Structure-borne sound power injection from machinery into a structure is a more complex process than airborne sound transmission. In general, engineers seek methods of prediction for the former, which are as straightforward as for the latter and this has been the main driver for this research. The main impact of ARU research has been on the development of European Standards for building acoustics and these approaches are currently of interest to the aeronautical industry.

Circulation pumps are an important source of noise from domestic central heating systems as they generate airborne, liquid-borne and structure-borne sound. The structure-borne acoustic power delivered by the pump not only depends on the pump but also on the connected receiving system (i.e. a combination of pipes, valves and radiators). Pumps deliver fluid-borne and structure-borne acoustic power simultaneously and their relative contributions to the sound radiated from the pipe system are not obviously obtainable. Research has been carried out to estimate the emission from the pump into semi-infinite pipes [1]. It is shown that 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. Structure-borne power is compared with fluid-borne power measured directly by intensimetry. Follow-on research [2] considered wave mode conversion at pipe junctions. A central heating system was physically modelled as various combinations of pipes with a radiator. Experimental work was carried out on fixed pipe lengths where resonance effects dominated the system response. In order to consider only the effect of number and orientation of pipe junctions, an idealised pipe system was modelled such that the length of the connecting pipes could be varied stochastically, thereby randomising out resonance effects. For typical domestic pipe systems the mixing of wave types is such that radiated sound is determined by the largest component of the structure-borne power from the pump into the pipe system.

A practical structure-borne sound source characterisation has been developed for mechanical installations in buildings which will yield single values of source strength to predict sound power in the installed condition as well as sound pressure levels in rooms within the same building [3]. A novel reception plate method is proposed which yields the source activity in the form of the sum of the squared free velocities, over the contact points. The source mobility is obtained separately as the average of the magnitude of the effective mobility, over the contact points. Both quantities can be used to estimate the installed power for the range of receiver mobilities likely to be encountered in buildings. It is demonstrated that installed power can be estimated by reference to a high source mobility condition, a low source mobility condition, or to a matched mobility condition. For plate-like structures [4] 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 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.

A method for characterisation of structure-borne sound sources is proposed and investigated for machines in heavyweight buildings [5]. It is based on the reception plate approach where the total structure-borne sound power from the machine under test is assumed to be equal to the power dissipated by a plate to which the machine is attached. The method allows comparison of sources on a power basis and the resulting data is suitable for transformation into an installed structure-borne power to predict the sound pressure levels in other rooms. The method is validated by cross-spectral and mobility methods. The method is shown to provide appropriate input data for the prediction of installed structure-borne power and the resulting sound pressure levels in rooms other than the room containing the machine [6]. Case studies of two sources are described: a whirlpool bath and a water cistern. It is shown that the method can be incorporated into EN12354 prediction models and that sound pressure levels in buildings can be predicted. This research on the reception plate method formed the basis of the European Standard, EN 15657-1:2009, a laboratory method for measuring mechanical installations in heavyweight buildings. 

Taps and valves are major sound sources in piping systems and can cause unacceptable noise levels. The noise results from the fluid-, structure- and air-borne sound emission. Research was carried out into methods of characterizing water appliances as sources of structure-borne sound considering taps in isolation and attached to a basin [7]. The concepts of mobility and free velocity are employed for source characterisation based on power as well as a reception plate method. These two methods each provide an independent characterisation of a structure-borne sound source as a single value. Predicted values of the sound pressure level caused by a wash-basin installed in an adjacent room show good agreement with measurements. Further work [8] determined structure-borne sound power from pipes to supporting walls using intensity measurement of the fluid-borne sound power. 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. A reception plate method is also proposed, which avoids the necessity for intensity measurements and allows prediction of sound pressure levels in other rooms.

A study has been carried out on a practical method of predicting the installed structure-borne sound power from mechanical installations with multiple contact points that are fixed in lightweight buildings [9]. 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. Source data has been 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. The single equivalent values are used for predicting the structure-borne power in the installed condition.

A study of multi-contact machinery sources in buildings has been considered to investigate the trade-off between accuracy and simplicity [10]. The study concentrates on measurement and calculation procedures for sources and calculation procedures for receiver structures, particularly lightweight building elements. The findings point to the limitations of simplified methods, specifically the uncertainties likely as a result of reducing the data sets and computational effort, and the discrepancies resulting from simplifying assumptions.

Isolated reception plates provide an engineering approach to quantify the structure-borne sound power input from machinery. However, for applications in building acoustics there are practical and economic reasons to consider using coupled reception plates formed by solid heavyweight walls or floors that are structurally coupled to other building elements. Research using transient and steady-state statistical energy analysis have been used to investigate how the errors depend upon the building structure to which the coupled reception plate is connected [11]. It is shown that the problem is twofold. Firstly, in the low- and mid-frequency ranges, the steady-state velocity level on the coupled reception plate is increased by energy returning from other coupled plates. Secondly, the structural decays on the coupled reception plate have significant curvature due to returning energy; hence short evaluation ranges are needed to minimise the error when determining the total loss factor. This leads to a problematic situation where the coupled reception plate appears to give the correct answer due to the error in the energy cancelling out the error in the total loss factor. It is shown that the latter error can be minimised using short evaluation ranges for the structural reverberation time.

Methods are considered in [12] for the indirect determination of the mobility of structure-borne sound sources. Instead of performing measurements on the source in the free state, the source mobility is obtained from measurements made in-situ. This approach is beneficial if the source is difficult to suspend, or if it contains nonlinear structural elements. Two formulations for an indirect source mobility are derived theoretically. The first requires measurement of velocities at or near to the contact points. The second involves measurement of remote velocities only. Neither of the methods requires excitation at the contacts in the coupled state. Numerical simulations of coupled beams are used to validate the two methods and investigate their accuracy and reliability with respect to typical measurement errors, such as background noise and inaccuracies in sensor positioning. It is found that these can have a significant effect on the methods considered. Several experimental case studies with single-contact and multi-contact sources are performed. The results confirm the validity of the two methods in principle, but highlight their sensitivity to experimental errors. In a representative case study with a fan unit, average errors range between ±5 dB and ±10 dB, with occasional errors of up to 30 dB.

An approximate approach is described in [13] to obtain 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. 

The determination of the structure-borne noise from operational equipment in airplanes is a complex process that requires much source and receiver component information in the analysis. Different test setups and instrumentation usually are required to obtain these quantities separately; for example, free velocity, blocked force, source and receiver mobilities, isolator properties and transmitted power. A previously proposed approach, combining the inverse force method (IFM) and the reception plate method (RPM), and enabling dual force-power measurement from a single test platform, was demonstrated in laboratory tests. This paper [14] reports on the measurement variations of blocked force and transmitted power from the integrated test setup. To gauge the practicality and readiness of the test methods, an experimental round-robin evaluation was arranged and coordinated with four industrial participants. The same source was used in the round robin evaluation with controlled mounting details, in order to investigate the source installation sensitivity. In general, good agreements were observed between powers obtained by the two methods from data acquired at each test site; larger variations were observed in measurement across test sites. Work continues to determine the test method uncertainty; however, both test methods are considered acceptable and ready for wider industry applications.

Vibrating sources, such as building service equipment, are major contributors to noise in buildings. In order to predict and subsequently reduce the sound pressure levels generated by these devices, it is necessary to first predict the total vibrational power injected by them into the supporting building structure. Whilst simplified methods are available for the calculation of the total power through all contacts, it would be beneficial to have more detailed knowledge of the dominant contact powers. For sources on low-mobility building elements, the contact powers are determined by the blocked force, along with the real part of the receiver mobility at each contact. Our research describes a novel inverse method to obtain the blocked forces at each contact [15]. The method employs an instrumented reception plate, which is numerically modelled to allow optimum accelerometer positions to be selected, for any source and any location. The underlying theory and measurement procedure are described, and experimental validations are presented.

Selected publications

[1] Qi N and Gibbs BM (2003) Circulation pumps as structure-borne sound sources: Emission to semi-infinite pipe systems. Journal of Sound and Vibration vol 264 pp 157-176.

[2] Gibbs BM and Qi N (2005) Circulation pumps as structure-borne sound sources: emission to finite pipe systems. Journal of Sound and Vibration vol 284 pp 1099-1118.

[3] Gibbs BM, Qi N, Moorhouse AT (2007) A practical characterisation for vibro-acoustic sources in buildings. Acta Acustica united with Acustica vol 93 issue 1 pp 84-93.

[4] Gibbs BM, Cookson R, Qi N (2008) Vibration activity and mobility of structure-borne sound sources by a reception plate method. Journal of the Acoustical Society of America vol 123 issue 6 pp 4199-4209.

[5] Spaeh MM and Gibbs BM (2009) Reception plate method for characterisation of structure-borne sound sources in buildings: Assumptions and application. Applied Acoustics vol 70 pp 361-368.

[6] Spaeh MM and Gibbs BM (2009) Reception plate method for characterisation of structure-borne sound sources in buildings: Installed power and sound pressure from laboratory data Applied Acoustics vol 70 pp 1431-1439.

[7] Alber TH, Gibbs BM, Fischer HM (2009) Characterisation of valves as sound sources: Structure-borne sound. Applied Acoustics vol 70 pp 661-673.

[8] Alber TH, Gibbs BM, Fischer HM (2011) Characterisation of valves as sound sources: Fluid-borne sound. Applied Acoustics vol 72 pp 428-436.

[9] Mayr AR and Gibbs BM (2012) Single equivalent approximation for multiple contact structure-borne sound sources in buildings. Acta Acustica united with Acustica vol 98 pp 402-410.

[10] Gibbs BM (2013) Uncertainties in predicting structure-borne sound power input into buildings. Journal of the Acoustical Society of America vol 133 issue 5 pp 2678-2689.

[11] Hopkins C and Robinson M (2014) Using transient and steady-state SEA to assess potential errors in the measurement of structure-borne sound power input from machinery on coupled reception plates. Applied Acoustics vol 79 pp 35-41.

[12] Hoeller C and Gibbs BM (2015) Indirect determination of the mobility of structure-borne sound sources. Journal of Sound and Vibration vol 344 pp 38-58. 

[13] Gibbs BM and Mayr AR (2016) Approximate method for obtaining source quantities for calculation of structure-borne sound transmission into lightweight buildings. Applied Acoustics vol 110 pp 81-90.  

[14] Lai HK, Moorhouse A and Gibbs BM (2016) Experimental round-robin evaluation of structure-borne sound source force-power test methods. Noise Control Engineering Journal vol 64(2) pp 170-180.

[15] Holler C and Gibbs BM (2017) Inverse Method to Obtain Blocked Forces of Vibrating Sound Sources in Buildings. Acta Acustica united with Acustica, vol 103(4) pp 639-649.