have proven that maximum performance is obtained when the diameter of the colander is equal to or slightly larger than the diameter of the jet. In view of the many engine configurations presently operational and in view of continuous changing engine types, maximum utilization of a single colander cannot be realized. In addition to degradation in acoustic performance, mismatching also engenders other problems. In the case of an oversized colander, “boom” phenomena have been observed. In addition, backflow is created. This backflow causes temperature rise that complicates engine calibration and may even damage engine accessories. Aside from the problem of mismatching, present colander‐type suppressors have limited use. During afterburner operation, exhaust temperatures reach the neighborhood of 3000°F. Therefore, it has been the practice to employ the MULTi‐JETS only during the so‐called military operation of the engines. This limitation has persisted, notwithstanding the attempted uses of a liquid coolant, since severe thermal shocks generated by application of coolant produce rapid suppressor deterioration. The aeroacoustic‐flow diffuser evolved from a series of design and through numerous field tests has resolved the three major problems: acoustic performance, aerodynamic capacity, and structural durability. Field experience and test data relevant to the final design of a water‐cooled aeroacoustic diffuser are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1937326

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

Low‐frequency noise of physiological origin has been measured in a cavity enclosing the ear. The measurements were performed with a good condenser microphone resiliently mounted in a special enclosure of effective volume approximately 60 cm3sealed to the head with a circumaural liquid‐filled cushion. The over‐all noise level in the enclosure and levels in certain bands rise and fall by a few decibels at the heart‐pulse frequency, indicating that the noise is generated by the circulatory system. The level per13octave band averaged over six subjects is approximately 70 db re 0.0002 dyne/cm2at 16 cps falling steadily to 34 db at 125 cps and 12 db at 250 cps. With the 12‐cm3effective volume of a TDH39 and MX41AR cushion, one would anticipate physiological noise levels 14 db higher than these or 48 db at 125 cps and 26 db at 250 cps. The levels per critical band might be somewhat higher yet. Since the tentative ISO hearing threshold levels at 125 and 250 cps are 49 and 27 db, respectively, one is led to suspect that these levels may have been raised by the masking effect of physiological noise.

ISSN:0001-4966    

DOI:10.1121/1.1937329

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

A battery of 21 contact‐detection tests was prepared consisting of a variety of signals immersed in one of three types of background noise. Signals were pure‐tone pulses, filtered clicks, speech, and several other complex sounds. Backgrounds were either white noise, a frequency‐modulated noise band, or an interrupted noise. The fundamental question is whether performance in handling tasks involving unfavorable S/N ratios is an unitary trait or whether several different (though perhaps related) abilities are involved. Two groups of young men, of average or better intelligence, were given the battery: a group of 72 given special motivation and more prolonged testing, and a group of 700 given more‐routine treatment. Four factors emerge from a Thurstone centroid treatment: an ability to detect complex noises, an ability to understand masked speech, an ability to detect tone pulses in a noise, and an ability to detect clicks in an interrupted noise. Interpretations of these factors are offered. Evidently, the area of contact detection is factorially complex; parameters involved in any particular experimental situation may not necessarily be involved in the same way in other situations.

ISSN:0001-4966    

DOI:10.1121/1.1937335

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP

摘要:

An investigation was conducted to determine the effect of a normal pressure on the transmission of ultrasound across an interface. The results are presented.

ISSN:0001-4966    

DOI:10.1121/1.1937337

出版商:Acoustical Society of America    

年代:1962

数据来源: AIP