739This frequency equation was derived by Mirsky and Herrmann •7 but since they were interested in cylindrical shells no frequency calculations were made. Table VIII includes a tabulation of the frequency coefficients for this case. D. Hollow Circular Bar Clamped along Its Outer Surface The boundary conditions, Eqs. (14a) and (14b), result in the following frequency equation' J,•(f2nr•) It.' •2. --J,•' •2,.,• r•(gnm) = 0. (19)A typical single wall is characterized by a free flexural wave speed, cB, which increases with cot. When cB approaches co (the speed of sound waves in the surrounding medium) the impedance of the wall to incident sound wave ceases to be mass-like because of the coincidence effect and the transmission loss will be less than that given by mass law. Since ½•= (B/M)t(co)• for homogeneous walls, the deleterious effects of the coincidence of the flexural wave speed with the phase velocity of sound in the medium can be avoided for homogeneous walls by reducing the ratio of B/M. (B is the dynamic bending stiffness of the wall; M is the mass per unit area of the wall.) Thus the problem of good acoustical performance in homogeneous walls is seen to be in essential conflict with the need for structural rigidity.A new wall design has been found in which the ratio of the static to the dynamic stiffness can be in excess of 1000:1 and where the stiffness changes from the static to the dynamic value in such a way that the acoustical behavior is nearly that of a perfectly limp wall. If desired, the loss tangent of the wall can be made large and nearly constant over a wide range of frequencies. INTRODUCTIONHE sound transmission loss (TL) of fairly large, single solid walls is essentially determined by the following physical characteristics•.2: 1. The mass per unit area of the wall. 2. The dynamic bending stiffness of the wall. 3. The internal damping or loss tangent of the wall material. An alternative set of physical properties which can be related to the above set is: 1. The mass per unit area. 2. The speed of propagation of free transverse waves on the wall. The decay, or damping of these free waves per unit distance along the wall.It is generally accepted that a high value of mass per unit area is required for high TL. The role of the bending stiffness of the wall (or equivalently, the speed of transverse waves) is not so well known but is fully as important as that of the mass. When the dynamic stiffness of a wall is great enough (or when the speed of transverse waves is high enough) then the wall tends to lose its massive character and the TL is reduced. The amount of internal damping of the wall material generally is of less importance than the mass or stiffness as far as the TL is concerned.Other, equally annoying "side effects" also occur in the case of a relatively stiff wall which are largely absent in a relatively limp wall of the same weight. For example, structure-borne vibrations transmitted from a distant point (e.g., machinery noise, heel clicks, etc.) will radiate much more efficiently from ...
Measurements have been made in four concert halls, and on scale models of the seats alone, of the attenuation of sound passing at grazing incidence over the seating area. For low-frequency sound, we find attenuation in excess of inverse-square loss, which amounts typically to 15–20 dB; the attenuation is considerably less at frequencies lower and higher than about 150 cps, the frequency of maximum attenuation. This attenuation attains its maximum severity after about 12 seat rows, beyond which little further degradation occurs. It appears to be an interference effect and does not depend markedly on the absorptive properties of the seats. The frequency of maximum attenuation is related primarily to the height of the seats and only secondarily to the row-to-row spacing. This low-frequency loss occurs in all the halls that we have measured, and we believe it to be typical of the main-floor sound of all concert halls.
Obtaining adequate speech privacy in modern buildings is one of the important goals of the architect and consultant. This paper deals with the development of a rating method which takes into account the several factors influencing speech privacy. Our work in this area began with a brief laboratory study. The results indicated that speech privacy is related to speech intelligibility rather than to level. The initial experiments were supplemented with an analysis of about 40 case histories representing about 400 pairs of spaces in different kinds of buildings. There appears to be good correlation between the articulation index of intruding speech sound and the reactions of building occupants.
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 fairly 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. LIST OF SYMBOLSCb CO C• ratio of frequency to critical frequency very stiff, light walls may lie below this region. 898 Redistribution subject to ASA license or copyright; see http://acousticalsociety.org/content/terms. Download to IP: 129.105.215.146 On: Mon, 22 Dec 2014 11:33:08 TL OF SOME MASONRY WALLS 899Clearly, the mass law is only a convenient, rough approximation to the performance of single walls. During the past 20 or so years, a more complete theory for single walls has been developedJ -7 This theory, often called the "coincidence effect" theory, takes into account not only the mass but also the stiffness and internal damping of a wall panel. While this theory is not exactly applicable to the usual case of sound transmission between typically sized rooms (the theory treats only very large walls, neglects edge effects, panel resonances, room modes, etc., as does the 2 S•N 8 (c) Fro. 2. (a) Section through wall at rest, (b) exaggerated sketch of wall excited by a low frequency sound wave, (c) wall excited by a high frequency wave. mass law), it has proved to give at least a good qualitative explanation of the measured performance of typical walls.
A summary of general considerations in active vibration isolation obtained from our experience in developing a testbed system is given. Application of this perspective is illustrated by a description of the testing facility and the results achieved to date. These include broadband active vibration isolation of 20 dB over a decade wide frequency band on the nearly full-scale testbed. Significant features of the testing facility are a s m a l l diesel engine mounted on a representatively complex structural foundation, the correspondingly complex open-loop transfer h c t i o n of the system, and the use of a hybrid analog/digital controller, to partition the compensation filter frequency coverage.
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