摘要:

Utilizing a formulation of output power of a digital beamformer with input quantizers ofunequalinput and output step sizes reported elsewhere [H. S. C. Wang, Quantizer Functions and Their Use in the Analyses of Digital Beamformer Performance (to be published); also see J. Acoust. Soc. Amer.51, 135(A) (1972)], the optimum quantizer step sizes that yield maximum processing gain are searched by the method of “steepest ascent” with a digital computer. For the case of 2 bits/channel quantization with small input S/N (−10 dB), no maximum can be found, and, as the input S/N exceeds −4 dB, the array gain increases monotonically with respect to both input and output step variables. These phenomena were clarified by a numerical study of a “3‐level quantizer” as a limiting case. The large theoretical processing gain at large step sizes is attributed to a threshold effect of the input quantizers.

ISSN:0001-4966    

DOI:10.1121/1.1982393

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Explosives are often used as a source of acoustic energy in transmission measurements in the ocean because of the broad‐band spectra of underwater explosions and the wide range of levels that can be generated. At the same time, this broad‐band spectrum can complicate a transmission loss measurement because the spectral character of the shock wave and subsequent bubble pulses of the shot can be very similar to the spectral character of the loss that is due to the multipath structure of the propagation. This complication can be avoided by the proper choice of the detonation depth and the charge weight. When these choices have been made, the properties of the analyzing filter of the data processing system must then be selected; this must be done with regard for the nature of that portion of the multipath structure to be resolved as well as the nature of the charge spectrum. This paper discusses the principles involved in making these choices and gives illustrations of both short‐ and long‐range measurements.

ISSN:0001-4966    

DOI:10.1121/1.1982397

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

A convenient laboratory method for producing highamplitude elastic stress waves in rods and pipes is described and compared with existing impact methods [C. S. Yang and R. J. Hassett, “Transient Stresses in Axisymmetric Bodies of Varying Area,” Exp. Mech.12, No. 7 (July 1972)]. The method employs electromagnetic repulsion of good electrical conductors to convert electromagnetic energy to mechanical energy stored in the elastic stress wave. A description of the apparatus and its design features is given as well as equations that predict the amplitude and duration of the elastic waves. The apparatus was used to study the stress wave characteristics of pipes. In particular, a stress wave with speed of propagation higher than the longitudinal phase velocity in elastic isotropic solids predicted by Chang, Cakmak, and Lee [K. P. Chang, P. C. Y. Lee, and A. S. Cakmak, “Propagation of Axially Symmetric Waves in Hollow Elastic Circular Cylinders Subjected to a Step‐Function Loading,” J. Acoust. Soc. Amer.49(1971)] was found experimentally using the new method.

ISSN:0001-4966    

DOI:10.1121/1.1982405

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

A literature review is prepared to summarize the present state of knowledge concerning the field performance of barriers, with particular reference to screen‐type barriers on finite impedance ground, to barriers of finite thickness, to right‐angle wedges, to multiple barriers, and to effects of wind. The attention is focused on practicabilities and limitations of prediction schemes applied for the design of barriers, which shall reduce the noise ontdoors from vehicles on the ground and from stationary installations.

ISSN:0001-4966    

DOI:10.1121/1.1982407

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

A study that has received added consideration in the past few years is the propagation of sound in the atmosphere. This was occasioned by the introduction of jet aircraft and the resulting noise which required locating airports in such a position so as to minimize the undersirable effects. A study of the acoustical ray paths in the atmosphere would provide criteria for selecting an appropriate location. A computer program has been written for the CDC‐3800 computer to predict the acoustical ray paths in the atmosphere, as well to determine the transmission loss in air. The program accepts data in the form of a temperature or sound speed versus height profile. A wind velocity profile may also be entered. The program will produce a printplot of the ray trajectories, as well as a printplot of the transmission loss. A CALCOMP plot of the ray trajectories may also be obtained. By observing the locations where the rays strike the surrounding terrain and the resulting transmission losses, a suitable location can be obtained to produce the least annoyance.

ISSN:0001-4966    

DOI:10.1121/1.1982411

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

It is vitally important to know accurately the ultrasonic energy levels of electronic equipment that generates and sends ultrasonic energy into the human body. Therefore, a radiation force measuring technique was developed to measure the ultrasonic energy generated by such equipment. Ultrasonic energy levels of less than 1 mW/cm2have been measured, with good repeatability. The equipment used is a Mettler Balance, model H‐20‐T, with a readable sensitivity of 0.01 mg and a target that detects the incident radiation force. The target is attached to the balance weighing pan and suspended in water under the balance by a nylon line. The balance has a built‐in tare, used to counteract the ultrasonic energy force that illuminates the target. In addition the tare is adjusted to counteract the surface tension that normally occurs at the nylon support line, which would interfere with the accuracy of power measurements. The target consists of an air‐backed reflector that has a half angle of 45°. This angle and the air space behind the thin metal surface makes the target a nearly perfect reflector. To make certain that no ultrasonic energy is reflected from the walls of the water tank, the ends of the tank are tilted about 20°, and all the inside surfaces are lined with ultrasonic energy absorbing material. This equipment has been in use now for over a year. Ultrasonic energy power measurements have been made on many different sizes, configurations, and frequencies of ultrasonic transducers. Details of this technique of measuring ultrasonic power will be reviewed in the paper.

ISSN:0001-4966    

DOI:10.1121/1.1982420

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The technique of reciprocity calibration, which allows the absolute determination of sound quantitaties, has been effectively used for many years at low frequencies. The self‐reciprocity modification of this technique requires fewer measurements and has higher accuracy. Present day pulse echo systems have been used to perform this calibration on their own transducer. These transducers are available with wide bandwidths and can be used to measure both peak and average fields. The technique is discussed from the standpoint of energy density, from which the self‐reciprocity conditions and derived in a convenient form. The choice of auxiliary measurements of impedance or open‐circuit voltage is also discussed. No acoustical standards are required. The technique appears to be suitable for average power measurements on pulsed and CW systems. Corrections for short‐pulse operation of typical medical systems require further investigation.

ISSN:0001-4966    

DOI:10.1121/1.1982423

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

Free‐flooding piezoelectric ring transducers have good efficiencies, are capable of radiating high acoustic power, are of simple design, and have the mechanical strength required for deep underwater submergence operation. However, the omnidirectional acoustic radiation in the plane of a ring transducer is undesirable for applications where directional beams are required. Directional acoustic beams may be obtained from nondirectional transducers by the use of reflectors that are bulky, costly, and difficult to maintain for large low‐frequency arrays or by using volume arrays in which the transducer elements are spaced and phased in such a manner that acoustic radiation from all transducers adds in the direction along the acoustic axis of the array and cancels in the opposite direction. Since the technique used to get directionality from biplanar or volume arrays controls acoustic radiation only along the 0° and 180° but not the 90° and 270° directions associated with beam pattern charts, the system works best when transducers with dipole (figure‐eight type) beam patterns are used. The design of a dipole ring and spherical transducer will be discussed and measurement data presented.

ISSN:0001-4966    

DOI:10.1121/1.1982427

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

The STL‐ionophone consists of a dc discharge in air. The discharge emits sound when an ac current is superimposed on the dc current. It represents a small sound emitter with high internal impedance and constant source strength from low to ultrasound frequencies. It can also register air motions as a superimposed voltage and works in this function from zero to ultrasound frequencies. It has been employed as a sound source for measurements on woodwind instruments and models of the human vocal tract. As a receiver, it has been employed to study the sound field at the embouchure of flutes. Fundamental properties registered and applications will be discussed.

ISSN:0001-4966    

DOI:10.1121/1.1982432

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP

摘要:

A total of 18 young deaf adults read a 181‐word passage, and distributional characteristics of fundamental frequency (f0) were investigated using a period‐by‐periodf0analysis program. The results of analysis showed significantly higherf0means and smaller standard deviations and mid‐90% range than those for normal‐hearing subject. Deaf speech was characterized, furthermore, by significantly less skewness of fundamental frequency distribution. In addition, bimodality or multimodality off0distributions commonly observed in speech of normal‐hearing subjects were also less clear in the deaf speech samples. The results indicate limited, monotonous pitch variability and almost constantly strained type of phonation in deaf speech. [This research was supported in part by Air Force Cambridge Research Laboratories, OAR, and by National Institute of Dental Research.]

ISSN:0001-4966    

DOI:10.1121/1.1982440

出版商:Acoustical Society of America    

年代:1973

数据来源: AIP