摘要:

The space absorbers were long cylindrical shells of perforated metal, containing glass fiber. The absorbers were to be used in an aero‐engine test cell, and these conditions were simulated in the measurements by mounting a few absorbers in a long concrete pipe, so that the cross‐sectional ratio (absorber/enclosure) was the same in each case. The direction of propagation of sound was parallel to the axis of the cylinders so that absorption occurred at grazing incidence. The concrete pipe was terminated by a rigid reflector, and the attenuation figures were deduced from the resulting interference pattern using impedance tube theory. The theory was extended to the case of octave bands of noise, when the interference pattern for the mean squared pressure is(p2) = [cosh2ax sinh(2ax/3)]/(2ax/3) + [cos(2kx) sin(2kx/3)]/(2kx/3), whereais the sound attenuation constant,kis the wave number at the center frequency of the band, andxthe distance along the pipe measured from the reflecting end. Experimental results were obtained which followed this pattern, and the required sound attenuation constant was obtained by a graphical method. Measurements were made at low frequencies only, 300 cps and below.

ISSN:0001-4966    

DOI:10.1121/1.1908446

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The author has constructed a device which gives pointer indications of acoustic intensity over a 50‐db range at frequencies up to 10 000 c/sec.The signals from a pair of pressure‐sensitive condenser microphones, mounted back to back in a space roughly the size of a dime, are treated electronically to provide pressure and velocity signals which are then multiplied and averaged in a specially designed moving‐coil dynamometer to give precise pointer readings.The theory of operation of the apparatus is discussed and the paper concludes with a brief account of the test results of the acoustic wattmeter.

ISSN:0001-4966    

DOI:10.1121/1.1908448

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The application of lateral forces by a relatively stiff structure such as a small iron vise to one end of a longitudinally vibrating crystal or composite bar, e.g., a crystal and backing plate, has been used to control and vary the resonant frequency of the system. A change in resonant frequency of over an octave for a composite system has been measured. It is believed that the phenomenon depends upon the (dynamic) elastic range of friction as first noted for the static case by J. S. Stevens in 1899, in addition to the elastic parameters. It is noted that for a given crystal (material and dimensions) and backing plate (material) there are optimum backing plate lengths to obtain both maximum and minimum changes in resonant frequency. For example a one inch 45° Y‐cut Rochelle salt crystal with a one‐half inch steel backing plate can undergo a continuous change in resonant frequency from 26.5 to 66.7 kc. The same crystal with a three inch backing plate will only change from 22.2 to 24.4 kc. It was found that the type of clamping herein described results in an additional mode of vibration which may be very low in frequency depending on the lateral force.

ISSN:0001-4966    

DOI:10.1121/1.1908450

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

A self‐reciprocity free‐field response calibration of a resonant electroacoustic transducer may be determined in a small water‐filled tank by only two electrical impedance measurements at each discrete frequency point. The use of a pulse technique permits the measurement of electrical impedance under two conditions: (1) in a free field and (2) under the influence of a totally reflected signal. The vector difference between these two impedances together with the reciprocity parameter (J) establish the free‐field response. No measurement of voltage, current, or power is required; the ratio of the hydrophone voltage to the projector current is measured as an impedance vector. This method is particularly applicable in the ultrasonic frequency range.

ISSN:0001-4966    

DOI:10.1121/1.1908453

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

Mathematical expressions are derived for the motion of a rigid, uniform sphere in an underwater sound field. The results show that at low frequencies a neutrally buoyant sphere of this type has a velocity equal to the particle velocity of the water at the same location when the sphere is removed. A velocity pickup may be mounted inside such a sphere to form a hydrophone sensitive to particle velocity. Two practical design applications are presented. A low‐frequency hydrophone with a diameter of five inches was constructed having an essentially flat response between 15 and 700 cps with a sensitivity of 0.18 volt/cm/sec working into a 500‐ohm load. A higher frequency design with a 2.5‐inch diameter operated from 70 to 7000 cps with somewhat lower sensitivity and impedance level. Velocity hydrophones of this type are easy to build, easily calibrated, stable, and possess good directionality.

ISSN:0001-4966    

DOI:10.1121/1.1908455

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1908457

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1908459

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1908465

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The recent paper by Hueteret al.on ultrasonic lesions in the central nervous system, which appeared in a recent issue of this Journal (March, 1956), presents a rather limited view of this field and may lead to some misunderstandings.This note is a critical analysis of the Hueter paper. Some of the more important points discussed are: (1) The unique advantages of producing ultrasonic lesions in the brain by using dosages appropriate for selective action combined with focusing of the beam in contrast to the use of focusing alone. (2) The choice of an irradiation procedure in experiments designed for the elucidation of physical mechanisms of the action of ultrasound on tissue. (3) The specification of the acoustic variables required for a dosage description. (4) The inapplicability of Eyring's theory of viscosity, plasticity, and diffusion to the elucidation of observed dosage relations. (5) The clarification of the usage of the terminology, nontemperature effects. (6) The applicability of the reciprocity method for the calibration of thermo‐couple probes.

ISSN:0001-4966    

DOI:10.1121/1.1908467

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP

摘要:

The use of the ultrasonic pulse technique as a means of measuring ultrasonic absorption in an aluminum alloy (2S) at high temperatures has been considered. The absorption for both the longitudinal and shear waves was computed. These measurements show that the absorption is relatively constant up to approximately 200°C where it then experiences a large increase. The major part of the absorption at high temperatures is found to be due to thermal effects.

ISSN:0001-4966    

DOI:10.1121/1.1908469

出版商:Acoustical Society of America    

年代:1956

数据来源: AIP