Reverberation Room Measurements Research Articles

TM Spectra: A Measure of Diffuseness of Reverberant Sound

Diffuseness of reverberant sound is a sine qua non for accurate reverberation room measurement of sound absorption, power, and transmission loss. Most authorities regard it as important for the concert hall as well. And yet, technique for the measurement of diffuseness is not well developed. In this connection, the recently proposed technique of TM (traversing microphone) spectroscopy shows promise. The technique exploits information on diffuseness contained in the spatial correlation function of instantaneous sound pressure. Mathematically, the TM spectrum is the Fourier transform of the spatial correlation function. Physically, it can be interpreted as the power spectrum obtained at the terminals of a small microphone traversing the room at constant speed. For an ideally diffuse field of narrow bandwidth, the TM spectrum is, conveniently, a perfect rectangle. For a less diffuse field, deviations from the rectangular shape will occur. Supporting experimental and analytical results are shown. The limitations and promise of this new technique are discussed. [This work supported by the Office of Naval Research, Code 468.]

Noise Measurements on Desk Calculators and Typewriters

To get information on the state of the art and to find a basis for the preparation of a measuring standard, the noises of 32 desk calculators and 14 electric typewriters have been measured. Measured quantities were the sound-power level in the reverberation room and the mean A-weighted impulse sound level in the anechoic room. The mean level was derived from the dc-output of the impulse sound-level meter by a simple continuously operating device and was taken over one operation cycle of the machine under test at five microphone positions, each at a distance of 1 m from the surface of the machine. The results range from 61 to 73 dBAI for desk calculators and from 65 to 74 dBAI for typewriters. Since impulses like the printing noise of a calculator are not suited for reverberation room measurements and the directivity of the sound radiation is sometimes of interest, it seems appropriate to recommend for these measurements free-field conditions and the impulse sound level.

Absorption of Sound by a Strip of Absorptive Material in a Diffuse Sound Field

The random incidence sound absorption coefficient is calculated for a narrow strip of absorbing material set in an otherwise reflecting plane. The assumptions made are that the material is of the locally reacting type, with a real normal admittance. The effect of an imaginary component of admittance is discussed qualitatively. The results show the increase in absorption coefficient that occurs at small widths due to diffraction. Consideration is also given to the absorption of rectangular patches having both dimensions in the diffraction range. The standard reverberation room measurement of sound absorption is examined.

Interference Patterns in Reverberant Sound Fields

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) where x is the distance from the surface and k is 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.

Interference Patterns at Boundaries in Reverberant Sound Fields

Starting with the definition of a diffuse sound field as having equal energy density at all points, the waves converging on any point being random in phase and direction, an analysis is given showing that a reflecting boundary large compared with the wavelength destroys the diffuseness of such a field by setting up an interference pattern. For a perfectly reflecting plane surface, the interference pattern can be expressed analytically as a mean square sound pressure varying as [1 + (sin2kx/2kx)] where x is the distance from the surface and k is the wave number. Similarly, the mean square particle velocity varies as 1 − sin2kx2kx + 1k2x2(sin2kx2kx − cos2kx), and the combination of these two expressions gives a mean energy density variation with distance proportional to 1 + (1/2k2x2)[(sin2kx/2kx) − cos2kx]. Thus the energy level at the surface is about 2.2 db higher than at points further away where the interference pattern is negligible. The effects of such interference patterns on transmission loss and reverberation room measurements are discussed. The patterns are not much affected by the frequency band widths habitually used in room acoustics. Experimental confirmation of the theory is given.

A Long-Tube Method for Field Determinations of Sound Absorption Coefficients

To determine whether acoustical materials installed in Government buildings meet specifications of acoustical properties and to determine the effect of various methods of maintenance on the acoustical properties of acoustical materials, a modification of the familiar laboratory impedance tube has been adapted for field measurements of the absorption coefficient at 512 c.p.s. only. The absorption coefficient for reverberant sound is calculated on the basis of a semi-empirical development described at the last meeting by A. London. The apparatus will be described and results of simulated field tests on several acoustic materials will be presented giving a comparison between the reverberation room measurement and the simulated field test as well as the standard deviation of several tests made on the same material. A means of allowing for the errors inherent in the method in calculating the absorption coefficient for reverberant sound will be indicated.