摘要:

The propagation of plane waves in a piezoelectric medium has been studied. A simultaneous solution of Maxwell's field equations and Newton's law of force gives the necessary conditions for wave propagation in the infinite medium. By piezoelectric coupling, every electromagnetic wave has associated with it an elastic wave. Nonpiezoelectric crystals are, at best, birefringent electromagnetically and trirefringent acoustically. In piezoelectric media, the electromagnetic and elastic modes are coupled, thus giving five phase velocities for both the electromagnetic and acoustic waves. These media are therefore electromagnetically and acoustically pentrefringent. The slower electromagnetic waves are the field variations accompanying the elastic waves, while the fast elastic waves are the mechanical deformations accompanying the electromagnetic wave.The displacement in the elastic wave will differ in phase by ninety degrees from the electromagnetic wave, its amplitude being proportional to the electric field strength and inversely proportional to the frequency of vibration. The flux of energy will, in general, be in directions different from that of phase propagation. The generalized Poynting vector will be the sum of two vectors representing separately the flux of electromagnetic energy and the flux of elastic energy.Analysis was made of propagation in a particular direction in sphalerite. The orientation of the vectors involved has been calculated for the fast and slow modes of vibration.A study of specific boundary value problems gives interesting results. The piezoelectric effect gives a contribution to the reflection of electromagnetic waves at an interface. This results in a small correction to be applied to the familiar expressions for the reflection coefficients. This correction might be used to determine piezoelectric constants, providing it is possible to measure accurately the reflection coefficients and other constants involved.Slabs of piezoelectric media may be set into vibration by an incident electromagnetic wave. At particular frequencies the crystal may be set into resonance. At resonance for the elastic wave there is almost perfect reflection of the incident electromagnetic wave. It should be possible to detect these resonances by absorption measurements.The piezoelectric slab, if set into resonance by an acoustic wave, emits a weak electromagnetic signal.

ISSN:0001-4966    

DOI:10.1121/1.1906490

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

The absorption of a plane wave of sound by a sphere is computed. The calculations are based on the assumption that the complex ratio of sound pressure at a point on the sphere's surface to the normal component of particle velocity is a constant independent of the direction of incidence (“normal impedance assumption”). Absorption measurements were made on hair felt‐covered spheres placed in a reverberation room, and were compared with the computed absorption by means of the reverberation room statistics appropriate for spheres. The theory and measurements both show that absorbent spheres can have absorption coefficients greater than unity. The discrepancies between theoretical and experimental coefficients seem to indicate that the normal impedance assumption is not valid for the hair felt used in the experiments.

ISSN:0001-4966    

DOI:10.1121/1.1906491

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

Improvements to an ultrasonic interferometer are discussed. As a result a greater sensitivity to acoustic reactions is obtained and the reproducibility of the data is greatly improved. Velocity measurements in driedCO2‐free air, driedN2, and driedH2are given. Amplitude absorption coefficients forH2,N2, andCO2are also included. Measurements are made over the temperature range from 9°C to 36.6°C and over the pressure range from 82.17 cm Hg to 0.45 cm Hg. The frequencies extend from approximately 500 kc to 2.16 mc. A maximum value for the absorption inH2, attributed to molecular absorption, is located at anf/pratio of approximately 10 mc/atmos.

ISSN:0001-4966    

DOI:10.1121/1.1906492

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

Supersonic velocities and adiabatic compressibilities of a series of mixtures of water‐ethyl alcohol‐sodium chloride have been found. In general, maxima of sound velocities and minima of adiabatic compressibilities were obtained. With increase of salt concentration, the depth of the minimum of compressibility decreased and shifted towards the value for alcohol free‐salt water, and for 3Nconcentration of salt, no minimum at all remained. The phenomenon has been explained as caused by the depolymerization of water by sodium chloride. The explanation supports earlier results on the water alcohol system.

ISSN:0001-4966    

DOI:10.1121/1.1906493

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

The motional impedance of a magnetostrictive element is determined by a unique method involving embedding of the element in a block of Permafil, a non‐magnetic plastic. The method used can be adapted to determine the motional impedance of any odd‐shaped magnetostrictive element. The possible application of this method to the determination of magnetostrictive constants is pointed out.

ISSN:0001-4966    

DOI:10.1121/1.1906494

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

An analysis of the substitution method of measuring the open circuit voltage generated by a microphone is given which shows that the “normal” substitution voltage equals the open circuit voltage for all types of acoustic measurements and for any value of electric impedance loading the microphone. It is shown that the method recently proposed by some authors of removing the acoustic load from the microphone when applying the substitution voltage results in a substitution voltage which does not equal the open circuit voltage. It is also shown that a formula for the response of a transducer derived for a system in which the microphones are open‐circuited may be used when the microphones are terminated by finite electrical impedances, by replacing the generated open circuit voltages in the formula by the corresponding “normal” substitution voltages.Consideration is given to the restriction in the definition of the pressure response of a transducer made necessary by the fact that the pressure on a microphone diaphragm is a function of the electrical impedance terminating the microphone.An experiment is described which involves a microphone coupled to a chamber, the acoustical impedance of which is high relative to that of the microphone. The results of this experiment agree with the conclusions of the analysis.

ISSN:0001-4966    

DOI:10.1121/1.1906495

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

The earlier work on the comparison of the JRB and the Massa Artificial Ear couplers has been continued to seek the explanation of certain differences observed. The later work, for the most part, has been undertaken on couplers under active consideration for adoption by the American Standards Association.The difference in pressure in 6‐cc couplers as measured by the Western Electric 640‐AA and the Massa M‐101 microphones appears to be identified with the strong earphone coupler resonance peak rather than directly with the shape of the coupler. The effect is apparently associated with the diaphragm impedance of the microphone more closely when the microphone is in use as a measurement device than when merely present, but inactive, in the coupler, except in the case of the M‐101 microphone whose presence gives rise to an unexplained increase in pressure.For general earphone calibration work, the results obtained by means of any of the coupler systems will be satisfactory. For some physical purposes, however, the effects may lead to significant errors.

ISSN:0001-4966    

DOI:10.1121/1.1906496

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

The conditions under which titanate ceramics may be made to have piezoelectric properties suitable for application to transducers are described. These conditions involve a selection of the titanate material to give a Curie point well above the working temperature, polarization from an external voltage source within certain limits of potential gradient and time duration, and mechanical structure. Most common transducers have a mechanical impedance far below that of a solid block of ceramic. Multilamellar strips have been developed to provide the mechanical impedance required in a phonograph pick‐up, microphone, or the like. These consist, in general, of a metal strip with a thin plate of ceramic soldered to one side or with thin plates soldered to both sides. As the multilamellar strip cools below the solidification point of the solder, the ceramic becomes pre‐stressed in compression. This strip is then used in flexure in a transducer. It has many unique properties, especially for applications involving extreme conditions of humidity.Applications of this strip for a phonograph pick‐up and to a microphone are shown.Precautions involved in controlling production of a pick‐up are discussed.

ISSN:0001-4966    

DOI:10.1121/1.1906497

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

Two sources of non‐linear distortion in a dynamic loudspeaker are considered in this discussion; both are related to the magnetic characteristics of the driving mechanism The first type of distortion arises because of a force of attraction between the voice coil, carrying a current, and the iron of the field structure. This force varies as the square of the current and produces second harmonic distortion. The force may be related to the space rate of change of self‐inductance of the voice coil as it moves in the air gap. The magnitude of the distortion produced in this way may be several tenths of one percent, and is greater for low frequencies and large currents. This distortion may be reduced by proper proportioning of the voice coil and field structure and by using a short‐circuited winding on the field structure.The second type of distortion arises due to non‐uniformity of the magnetic field in which the voice coil moves. This effect is well known qualitatively, but equations are given here for its quantitative evaluation. These equations indicate that the distortion characteristically is less than one percent, and is greater for large amplitudes of motion. If the voice coil is centered in a symmetrical field, only odd‐order distortion is produced. If the field is not symmetrical about the voice coil, even‐order distortion is also present. This distortion may be reduced by proportioning the voice coil and field structure so that the mean field in which the coil moves remains as constant as possible.

ISSN:0001-4966    

DOI:10.1121/1.1906498

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP

摘要:

In order to isolate some of the factors affecting the internal tuning of brass instruments several differently tapered bells were made and a trumpet made up such that the different bells could be easily attached and detached. Tests were then run on the internal tuning using both a physical measure of the trumpet air column resonant frequency and a group of five players. The results were analyzed statistically and indicate that changes in the shape of the bell taper as great as those used for these tests caused significant changes in internal tuning, and that these changes were functionally related to the type of taper. Secondary results of the analysis gave measures of the magnitude of the differences among players and trials and showed the relationship between the player average and the physical measure of air column resonance.

ISSN:0001-4966    

DOI:10.1121/1.1906499

出版商:Acoustical Society of America    

年代:1949

数据来源: AIP