Sound Diffusion Research Articles

Determination of Sabine Coefficient from Acoustic Impedance Measurement and Its Application in Studio Design

ABSTRACTValues of absorption coefficient α0 and αs for normal and random incidence of sound respectively have been derived from the measured acoustic impedance of sound absorbing materials. It has been found that α0 is maximum and greater than αs only when a material has specific acoustic impedance | (or mid);Z| (or mid);/ρ C < 1, otherwise αs is always greater than α0. The values of α0 and αs, so measured, have been compared with the values of absorption coefficient α, derived from reverberation measurements in the studios. It has been found that when random sound field exists in a studio, αs is same as α, otherwise its value lies between α0 and αs depending upon the conditions of sound diffusion inside the studio.

Novel Uses of Digital Computers in Room Acoustics

The advent of large, high-speed digital computers has opened up new approaches to the study of complex problems in room acoustics. Digital computers can simulate the transmission of sound in rooms using speech and music as signals. Thus, it is possible to subjectively evaluate and redesign proposed concert halls and theaters prior to construction. More basic problems such as, for example, the influence of delays and intensities of the first reflections on subjective quality can likewise be studied without restriction to the limited choice offered by existing rooms. Other interesting applications of digital computers result from the simulation of frequency and space responses of rooms. By generating and evaluating these responses on the computer according to a Monte Carlo method, many difficult analytical problems have been solved. Among the questions which have been answered by this method are the following: What is the permissible maximum gain of a public address system in a room of given reverberation time to avoid instability (howling)? By how much can the gain be raised if an electronic frequency shifter is inserted between microphone and loudspeaker? How does frequency irregularity depend on reverberation time and the ratio of direct-to-reverberant sound intensity? What is the relation between space irregularity and sound diffusion? Examples of both Monte Carlo computation and simulation of actual sound transmission in rooms will be discussed to illustrate the application of digital computers in room acoustics.

Sound diffusion in broadcasting studios

Sound diffusion in broadcasting studios

Acoustical Features of the Addition to the Physics Building at the University of Texas

Detailed description is given of the acoustic designs used in the addition to the Physics Building at the University of Texas. Sound absorption is included in the lecture rooms in the form of an acoustic blanket behind a pierced glazed brick wall to give a broadly resonant absorption with maximum coefficient at 500 cps of 0.60, reducing to 0.40 at 125 cps and to 0.25 at 4000 cps, and in the form of a slit absorber having a resonance at about 400 cps with coefficient of 0.55, reducing to 0.45 at 125 cps and 0.33 at 4000 cps. Sound diffusion and projection in these lecture rooms is secured by sound reflectors over the speaker's position, nonparallel side walls, and light fixture coves in the ceiling. Sound absorption in some of the laboratories is provided by use of functional absorbers each using an acoustic blanket free on one side and confined by a perforated hardboard on the other. Measurements on two such rooms show a resonant absorption is obtained with peak frequency at 500 cps, and with apparent absorption coefficient near unity at the peak, dropping to 0.70 at 125 cps and to 0.50 at 4000 cps.

Diffusion of sound in small rooms

The paper describes an investigation of some problems of sound diffusion in rooms, with particular reference to small sound and television studios. The experimental work shows that it is possible to measure the degree of diffusion in a room by fairly simple practical techniques. Quantities based on the frequency variation of reverberation time and double reverberation decay constants are the most promising for use in small broadcasting studios.It is also shown that uniform distribution of absorption can be as effective as other means of attaining conditions of good diffusion. Rectangular diffusers are particularly effective in improving conditions where the distribution of absorption is poor.

Transmission Loss of Some Masonry Walls

The transmission loss of very large, single, solid walls is given by the “coincidence-effect” theory as presented by L. Cremer and others. The results of this theory have been compared with data from field measurements on some typical, masonry walls and are found to be in fairy good agreement. The differences which exist are believed to reflect the inapplicability of the theory to finite sized walls and the imperfection of the sound diffusion in the test rooms. An empirical design technique is presented which agrees more closely with typical field results. The hollow masonry block walls which were studied behave much like solid walls with the same surface weight and bending stiffness. Laboratory techniques are described for measuring the physical constants of these and other materials. Tests made on a typical lightweight aggregate masonry block show that the porosity through the block faces does not greatly reduce the transmission loss. However, painting such a block may increase the bending stiffness of the wall and slightly alter the transmission loss.

Application of acoustical engineering principles to home music rooms

This paper describes the important factors to be considered in the construction of a home music room. The effects of reverberation, room resonances, sound diffusion, and noise are considered. Special consideration is given to the control of acoustical conditions, and a room in which many of these factors have been taken into account, is described.

Perturbation of Sound Waves in Irregular Rooms

A recently developed boundary perturbation theory is applied to the solution of certain problems in room acoustics. Wave patterns and normal frequencies can be calculated for rooms having walls which are distorted by shape irregularities, by non-uniformities of boundary absorption, or by a combination of these. The method is quite general; it is applicable to any form of irregularity if the amount of distortion is “small,” provided solutions for the basic form are obtainable. Specific formulas are derived for trapezoidal shapes and calculations agree closely with experimental patterns and frequencies reported previously. An experiment has been devised to test the range of validity of the perturbation formulas for one case. A small test chamber was built with walls which could be shifted continuously in plan to give trapezoids of constant area, having any desired degree of deformity. A number of normal frequencies were traced through the whole range of shapes. For wall angle shifts up to about 15° the perturbation calculations hold well. The perturbation theory yields certain general conclusions regarding the problem of sound diffusion in irregular rooms. These appear to be of particular interest in the light of recent experience in applied acoustic design.

A Sound Measurement Room Utilizing the Live End-Dead End Studio Principle

The requirements of a suitable sound test room for the measurement and analysis of sound produced by household appliances are discussed. It is shown that neither the highly reverberant room nor the highly absorptive room will meet all requirements due to the room resonant response of the former and the high sensitivity to directional effects of the latter. A sound room is described which makes use of a reflective floor, absorptive walls and a highly absorptive ceiling with the microphone placed near the ceiling. The sound source is moved in a reciprocating manner and a sound diffuser is rotated slowly during measurements to average out the space pattern effects. A high degree of sound reduction through the walls is achieved by massive construction and a large reduction of transmitted vibration is effected by a coil spring suspension. The test room has been used over an extended period of time and has been of great assistance in sound analysis investigations directed toward sound reduction on electric cleaners.

On the Perturbation of Normal Modes of Vibration

A recently developed boundary shape perturbation theory has been applied to the calculation of acoustic normal frequencies and wave patterns in enclosures which deviate somewhat from rectangularity. The method is quite general; it is applicable to any form of irregularity if the amount of distortion is “small,” provided solutions for the basic form are obtainable. Specific formulas have been derived for trapezoidal and parallelogram shapes. Calculated frequencies and wave patterns are found to agree closely with experimentally measured values reported previously. An experiment has been devised to test the range of validity of the perturbation calculations for one case. A small test chamber was built with walls which could be shifted continuously in plan to give trapezoids of constant area, but having any desired degree of deformity between a rectangle AB and an isosceles triangle of base 2A and height B. A number of normal frequencies were traced through the whole range of shapes. For wall angle shifts up to about 15° the perturbation calculations hold very well. The perturbation method promises to yield information bearing on the problem of sound diffusion in irregular rooms.

The measurement of sound absorption coefficients

Standardised absorption measurements in reverberation rooms suffer from relatively low reproducibility, especially at low frequencies and for highly absorptive samples. In these conditions, complete sound field diffusivity cannot be achieved. However, recent research has demonstrated that the theoretical diffuse absorption emerges as the ensemble average across a wide range of rooms with different geometries, even at very low frequencies. The present study aims to investigate the influence of the room geometry, sound source positioning, and presence of panel diffusers on the sound absorption values obtained in a specific reverberation room, as well as on the difference between those values and the theoretical diffuse values. The focus is on the lowest frequency bands.A numerical simulation approach is proposed, in which the room without sample is modelled in full detail using the finite element method and coupled to the sample with a Rayleigh–Ritz approach. The measurement of diffuse sound absorption is simulated in an efficient way using a stationary power balance approach. This approach is validated against measured data and against a detailed simulation of the impulse response. From the parametric study conducted with the model, it can be concluded that, for highly absorptive samples, good agreement with the theoretical diffuse absorption values can be obtained for certain room designs. In these rooms, the measured absorption is also less sensitive to the number and positioning of sources and diffusing elements.