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In wood framed buildings, floors are formed by fastening wood sheathing to joists spaced at a regular interval. The sheathing is typically fastened to the joist using screws so the resulting system is a complicated periodic point-connected plate-rib structure. It is shown that the point force mobility varies significantly with position. A machine installation generally is close to discontinuities, such as at: floor edges, joist-screw locations, joints between the sheathing plates, and due to workmanship. In such cases, moment excitation might become important at some frequencies and the neglect of moments a priori can ead to inaccurate prediction of the total emission. This paper presents measured point moment mobilities with respect to the distance to discontinuities for a timber joist floor where a single layer of chipboard forms the sheathing. It is shownthat the measured point moment mobility indicates an infinite plate behaviour. This includes for positions above a joist but between screw positions. To determine the relative contribution of moments and perpendicular forces to the total structure-borne sound power, case studies of two sources, a fan unit and a whirlpool bath, are described, for various locations on a timber joist floor.
An approximate approach is described, for obtaining the source quantities required for the calculation of structure-borne sound power from machines into supporting lightweight building elements. The approach is in two stages, which are based on existing international Standards for measurement. The first stage involves direct measurement of the source free velocity at each contact, to give the sum of the square velocities. The second stage is based on the reception plate method and yields the single equivalent blocked force, which approximates the sum of the square blocked forces. The applicability of the source data obtained has been investigated in a case study of a fan unit on a timber joist floor. The approach contains several significant simplifying assumptions and the uncertainties associated with them are considered. For the case considered, the power transmitted into the floor is estimated by the approximate method to within 5 dB of the true value, on average.
Vibrational active sources introduce structure-borne sound power to the supporting structure through all contact points and by up to six components of excitation. The total structure-borne sound power is a function of source activity, source mobility and receiver mobility and receiver mobility is expected to vary significantly with source position on lightweight structures such as timber joist floors. A case study is presented, in which the spatial variation in structure-borne sound power is examined for a fan unit mounted on a timber joist floor. The exact structure-borne power is obtained from measured source free velocity, and measured source and receiver mobility, where forces are assumed the dominant excitation mechanism. The exact calculations are computationally intensive and require large data sets, and this is generally not a practical approach for prediction purposes. In addition, manufacturers view their products as single entities and desire corresponding single values of source strength. A laboratory reception plate measurement procedure has recently been proposed which yields single equivalent values of source strength and source mobility. This paper investigates if this collapsed data format appropriately describes the dynamic behaviour of the source and especially inhomogeneous receiver structures, to obtain estimates of total structure-borne sound power within acceptable accuracy. Since measured receiver data is usually not available, an approximate method is described, of estimating the point and transfer mobility, based on infinite beam and infinite plate behaviour. The single equivalent receiver mobility then is easily obtained and, when used in combination with single equivalent source data, yields an approximate prediction of the total installed power. Single equivalent estimates of structure-borne sound power are compared with exact values obtained from full mobility formulations. Results indicate that the accuracy of the approximate prediction may be sufficient for such installations.
Single equivalent excitation for the structure-borne power from machines on ribbed-plate structures
(2009)
Machines impart structure-borne power into connected and supporting structures through all contact points and by up to six components of excitation. It is recognised that considering all transmission paths for prediction purposes generally is not a practical approach, due to the large data and computational requirements. Reduced data sets and less computationally demanding procedures are sought which give the total power to the accuracy appropriate for the particular engineering design requirement. In this paper, single equivalent source and receiver values are considered for machines on ribbed-plate structures. In this case, large variations in the structural dynamics at the contact positions can be expected, and spatial averaging will lead to loss of accuracy. Single equivalent values have been assembled from measured source and receiver data. Case studies are described, for a fan unit and a whirlpool bath, on a timber-joist floor, which is a complicated periodic point-connected plate-rib structure. Approximate estimates of structure-borne power are compared with exact values obtained from full mobility matrix formulations. The case studies presented indicate the loss of accuracy of the prediction, which, in many situations, may be sufficiently small to allow such simplifications.
The work reported in this paper addresses the problem of structure-borne sound transmission between vibrating sources and ribbed-plate receiver structures. Vibrating sources, such as pumps, motors, fans, etc., transmit vibro-acoustic power, causing noise complaints by occupants in cars, trains, aircraft, buildings and/or material fatigue and damage. The transmission process is complicated in that sources transmit power through several contacts and by up to six components of excitation at each contact. The structure-borne sound power is a function of source activity, source mobility and receiver mobility, and all three quantities must be known to some degree. For non-homogeneous receiver structures, such as thin-plate cavity constructions or lightweight framed constructions, the sheathing plates are typically fastened to the framing members using bolts, screws or spot-welded joints. Hence the resulting system is a point-connected ribbed plate structure and the receiver mobility is expected to vary significantly with position. Since measured receiver data seldom is available for prediction purposes, a method of estimating the point and transfer mobility of point connected ribbed plates is required. In this paper, an approximate approach, based on substitution forces, is described. The model uses infinite beam and infinite plate behaviour as input quantities. Estimates of point and transfer mobility are compared with measured results using a timber joist floor construction as an example.
Single equivalent approximation for multiple contact structure-borne sound sources in buildings
(2012)
The work reported in the paper addresses structure-borne sound transmission between multiple contact sources and non homogeneous plate receiver structures. This study concentrates on a practical method of predicting the installed structure-borne sound power from mechanical installations in lightweight buildings. The structure-borne sound power is a function of source activity, source mobility and receiver mobility, and all three quantities must be known to some degree. It is rarely practical to consider all transmission paths individually and in detail, and therefore, reduced data sets and less computationally demanding procedures are proposed. The paper examines how source data can be used to assemble single equivalent values, using spatial averages and magnitudes. Single equivalent values of receiver mobility also are proposed for lightweight, point-connected ribbed plate constructions. In case studies, the single equivalent values are used for predicting the structure-borne power in the installed condition.