Reception plate method for characterisation of structure-borne sound sources in buildings: Installed power and sound pressure from laboratory data

In a companion paper, a laboratory method is described to obtain the structure-borne sound power of machines before they are installed in heavy-weight buildings. The laboratory method is based on the concept of the reception plate. In this paper, the method is shown to provide appropriate input data for the prediction of the installed structure-borne power, and thence the resultant sound pressure level in rooms removed from the room containing the machine. Case studies of two common sources are described: a whirlpool bath and a water cistern. It is shown that the method can be incorporated into recently proposed standard prediction models and that sound pressure levels in buildings can be predicted.     

Reception plate method for characterisation of structure-borne sound sources in buildings: Assumptions and application

A method for characterisation of structure-borne sound sources is proposed and investigated for the special but common case of machines in heavy-weight homogeneous building structures. The method is based on the concept of the reception plate where the total structure-borne sound power from the machine under test is assumed equal to the power dissipated by a plate attached to the machine. The method is relatively simple and allows comparison of sources on a power basis, and of tests results at different laboratories. Additionally the data obtained is in a form suitable for transformation into an installed structure-borne power and thence for the resultant sound pressure generated in buildings. The method is validated by cross-spectral and mobility methods. A study of the uncertainty of the power estimate was performed by numerical modelling and measurement.     

Circulation pumps as structure-borne sound sources: emission to semi-infinite pipe systems

Circulation pumps are an important source of noise from domestic central heating systems. Pumps can generate airborne, liquid-borne and structure-borne sound and although standards exist for airborne and liquid-borne sources, none do for structure-borne sources. This is primarily because the structure-borne acoustic power delivered by the pump not only depends on the pump but also on the connected receiving system, which can be a complicated combination of pipes, valves and radiators. Also pumps deliver liquid-borne and structure-borne acoustic power simultaneously and their relative contributions to the sound radiated from the pipe system is not obviously obtainable. The approach proposed is to estimate the emission from the pump into semi-infinite pipes of material and cross-section typical of heating systems. Then to estimate the ‘mixing’ effect of bends, joints and other pipe discontinuities, due to wave mode conversion, as described in a companion paper. In the present paper, it is demonstrated that the structure-borne power can be calculated from the measured free velocity and mobility of the pump for each component of vibration and from receiver mobilities of idealized pipe systems. The structure-borne power is compared with the liquid-borne power measured directly by intensimetry.     

Vibration activity and mobility of structure-borne sound sources by a reception plate method

This paper considers a practical structure-borne sound source characterization for mechanical installations, which are connected to plate-like structures. It describes a laboratory-based measurement procedure, which will yield single values of source strength in a form transferable to a prediction of the structure-borne sound power generated in the installed condition. It is confirmed that two source quantities are required, corresponding to the source activity and mobility. For the source activity, a high-mobility reception plate method is proposed which yields a single value in the form of the sum of the squared free velocities, over the contact points. A low-mobility reception plate method also is proposed which, in conjunction with the above, yields the source mobility in the form of the average magnitude of the effective mobility, again over the contact points. Experimental case studies are described and the applicability of the laboratory data for prediction and limitations of the approach are discussed.     

Characterisation of valves as sound sources: Fluid-borne sound

The sound pressure level in receiving rooms, caused by taps at the ends of pipe systems, is considered. The structure-borne sound power, from the pipes to the supporting wall, was obtained from intensity measurement of the fluid-borne sound power of the tap. The fluid-borne sound power is combined with a ratio of structure-borne sound power to fluid-borne sound power, obtained from laboratory measurements of similar pipe assemblies. Alternatively, a reception plate method is proposed, which avoids the necessity for intensity measurements. The structure-borne power into walls, to which the pipe work is attached, provides input to the standard building propagation model, which yields the predicted sound pressure level in the adjacent room.     

Experimental example of the pseudo-forces method used in characterisation of a structure-borne sound source

In the analysis of machinery noise the aspect of sound source characterisation is of importance. Unlike for airborne sources, no widely applicable methods are available yet for structure-borne sound sources. In previous work a ‘pseudo-forces' methodology was suggested. In this approach fictitious forces on the outer surface replace the internal excitation in a source. The application of this approach is illustrated by experiments using a small air-compressor. In this example the pseudo-forces gave a good reconstruction of the response field (within 3 dB). Confidence intervals are derived for the results, which are 2-3 dB wide. This spread is separated into spread caused by inefficiencies of the method, spread caused by random measurement errors and the spread which is inherently generated by the source itself. These causes together enable the explaination of most of the deviations. Therefore, the pseudo-forces method is expected to serve well for particular structure-borne noise related questions.     

充液管路系统流体声与结构声的复合有源控制

采用基于谐频自适应控制算法的有源消声与消振系统对充液管路系统突出的低频线谱噪声进行有源控制实验研究.建立了泵水循环管路实验系统,在管路中安装有源消声器对流体声进行控制,在管路出口障板上采用8×8通道有源消振系统控制结构声辐射。开展的低频线谱噪声与振动有源控制实验结果表明,在50~200 Hz频带内,通过结合有源噪声与振动控制可在多数频点取得10 dB以上的降噪效果。针对该实验系统,通过分别控制流体声和结构声分析了两者的贡献.实验结果验证了有源消声与消振系统具有较好的降噪性能,各频点处流体声与结构声占比情况不同,需要综合控制流体声与结构声才可以取得显著的降噪效果。      

Predicting transmission of structure-borne sound power from machines by including terminal cross-coupling

Structure-borne sound generated by audible vibration of machines in vehicles, equipment and house-hold appliances is often a major cause of noise. Such vibration of complex machines is mostly determined and quantified by measurements. It has been found that characterization of the vibratory source strength and the prediction of power transmission to a supporting structure or the machine casing itself can be greatly simplified if all mobility cross-terms and spatial cross-coupling of source velocities can be neglected in the analysis. In many cases this gives an acceptable engineering accuracy, especially at mid- and high-frequencies. For structurally compact machines, however, the influence of cross-coupling cannot always be ignored. The present paper addresses this problem and examines the transmission of structure-borne sound power by including spatial cross-coupling between pairs of translational terminals in a global plane. This paired or bi-coupled power transmission represents the simplest case of cross-coupling. The procedure and quality of the predicted transmission using this improved technique is demonstrated experimentally for an electrical motor unit with an integrated radial fan that was mounted resiliently in a vacuum cleaner casing. It is found that cross-coupling plays a significant role, but only at frequencies below 100 Hz for the examined system.     

Point and transfer mobility of point-connected ribbed plates

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.     

Reduction of structure-borne sound in simple ship structures: Results of model tests

A method for the prediction of the transmission of structure-borne sound in ship structures is presented. Various methods to decrease the noise levels in the accomodation spaces in superstructures are investigated in model tests. The attenuation of structure-borne sound in the propagation path between source and receiver is increased by means of damping layers, resilient mounts and changed boundary conditions between main deck and superstructure. Damping layers are found to have only a local effect. Resilent mounts between superstructure and main deck can reduce the noise levels in the superstructure by the order of 10 dB(A).     

Research on the procedure for analyzing the sound quality contribution of sound sources and its application

Transfer path analysis (TPA) plays an important role for identifying and quantifying the contribution by airborne and structure-borne in the automotive industry. The main bottleneck of the TPA method is the test time consumption and complex procedure. It becomes a key target in many applications to find out the source with dominant contribution to overall noise rather than to identify each source. In recent years the contribution pattern of sources to the vehicle overall interior noise has changed with the reduction of engine noise, which masks all other sources. The panel radiation noise of vehicle body could not be ignored. There is an increasing demand for analyzing the sound quality contribution of sound sources in simple ways. The procedure for analyzing sound quality contribution of panel radiation noise is suggested in this study, in which an operational path analysis (OPA) method combined with partial singular value decomposition (PSVD) analysis is applied and sound quality objective assessment is introduced. The experimental research for verifying the procedure is finished, from which the source with largest sound quality contribution is picked up from three sources. For engineering application, the sound quality contributions of panels to the interior noise of a micro commercial vehicle are analyzed by using the procedure. By investigating the contributions of sound sources to each sound quality attribute, the dominant sound source is determined.     

A road traffic noise pattern simulation model that includes distributions of vehicle sound power levels

Annoyance, sleep disturbance and other health effects of road traffic noise exposure may be related to both level and number of noise events caused by traffic, not just to energy equivalent measures of exposure. Dynamic traffic noise prediction models that include instantaneous vehicle noise emissions can be used to estimate either of these measures. However, current state-of-the-art vehicle noise emission models typically consider a single emission law for each vehicle category, whereas measurements show that the variation in noise emission between vehicles within the same category can be considerable. It is essential that the influence of vehicles that are producing significantly more (or less) noise than the average vehicle are taken into account in modeling in order to correctly predict the levels and frequency of occurrence of road traffic noise events, and in particular to calculate indicators that characterize these noise events. Here, an approach for predicting instantaneous sound levels caused by road traffic is presented, which takes into account measured distributions of sound power levels produced by individual vehicles. For the setting of a receiver adjacent to a dual-lane road carrying free flow traffic, the effect of this approach on estimated percentile levels and sound event indicators is investigated.     

Estimation of directivity and sound power levels emitted by aircrafts during taxiing, for outdoor noise prediction purpose

Integrated noise model (INM) is the most internationally used software to calculate noise levels near airports. Take off, landing or pass by operations can be modeled by INM, but it does not consider aircrafts taxiing, which, in some cases, can be important to accurately evaluate and reduce airports’ noise assessment.Aircraft taxiing noise emission can be predicted using other prediction tools based on standards that describe sound attenuation during propagation outdoors. But these tools require data inputs that are not known: directivity and sound power levels emitted by aircraft during taxiing.This paper describes methods used to calculate directivity indexes and sound power levels, based on field measurements made in Madrid-Barajas Airport (Spain). Obtained results can be used as inputs for general purpose outdoor sound prediction software, which will be able to evaluate noise at airports vicinity as industrial noise.Directivity and sound power levels have been estimated in octave and third octave band terms, for several aircraft families.     

Sound radiation from a double-leaf elastic plate with a point force excitation: effect of an interior panel on the structure-borne sound radiation

The sound radiation from a double-leaf elastic plate subjected to a point force excitation is investigated theoretically, to gain a fundamental insight into the sound radiation from an interior panel of a double-leaf structure in buildings. The effects of the interior panel on the sound radiation, which show a negative effect at low frequencies due to the mass-air-mass resonance, are discussed in detail. The theory is validated experimentally. As a measure of the efficiency of the interior leaf in reducing noise radiation, the radiation reduction is defined in this study, and it is found useful for predicting the sound radiation due to the structure-borne sound in building elements. Parametric studies through theoretical results are made to clarify the effects of the parameters of the sound radiation system, and to gain a fundamental insight into the control of structure-borne noise radiation. It is shown that it is difficult to reduce the radiated sound power by an interior panel alone, even if its mass is increased.     

Influence of support properties on the sound radiated from the vibrations of rectangular plates

The control of the forced vibration response of structures through the optimal tuning of its supports is desirable in many applications. Tuning may enhance the dissipation of vibration energy within the supports, thereby reducing fatigue and structure-borne noise. Two different models were developed to calculate the optimal support stiffness that minimizes the velocity response of homogeneous plates. The first model, based on the wave propagation at the edge, yields a good first cut approximation of the optimal properties. The optimal viscous and viscoelastic support stiffness for minimal reflection at the edge was calculated. Maximum absorption of the incident waves occurs when the viscous support stiffness matches the characteristic mechanical impedances of the plate. The second model, based on the Rayleigh-Ritz method, yields more accurate estimates of the optimal support stiffness required to minimize the forced velocity response of the finite rectangular plate. The optimal support properties calculated from the two different methods were in good agreement. This suggested that the modal response of the plate is strongly influenced by the wave reflections at the edges. Finally, the effects of support properties on the sound radiated from the plate were investigated. The optimal support stiffness that minimizes the radiated sound power was found to be smaller than the value that minimizes the velocity response. The results show that both the velocity response and sound radiation are strongly influenced by dissipation of vibration energy at the edges, and demonstrate that support tuning can yield significant noise and vibration reduction.     

Noise generation by vacuum cleaner suction units. Part III. Contribution of structure-borne noise to total sound pressure level

Noise emitted by a vacuum cleaner suction unit consists of airborne and structure-borne noise. The airborne noise is generated mainly by the turbo blower and the structure-borne noise is generated mainly by the driving electric motor. The structure-borne noise depends on the suction unit design and on operating conditions, and is especially distinct at partial flow rates when rotating stall and surge appear. Among geometrical parameters, the stator of the blower and the electric motor, or metal shield if any, have the greatest effect on the structure-borne noise. Therefore, in this part of the paper, the effects of vibrations of the electric motor structure on the noise characteristics have been measured and analysed at the design and off-design operation. The contribution of structure-borne sound to the total sound pressure level becomes relatively less important at higher flow rates and with a vaned diffuser built-in the blower.     

Ventilation duct with concurrent acoustic feed-forward and decentralised structural feedback active control

This paper presents theoretical and experimental work on concurrent active noise and vibration control for a ventilation duct. The active noise control system is used to reduce the air-borne noise radiated via the duct outlet whereas the active vibration control system is used to both reduce the structure-borne noise radiated by the duct wall and to minimise the structural feed-through effect that reduces the effectiveness of the active noise control system. An elemental model based on structural mobility functions and acoustic impedance functions has been developed to investigate the principal effects and limitations of feed-forward active noise control and decentralised velocity feedback vibration control. The principal simulation results have been contrasted and validated with measurements taken on a laboratory duct set-up, equipped with an active noise control system and a decentralised vibration control system. Both simulations and experimental results show that the air-borne noise radiated from the duct outlet can be significantly attenuated using the feed-forward active noise control. In the presence of structure-borne noise the performance of the active noise control system is impaired by a structure-borne feed-through effect. Also the sound radiation from the duct wall is increased. In this case, if the active noise control is combined with a concurrent active vibration control system, the sound radiation by the duct outlet is further reduced and the sound radiation from the duct wall at low frequencies reduces noticeably.     

Multi-mode sound transmission in ducts with flow

Exhaust mufflers, large exhaust stacks, and turbofan engines are common examples of ducted noise. The most useful measure of the sound produced by these noise sources is the sound power transmitted along the duct. When airflow is present, sound power flow can no longer be uniquely determined from the usual measurements of acoustic pressure and particle velocity.One approach to sound power determination from in-duct pressure measurement, and the one discussed in this paper, is to predict the relationship between the sound power and pressure based upon an assumed mode amplitude distribution. This paper investigates the relationship between acoustic pressure and power for a family of idealized source distributions of arbitrary temporal and spatial order. Incoherent monopole and dipole sources uniformly distributed over a duct cross-section can be obtained as special cases. This paper covers the sensitivity of the pressure-power relationship to source multipole order, frequency and, in particular, flow speed. It is shown that the introduction of flow in a hard-walled duct can have a substantial effect on the behavior of the pressure-power relationship for certain source distributions. Preliminary experimental results in a no-flow facility are presented in order to verify some of the main results.     

An approximation for the point mobility at the intersection of two perpendicular plates

The problem of point excitation at a T-intersection of two perpendicular plates is studied in order to establish expressions for the point mobility. It is found that the theory for point excitation of the free surface of a semi-infinite elastic solid is applicable in the frequency range associated with structure-borne sound transmission. From this theory the mobility for an infinite system is derived. Based on this model and on an experimental investigation an estimation procedure for the point mobility in the finite dimension case is developed. The agreement with measurements performed in situ is quite acceptable. Both the theoretical and the experimental investigations reveal that the real part of this mobility is small, although it is larger in the experimental results. This indicates that other components of excitation are difficult to eliminate and may contribute to the power input in practice. Because of the small real part of the mobility it is advantageous with respect to the reduction of structure-borne sound power transmission to locate the contact points between a source and the receiver at such intersections. Corrections are deduced for the measured magnitude of the mobility for the case when separate force and motion transducers are used.