ISSN:0001-4966    

DOI:10.1121/1.1907504

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

Extensions of earlier studies [J. Acoust. Soc. Am.26, No. 2(1953)] on ventilation system noise are reported. It is shown here that in properly engineered ventilation systems the over‐all sound power level in decibelsre1013a watt is related to the horsepower rating of the fan motor by the formula:PWL = 100+10 1og10hp. The octave frequency band spectrum slopes off with increasing frequency at the rate of 5 decibels per octave. The level in the first band is one decibel below the over‐all level. Measured data on fourteen fans in actual installations yield a spread of about ±4 db around these relations.

ISSN:0001-4966    

DOI:10.1121/1.1907505

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

A 10‐gauge blank shotgun shell, exploded in a small cannon, has been used as an impulse source in performing a variety of acoustical measurements. The source has a peak power of the order of 10 000 watts with good reproducibility. The frequency spectrum simulates that of a typical turbojet engine. Instrumentation has been developed to permit simple and rapid analyses of measurements made with this source. Typical applications are described, and results obtained with the impulse source are compared to results obtained with continuous sources.

ISSN:0001-4966    

DOI:10.1121/1.1907506

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

A study has been made of the applicability of correlation techniques to the field of acoustic measurements. The development of an analog electronic correlator for this study is reported here while the application of the correlation technique to some acoustic measurements is reported in a companion paper.The analog correlator is divided into the following four main components and each one is discussed in detail: (1) variable time delay; (2) quarter‐squaring multiplier; (3) continuous and stepping integrators; (4) system for scanning and plotting the crosscorrelation function on both linear and logarithmic scales.The correlator operates over an input frequency range of 100 cps to 10 kcps, a range of relative time delay τ from −15 to 190 milliseconds andRCintegration times of 0.5 to 16 seconds. A dynamic range in the crosscorrelation function of approximately 50 db is achieved with ±1 db accuracy.Special attention has been given to the errors in the crosscorrelation function resulting from finite integration time and scanning of the correlation function. A significant increase in permissible scanning speed for a given signal‐to‐noise ratio has been shown to result from “matching” the equivalent integrator pass band to that of the spectrum common to the two signals applied to the correlator. Relations have been derived between scanning speed, signal‐to‐noise ratio, and input signal band width for integration performed by either a low‐pass filter or a "matched" filter.

ISSN:0001-4966    

DOI:10.1121/1.1907507

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

This paper discusses the application of an analog correlator which computes the crosscorrelation function between two sound pressures to such measurement problems as the localization of noise sources, the determination of transmission loss, and the reduction of microphone wind noise.The crosscorrelation function φ21(r) between two nonperiodic signals will have a peak in amplitude if a component of each signal originates from a common source. The value of time delay τ for which this peak occurs equals the difference in time required for the individual components to propagate from the common source to the two points under study. An analog electronic correlator described in a companion paper has been constructed which, by employing this property of the crosscorrelation function, can separate the acoustic signal at a given point into components according to: (1) their points of origin (assuming independent sources), (2) the transit time from source to the point in question, and (3) frequency.Preliminary experimental results show that correlation provides a practical method for determining the amount of sound contributed to the field at a given point by each of several sources. The correlator, by separating the signal transmitted directly through the structure from the flanking signal on the basis of arrival time, is also a useful tool for measuring the transmission loss of walls.

ISSN:0001-4966    

DOI:10.1121/1.1907508

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

In a reverberant sound field, where at all points equal mean energy flows in all directions, it is shown that the sound energy is distributed into interference patterns the reflecting boundaries. Thus the mean energy density is not uniform at all points in the field. For a perfectly reflecting plane surface that is large compared with the wavelength, the interference pattern can be expressed as a mean squared sound pressure varying as(1 + sin2kx/2kx)wherexis the distance from the surface andkis the wave number. Corresponding expressions are derived for the mean squared particle velocity and the mean energy density. The energy level at the surface is found to be 2.2 db higher than at points further away where the interference patterns are negligible. Similar expressions are derived for the interference patterns formed by two and three reflectors at right angles, as at the edges and corners of a room. The largest departure from uniformity occurs in a corner where the mean squared pressure is 9 db higher than at remote points. The effects of such interference patterns on transmission loss and reverberation room measurements are discussed briefly. The patterns are not much affected by the frequency band widths habitually used in room acoustics. Experimental confirmation of the theory is given.

ISSN:0001-4966    

DOI:10.1121/1.1907509

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

The vibration of an infinite elastic plate when driven by a localized driving force is studied theoretically. The dilation and shear potentials are expressed as Fourier integrals, the boundary conditions are applied, and the integrals evaluated by the calculus of residues. It is found that the motion of the plate may be represented by a discrete sum of nonorthogonal eigenmodes. These modes represent two types of waves, propagated and attenuated. The former are obtained from previous work on coincidence transmission. The latter are calculated by a graphical method, and presented in such a manner that they tie in continuously with the propagated modes. Certain unresolved features of the problem are discussed. One previous application of the theoretical results is disclosed.

ISSN:0001-4966    

DOI:10.1121/1.1907510

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

Approximate solutions are given for the fundamental frequencies of flexural vibration of flat triangular plates with all edges clamped. The plate equation is written in skew coordinates and the method of collocation is used to obtain the approximate solutions. Results are presented in simple graphical form so that one may determine rapidly a desired fundamental frequency.

ISSN:0001-4966    

DOI:10.1121/1.1907511

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

A device is described for measuring the intensity of an acoustic wave at a point. The instrument employs a crystal microphone as a pressure transducer, directional hot‐wire anemometer as a velocity transducer and an electronic multiplier and integrator. We can directly measure the intensity for any pressure or velocity wave form without having to assume any relationship between pressure and velocity. Several types of measurements have been made to indicate the present capabilities of the instrument.

ISSN:0001-4966    

DOI:10.1121/1.1907512

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP

摘要:

Given a wave duct with continuously variable parameters extending from y=0 to y=H, the reflection coefficients for originally plane downgoing and upgoing waves are written in the formR↓ = −exp(2iχ↓)andR↑ = minus;exp(2iχ↑), respectively. It is then shown that the characteristic equation governing the dispersion of undamped normal modes in the medium isχ↓+χ↑=mπ,mbeing an integer defining the mode number. χ↓ and χ↑ are solutions of nonlinear first‐order differential equations. If the medium consists ofnhomogeneous layers of finite thickness, the differential equation is replaced by a recursion relation, and the characteristic equation shown above includes one additional term.

ISSN:0001-4966    

DOI:10.1121/1.1907513

出版商:Acoustical Society of America    

年代:1955

数据来源: AIP