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Journal of the Acoustical Society of America

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Nov 1975

Volume 58, Issue S1, pp. S2-S132

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back to top Session X. Engineering Acoustics II
Contributed Papers
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Acoustic link reduction by digital processing (A)

John R. Gibson and Horst Arndt

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S44-S44 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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The work described in this paper is concerned with the digital simulation at Bell‐Northern Research, Ottawa, of the various acoustic transmission paths between a loudspeaker and a microphone in a room. There are a number of possible applications for this work in the telecommunications industry relating to speaker phones and teleconferencing. A reasonably accurate simulation has been found to produce a reduction in signal level following the microphone of the order of 20 dB for signals which originate from the loudspeaker. Other signals originating in the room pass through the system with no effect. The voice switch has been a necessary but undesirable part of the majority of speaker phones and teleconferencing systems. Its function is to permit adequate audio gains while preventing the oscillations which would normally occur in a two‐way conversation using loudspeakers and microphones. The voice switch effectively allows only one‐way transmission at any time. It is generally found that the voice switch wfil respond to background noise and other spurious sound souroes as well as to speech and in some applications this action is quite undesirable. The simulation system, on the other hand, can allow in excess of 20 dB of additional gain before oscillations would occur and in a suitably benign acoustic environment, this could be sufficient to dispense with the voice switch altogether. Further applications of this work in telecommunications include the reduction of ambient noise for the benefit of the far‐end user in either bandset of hands‐free operation. This would be a useful facility for persons wishing to communicate in high ambient noise conditions, such as in or around large pieces of equipment, or aircraft, etc.
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Experimental system for teleconferencing (A)

J. E. West, D. J. MacLean, and J. L. Flanagan

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S44-S45 (1975); (2 pages)

Online Publication Date: 11 Aug 2005

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Travel to distant conferences conferences consumes time, energy, and hemfin resources. An alternative, should it prove an adequate replacement for face‐to‐face communication, is “teleconferencing.” While the necessary and desirable elements of conferenoing by telephone are yet to be quantified, we describe a rudimentary system which provides a communication versatility that belies its simplicity. We establish a dedicated four‐wire connection between distant conference rooms. Conventional speakerphones (Model 4A) are modified by disconnecting the electronic hybrid to give separate transmit (microphone) and receive (loudspeaker) leads. We give individual instruments to each conferee. In a given conference room, we connect (sum) all individual transmit leads to the transmit path of the four‐wire. Similarly, we connect (distribute) all individual receive leads from the receive path of the four‐wire. The voice switch and the noise gain‐adjusting device (NOGAD) are left intact in each individual speakerphone. Complementary connections are made in the other conference room. As a result of these features, the teleconferencing system is able to cope well with room reverberation and background noise, both at transmit and receive locations, and to provide effective and economical conferencing.
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Method for measuring noise‐canceling microphone characteristics (A)

B. T. Mozo, J. H. Patterson, Jr., and R. T. Camp, Jr.

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S45-S45 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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A test method was developed using fast Fourier techniques to determine noise‐canceling microphone nearfield frequency response and noise‐canceling properties. This procedure utilizes transfer function analysis between a standard laboratory microphone and a noise canceling microphone. The standard microphone and the noise canceling microphone are mechanically coupled and move thru a pink noise sound field produced by an artificial voice. Transfer functions were measured at a point defined as the source of the sound field which yield the nearfield response and at fixed distances from that point which yield measures of noise canceling properties. A mapping of the sound field was completed by moving two standard laboratory microphones thru the sound field and measuring their transfer functions.
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Environmental effects on microphones of various constructions (A)

Gale R. Hruska, Edward B. Magrab, and William B. Penzes

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S45-S45 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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The change in the sensitivity of condenser, electret, and ceramic microphones was determined for numerous combinations of temperature (−20° to 50°C), percentage relative humidity (25% to 95% RH), and frequency (100–5000 Hz). To measure the sensitivity changes, three condenser microphones and the reciprocity technique in an acoustic coupler were used at each combination of temperature, relative humidity, and frequency to obtain a primary reference source. This reference source was then used to determine the sensitivity of each of the other microphones. Furthermore, insert voltage techniques were used for all the microphones, thus canceling the effects of the environmental conditions on the electronics. For the combinations of temperature and humidity at each frequency considered, the range of the change in sensitivity in decibels relative to sensitivity at 20°C and 44% RH was (1) for the condenser microphones: (−0.8, 1.0) and (−1.0, 1.4); (2) for the electret microphones: (−3.1, 1.1) and (−2.3, 1.9); and (3) for the ceramic microphones: (−2.1, 1.5) and (−5.0, 1.6).
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Fluctuating surface pressure measurements on USB wing:Two types of transducers (A)

James B. Reed and James A. Schoenster

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S45-S45 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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Measurement of noise levels in the vicinity of a jet exhaust, such as is desired on surfaces of propulsive lift devices, requires high‐temperature transducers that will withstand intense low‐frequency flow buffeting, yet will be sensitive to lower‐pressure loads at high frequencies. For flight test installation in an aircraft assembly. such as the USB YC‐14 prototype, rugged transducer integrity and attachment are critical for periodic installation and removal, and for flight exposure occurring in a test program of several months duration. Two transducer types used in such previous test environments were Kulite and Photocon units. These two transducers systems were compared by pairing three wing surface locations in a NASA Langley JT15D upper surface blowing (USB) static test. This USB static arrangement was an inverted simulation of the overwing section of a small airplane used in Langley wind tunnel USB experiments. In addition to transducer comparisons, the experience of measuring full‐scale surface‐fluctuating pressures of attached flow and jet interaction noise in the nearfield was desired. Some of the noise sources and characteristics of overwing nozzles and Jet interaction are discussed.
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Self‐consistent approach to the problem of membrane‐fluid coupling in condenser microphones (A)

Allan J. Zuckerwar

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S45-S45 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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The operation of a condenser microphone is characterized by strong coupling between membrane displacement and excess pressure of the fluid between the membrane and backplate. In solving the equation for the excess pressure [I.G. Petritskaya, Sov. Phys.—Acoust. 12(2), 193–198 (1966)], we assume a simple expression for the membrane displacement, containing a single unknown η0; then we calculate the membrane displacement from the membrane equation of motion, using the resulting solution for the excess pressure (which still contains η0). By requiring the assumed and calculated expressions to yield the same average membrane displacement, we find the unknown η0 and insure self‐consistent solutions to the membrane and fluid equations. We express the average membrane displacement and lumped parameters in terms of the physical properties of the microphone, including the configurations of holes and slots in the backplate. Effects of variation of microphone properties are demonstrated, both theoretically and experimentally. [Supported by NASA.]
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Measurement of acoustic impedance via reflected pulse (A)

J. D. Chalupnik and C. D. Clark

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S45-S45 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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This paper explores a technique for using a reflected acoustic pulse to determine acoustic impedance. A Fourier transform of the acoustic wave reflected by the test material is used to find the reflection coefficient as a function of frequency. An expression is derived which relates the reflection coefficient to the acoustic impedance of the sample. Experiments were performed to test the procedure on samples of typical acoustical materials. The acoustic pulse was generated by an electrical spark. The effect of the sphericity of the wavefront is taken into account in the derivation. The pulse was repeatable at most of the audible frequencies, for which the results of this study agree with values obtained by classical impedance tube techniques; however, at low frequencies, results were at variance with the impedance tube results. This is attributable to the low energy density in the test pulse at low frequencies. The pulse technique is unuseable.
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High‐power acoustic transmitter arrays (A)

Tant Priestley and Del Hunter

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S45-S45 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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A high‐power acoustic transmitter with design goals of producing a narrow bean (≈5° half angle to 3‐dB points), minimum side lobes (40 to 50 dB down) and high acoustic power (≈500 acoustic watts) was producted by NOAA's Wave Propagation Laboratories for use in the FAA sponsored wind shear measuring system. The antenna was required to operate under all weather conditions and to produce a 1250‐Hz pure tone for a duration of 0.1 to 0.5 sec with repetition rates of 6 to 8 sec. The transmitter design consists of an amplitude‐shaded array made up of 144 omnidirectional elements mounted in a 12×12 configuration on a fiat reflecting plane. Results of acoustical impedance measurements performed on a set of apertures which simulated the element openings and measurements aimed at compensating for element‐to‐element interaction are presented along with beam patterns from a full‐scale antenna.
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Transducer radiation pattern modification (A)

H. S. Hayre and M. S. Putcha

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S46-S46 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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Ultrasonic transducers were used to simulate radiation patterns of an electromagnetic cavity‐backed slot antenna when mounted on a flat ground plane as well as by itself in free space. A ½‐in. spherical transducer excited in radial mode was selected to provide essentially isotropic coverage generated by the antenna. The radiation coverage of this transducer was significantly distorted, when mounted on a flat surface just like the antenna. This distortion was caused by both the presence of the plane in the near field as well as due to reflection of the energy from the lower half of the spherical transducer by the plane surface, resulting in the interference or distorted pattern. Then the plane surface immediately underneath the spherical transducer was treated with a synthetic rubber adhesive, “Miracle Seal,” and the resulting surface was randomly shaped in order to diffuse the incident ultrasonic radiation; thus, utilizing our background in scattering, and reverberation of sound, was used in a reverse fashion in order to obtain a very smooth isotropic pattern. The resulting pattern did not contain any sharp nulls which were reduced to less than forty percent of the original number and were significantly attenuated (in decibels) by 75% on the average.
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Comparison of the reliability of an auraldome headset and a standard headset in obtaining pure‐tone audiometric threshold measures (A)

C. S. Vogen and A. B. Copeland

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S46-S46 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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An Auraldome headset and standard headset (TDH‐39/MX‐41/AR) were compared via threshold measures, probe tube microphony, and coupler measures. The same TDH‐39 receiver was used in each headset. Statistical and descriptive comparisons of headset performance were carried out. For each frequency, intersubject variances of the two headsets were compared statistically, as were their intersubject variances. Descriptive comparisons were also made of the probe tube and coupler data. The threshold data and physical data, considered together, permit comparison of the reliability of the two headsets, as well as allowing comparisons of their real‐ear‐to‐coupler errors. Results demonstrated the suitability of the Auraldome headset for audiometric purposes and of the probe tube mierophone methodology used in this study for transferring threshold standards from conventional to unconventional earphones.
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Surface, subsurface, and intercompartmental audio induction communication as influenced by large metallic surfaces, superstructures, and barriers (A)

Abraham B. Cohen, George J. Sebesta, and Alan Hofer

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S46-S46 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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Communication by induction systems on base audio frequencies may be considerably aided by the presence of ferrous metallic planes and enclosures, unlike equivalent deterioration for radio frequency links. The ferrous structures behave as bridging laminations of an otherwise large open air core audio transformer, with effects dependent upon the configuration of the ferrous surfaces and the degree of coupling of the induction coils to the structure. In practice, a master transmitting loop, either of one surround perimeter, or convoluted over several areas to be covered, is permanently fixed, while the subminiature receiving loop and amplifier is personnel carried over the area to be covered. Audio field patterns in, around, and through such ferrous structures are presented for a simulated typical shipboard installation, for cordless secure communication for moving personnel on board, and for underwater operations close to the ship.
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Role of acoustical technology in the development of new energy sources (A)

A. G. Jhaveri

J. Acoust. Soc. Am. Volume 58, Issue S1, pp. S46-S46 (1975); (1 page)

Online Publication Date: 11 Aug 2005

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During the past five years, considerable progress has been made in the control and abatement of excessive and/or harmful noise as well as vibration associated with the transportation, construction and industrial activities and systems. This has been a definite contribution of acoustics towards improving the quality of our environment and community life alike. A similar kind of concerted effort is required whereby the know1edge of acoustics can be advantageously utilized in the development of much‐needed new energy sources. Among the more challenging acoustical technologies available during exploration, detection, and identification of new conventional and non‐conventional energy sources are those utilizing ultrasonic, electroacoustics, underwater acoustics and geoacoustics, principles and/or disciplines. For example, echo‐sounding has been effectively used in the detection and assessment of marine resources. In the same way, acousti‐coring and seismic reflection techniques have been successfully employed during the exploration and detection phases associated with “mining geology” and “marine sediments.” However, the most attractive area of new energy source development, where acoustical technology can best be utilized, appears to be in the exploration of active "geothermal" sites. The ground noise (seismic) measurements utilizing sensitive and precision acoustical instrumentation can tremendously accelerate development of this important untapped and nonpolluting form of natural energy. This paper reviews the interaction of acoustics with new energy resource development programs.
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