摘要:

A definition of signal‐to‐noise ratio for magnetic recording systems is offered for the purpose of stimulating discussion. Factors involved in the choice of a medium for magnetic recording are then presented and discussed. Considerable attention is focused on modulation noise, a low value of which is necessary for high fidelity recording work. Background noise, crosstalk, and uniformity are the other factors which are presented as being significant in the choice of a magnetic recording medium. The ratio of coercivity to retentivity is suggested as a figure of merit for evaluating the high frequency response of a recording medium.

ISSN:0001-4966    

DOI:10.1121/1.1916495

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1916496

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

ISSN:0001-4966    

DOI:10.1121/1.1916497

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

The harmonic spectra of wooden and metal clarinets were obtained on an harmonic analyzer. The clarinets were blown artificially using a voicing technique similar to that used in certain reed organ pipes. The tests were performed on towers erected for the purpose in the open air. It is concluded that the following factors have no appreciable effect upon the steady‐state spectra: the respiratory tract and the‐wood or metal of which certain instruments are made. Factors which have important effects include the pressure on the reed and the position at which pressure is exerted; the blowing pressure; and the position of the note in the register. Observation of a vibrating reed under stroboscopic light verifies the conclusion that the reed is a beating reed.

ISSN:0001-4966    

DOI:10.1121/1.1916498

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

A modified form of an earlier design of impedance tube is described here. It is capable of measuring the normal impedance of a sample by the variable length, variable frequency, or traveling microphone methods without disturbing the sample in transferring from one test method to another. The tube also may be used to measure the transmission constant with the aid of a long probe tube. Modern graphical aids to the calculation of the impedance from the quantities measured in the tube are described. Comparison of data obtained by the variable length and traveling microphone methods is made, and good agreement is found over most of the frequency range. It is shown that the clamping effects of the tube walls on the edge of a lightweight, non‐rigid sample are serious enough to introduce large gyrations of the impedance curves at frequencies below 500 c.p.s.

ISSN:0001-4966    

DOI:10.1121/1.1916499

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

Soundproofing treatments for aircraft must be designed to provide high degrees of both attenuation of sound transmitted through the fuselage and absorption of sound within the cabin. At low frequencies, treatments of weights which are practical for aircraft provide little or no attenuation of transmitted sound, hence the sound absorption at low frequencies should be made as high as possible by spacing the treatment one to three inches from the dural. At intermediate and high frequencies, the sound absorption is approximately the same for most treatments which have a blanket of acoustical material at least one‐half inch thick on the side toward the interior of the cabin, and a highly porous trim cloth. If the trim cloth is not porous, the absorptive properties of the structure will be poor at low frequencies, which must be compensated for by sufficient absorption in the seat upholstering, carpets, etc. The flow resistance of the acoustical material used in the treatment should not be greater than about 500 g cm−2sec.−1per inch of thickness.Attenuation of sound by impervious panels, such as windows, dural, etc., at the intermediate and high frequencies is almost entirely dependent upon the surface (SS) density of the panels. The attenuation by an acoustically‐treated fuselage structure is at least as great as would be predicted from its surface density. The presence of air spaces, absorptive materials, and impervious septa in the treatment will tend to give greater attenuation than predicted by “weight law.” The relative “efficiency” of a treatment may therefore be characterized by a Merit Factor, which is the ratio of “db better than weight‐law” attenuation (at 5000 c.p.s.) to the surface density of the treatment.For both attenuation and absorption, the most effective of the simple treatments designed at this laboratory consists of two blankets of acoustical material with an impervious septum between, the treatment being mounted with an air space of one to three inches between it and the dural, and covered with a highly porous trim cloth.The choice of an acoustical material depends upon many mechanical and economic factors in addition to the acoustical requirements. Materials should be chosen on the basis of complete data on their acoustical transmission (attenuation) and absorption characteristics measured by the standard laboratory techniques referred to in this paper. A rapid estimate of the relative effectiveness of various materials in a given treatment may be obtained by measurement of the flow‐resistance of samples of equal surface density, at several thicknesses (obtained by compressing the samples). From a plot of flow‐resistanceR vs.thicknessT, the value ofRTxfor each material may be found. The material with the highest value ofRTxwill generally provide the highest attenuation in a given treatment.The validity of the laboratory measurements of acoustic attenuation and absorption discussed in this paper has been established by many measurements in airplanes of various types, in flight, with a wide variety of acoustical treatments.

ISSN:0001-4966    

DOI:10.1121/1.1916500

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

The results of calculations of the pressure distribution on the surface of a stationary rigid sphere and a stationary rigid circular cylinder of infinite length, when exposed to a plane progressive sound wave, are compared with experiment. A small probe microphone was used to measure the sound pressures on the surface of the obstacles in a room essentially free from acoustic wall reflections under a variety of experimental conditions. The sound pressurespon the surface are conveniently expressed relative to the free‐field pressurep0in the undisturbed incident wave.In the case of the sphere, reasonably good agreement was obtained between theory and experiment in the range of13 

ISSN:0001-4966    

DOI:10.1121/1.1916501

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

An optical means of measuring phase relations in the cochlea is described. With the stapes set into sinusoidal vibration, the phase relations, measured along the length of the cochlear partition, show that a traveling wave has been set up. The form of resonance which appears does not correspond at all with that to be found in a simple vibrating system. Measurements are presented for a frequency of 200 c.p.s. of the amplitude and phase of vibration at various places along the cochlear partition. From these is derived a curve showing the longitudinal bending of the partition with the traveling wave at two different points in time.Very small changes in the form of vibration can easily be demonstrated by the measurements of phase. By this means it is shown that the basilar membrane cannot be released from tension since it is not normally under tension. The elastic properties of the various distinguishable tissues within the cochlear partition were measured. These measurements show that only the elasticity of the basilar membrane changes through the length of the cochlear canal. The other portions of the partition are quite constant throughout the entire distance.Further, the non‐linearity in the vibration of the partition was investigated with results which indicate that the observed lowering of pitch of a tone when the loudness is increased cannot be produced by mechanical means.

ISSN:0001-4966    

DOI:10.1121/1.1916502

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

The paper deals with the ability of a submerged listener to hear sounds generated in the air above him, compared with their audibility when his head projects above the water. In a theoretical discussion it is shown that at 1000 c.p.s. a loss of the order of 45–55 db might be expected in the in‐water audibility relative to the in‐air value. This involves a number of assumptions, e.g., that there is no appreciable noise created by the listener's propulsion, and that the effect of hydrostatic pressure unbalance on the eardrum is negligible. A few measurements made at 1000 c.p.s. and 3000 c.p.s. yielded values which are not at variance with the theoretical analysis.

ISSN:0001-4966    

DOI:10.1121/1.1916503

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP

摘要:

By using a pulse method, attenuation and velocity measurements have been made for aluminum and glass rods in the frequency range from 2 to 15 megacycles. The sound pulses are generated by crystals waxed to the surface of the rod. This wax joint limits the band width of the transmitted pulse and measurements are made using long pulses which approach steady state conditions. The reflected pulses show evidence of several normal modes which can be minimized by using specially shaped electrodes. Longitudinal waves show delayed pulses of smaller magnitude that are caused by the longitudinal wave breaking up into reflected longitudinal and shear waves at the boundary. This effect is small if the diameter of the rod is 20 wave‐lengths or more.The measured losses for aluminum rods show a component proportional to the frequency and another component proportional to the fourth power of the frequency. The first component is the hysteresis loss found for most solid materials. The component proportional to the fourth power of the frequency is caused by Rayleigh scattering losses which are the result of differences in the elastic constants between adjacent grains caused by changes in orientation. Calculated scattering losses agree quite well with the measured values. The fourth‐power scattering law holds quite well until the grain size is equal to one‐third of a wave‐length. For higher frequencies the scattering loss increases more nearly with the square of the frequency. Glasses and fused quartz have a loss directly proportional to the frequency, showing that any irregularities must be of very small size.

ISSN:0001-4966    

DOI:10.1121/1.1916504

出版商:Acoustical Society of America    

年代:1947

数据来源: AIP