摘要:

This investigation is pointed toward obtaining some further measure of the acoustic properties of a room. First, a brief subjective study is made of listener response to sounds which have various degrees of modulation on their decay curves and to alternate sounds which differ only in the phase relation of one of the harmonics with respect to the fundamental frequency. Second, an objective study of the modulation of sound decay curves is carried out for rectangular rooms, with emphasis on the effect of the phase and amplitude of harmonics present in the source of sound. In this study, the sound source consisted of a Complex Wave Generator, a power amplifier, and a loudspeaker. The Complex Wave Generator provides output terminals for the fundamental frequency and for the first four harmonics at controlled phases and amplitudes. The power amplifier is a conventional resistance-coupled amplifier combined with a device designed to cut off the output at any predetermined phase of the fundamental frequency and with a minimum switching transient. The loudspeaker and the complete recording system are conventional.

ISSN:0001-4966    

DOI:10.1121/1.1917353

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

An 11‐Mc/sec “double‐crystal” interferometer for use with gases at low pressures is under construction at the National Bureau of Standards. Previous experience indicates that the principal difficulties associated with such an instrument arise from impurities in the gas (at low pressures), variable electrical cross‐talk between the transmitter and the receiver, and acoustical cross‐talk through the solid parts of the apparatus. The rather high operating frequency was chosen so that for a given dispersion a higher gas pressure would suffice than at a lower frequency. The sound is introduced into the gas through a long coupling rod so that the distance between transducers changes by only a small fraction during a run. The electrical cross‐talk is thus approximately constant and can be neutralized. The receiver crystal is mounted in a labyrinth of several concentric cylinders to provide a long path for leakage of sound between crystals. The output of the receiver is preamplified and converted to an intermediate frequency for further amplification and filtering. The interferometer screw is driven synchronously with two recorders, one of which records the phase of the output relative to that of the input, and the other the amplitude of the output on a logarithmic scale.

ISSN:0001-4966    

DOI:10.1121/1.1917358

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

If in the expressionpe = Aei(ωt−kz)for the excess pressure in a plane progressive harmonic sound wave it is assumed thatkis complex and we writek = kr+iki, the solution corresponds to a damped wave with absorption coefficient α = −ki, measuring attenuation of pressure amplitude withdistance. This may be calledspatialattenuation. If, on the other hand, it is assumed that ω is complex, withω = ωr+iωi, the solution corresponds to a damped wave with absorption coefficient ωi, measuring attenuation of pressure amplitude withtime. This may be calledtemporalattenuation. It is not commonly emphasized that the associated dispersion is in general quite different in the two cases. Thus, in absorption in a fluid due to heat conduction and viscosity the phase velocity increases with frequency for spatial attenuation but decreases with frequency for temporal attenuation. At low pressures in gases the difference might be detectable. Typical dispersion and absorption curves are presented for the above‐mentioned cases as well as for thermal and structural relaxation mechanisms in which the situation is rather different.

ISSN:0001-4966    

DOI:10.1121/1.1917363

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

During a study of the effect of molecular structure on sound velocity in liquids, a liquid was found in which the velocity of sound is 3000 meters/second at 20°C, as measured by a 3‐Mc ultrasonic interferometer. Previously the highest value known was less than 2000 meters/second, for glycerine. The new liquid is a viscous concentrated aqueous solution of sorbitol, prepared by heat dehydration of the commercial syrup “Arlex.” Sorbitol was selected for study, even though it is a solid, because its chemical structure is that of a double glycerine molecule. Concentrated sorbitol solutions also show a dispersion of sound velocity with frequency in the megacycle range (reminiscent of the behavior of polyisobutylene as described by Mason) and absorption coefficients many times larger than that of glycerine.

ISSN:0001-4966    

DOI:10.1121/1.1917368

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

Sound waves of finite amplitude are subject to progressive distortion or change in form during propagation. Theoretical analysis of this phenomenon for a plane wave neglecting dissipative forces leads ultimately to a multiple valued solution for acoustic pressure, displacement, etc. This is inconsistent with fact. R. D. Fay, introducing viscosity into the analysis, arrived at a solution in which the progressive distortion is balanced by the dissipative effects of viscosity. This wave, called the “stable” form, is essentially sawtoothed, varying in sharpness depending upon its intensity. Experimental measurements in a free spherically‐diverging sound field verify the existence of the stable wave by showing the progressive distortion of an initially sinusoidal wave to a sawtoothed stable form. Comparison of the experimental results (for the diverging field) with the theoretical results (for a plane wave field) reveals good agreement as to the harmonic content of the stable form, except that a higher sound intensity is required in the diverging field. Both experimentally and theoretically, for sufficiently high intensities, the pressure amplitude of thenth harmonic relative to that of the fundamental approaches 1/n.

ISSN:0001-4966    

DOI:10.1121/1.1917374

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

Instrumentation will be described for the measurement of the relation between the total acoustic energy emitted by a floor when subjected to a mechanical impact and the striking velocity of the impact hammer. In addition, the mechanical force generated during the impact is measured. Some experimental results on a wooden floor and steel plate will be described. The relation between these measurements and the problem of standardizing routine impact sound measurements on floors will be discussed.

ISSN:0001-4966    

DOI:10.1121/1.1917382

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

At the present time, standards of sound pressure corresponding to normal auditory threshold apply directly only to the particular type of earphone used in the original nationwide survey of hearing. Standards for new types of earphones are determined by subjective loudness‐matching against earphones for which threshold standards exist. A number of skilled observers with normal hearing must perform this test, working at a level very near auditory threshold. Since auditory threshold is defined in terms of sound pressure at theentranceto the ear canal, measurement by probe techniques of the sound pressure developed by an earphone at the entrance to the ear canal seemed to offer a means of determining equivalent standards for audiometer earphones without requiring subjective judgment. Data on individual observers‐obtained by both objective and subjective techniques will be compared.

ISSN:0001-4966    

DOI:10.1121/1.1917385

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

Electrical responses to supra‐threshold acoustic clicks are easily observed at the two extreme stations of the auditory nervous system of anesthetized cats: the round window and the auditory cortex. The neural components of the round window response and the cortical potentials have been shown to be particularly sensitive, amplitudewise, to masking noises and to previous exposure to loud sounds. The effects of repeated exposures and the effects of the intensity and duration parameters for given exposures have been investigated. We have also collected data regarding central factors in masking and fatigue. Present results indicate: (1) that the cortical response to supra‐threshold clicks delivered to the left ear remains essentially unaffected after the right ear has been exposed to loud sounds; (2) that the cortical response to a click delivered to the left ear is unaffected by a masking noise delivered to the right ear, even if the noise is sufficient to reduce (mask) the neural response to a right ear click stimulus. These results are valid only as long as there is no appreciable acoustic leakage from ear to ear.

ISSN:0001-4966    

DOI:10.1121/1.1917392

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

Theoretical and experimental studies of wave effects in vibration mounts have been made. From the standpoint of noise isolation, wave effects are important in the sense that the noise isolating properties of a mount are impaired by the presence of wave effects. The well‐known lumped parameter treatment of vibration mounts predicts that the vibration isolation of a mount increases at 12 db per octave for frequencies well above the resonant frequency of the spring‐mass system. This is true only when the wavelength of the elastic wave in the mount is large compared to the dimensions of the mount. Standing waves occur as would be expected and often impair the vibration isolation properties of the mount by as much as 20 db. For practical mounts, wave effects are often most deleterious in the audible frequency range 500 to 2000 cps. The theoretical and experimental treatments give good agreement and indicate various means for improving the vibration isolating properties of the mount.

ISSN:0001-4966    

DOI:10.1121/1.1917400

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP

摘要:

It was recently demonstrated that frequency was represented in the MES area by parallel overlapping bands of cortex rather than by points. Response curves for strychninized patches along a band revealed no differences in the intensity and frequency characteristics for the opposite ear. Response curves for the ear on the same side showed that the same frequency elements were used, but the intensity levels of the curves were elevated depending upon the position of the band tested. At the upper end of the band the intensity for the two ears was most similar, but at the lower end of the band the intensity for the ipsilateral ear was 40–80 db higher than for the contralateral ear. Intervening portions of the band showed intensity levels between the two limits. There appears to be an intensity representation along the band for the ipsilateral ear, but not for the contralateral. The gradient is about 0.2 mm/db. No information concerning the frequency and intensity of the tones can be found in the potential characteristics nor in the response or masking curves. From various evidence the only information present in the patches is whether an electrical change occurs or does not. These two conditions can be represented by 1 and 0. In the frequency axis of the area, 70 elements (0.2 mm or 0.1 octave wide) have been measured. Thus, 270or 1025pieces of information can be displayed on the area in the frequency dimension alone.

ISSN:0001-4966    

DOI:10.1121/1.1917402

出版商:Acoustical Society of America    

年代:1951

数据来源: AIP