Source Descriptor Research Articles

The load criteria for ship mechanical noise prediction in low frequencies and experimental validation

The loading method of the external excitations generated by the equipment directly affects the predicted result of the mechanical noise which should be the same under different excitation forms for the given equipment. In this paper, general load criteria are proposed to define forces/moments as the standard form and convert other forms of loads in the low-frequency domain. As the most typical form to characterize equipment excitation, acceleration load loading methods for different conditions are investigated. The equivalent formula between ideal accelerations and generalized forces establishes the first load criterion. The second load criterion is proposed to address the issue of an average acceleration loading, in which the phase and amplitude distribution are both absent, and cannot apply to the load identification. The upper and lower limits of the mechanical noise can be determined by the vibroacoustic transfer function of the three load models, and the energy-averaged value is used to represent the mechanical noise. Furthermore, the third criterion is used to handle the case where the acceleration load is given by the results of a bench test. According to the equipment source descriptor invariance, the conversion method is achieved between the bench test and the real ship based on the transfer function of a load model, and the mechanical noise is predicted by an equivalent energy method. Finally, a three-parameter method to quantitatively evaluate the well-fitting of experimental and numerical results, and the load criteria are well validated by underwater acoustic experiments of an experimental model.

Interface mobilities for characterization of structure-borne sound sources resiliently mounted via multiple contact points

This paper introduces an expansion of the interface mobility method for structure-borne sound sources that are resiliently mounted to a receiving structure at multiple discrete contact points. Subsystems of source, receiver and isolators are represented by Fourier transforms of their mobility matrices. Along with the source activity, a formulation for the transmitted power fed into the receiver is obtained. Furthermore, the source descriptor and a coupling function for resiliently mounted multi-point sources are obtained which both form recognized concepts for structure-borne sound source characterization. The method is based on a neglection of cross-order terms which describe a coupling between Fourier coefficients of different order. With regard to a subsystem of discretely placed isolators, cross-order terms were found to account for variations of isolator parameters along the contact points. In a set of numerical studies it was shown that errors introduced by neglecting cross-order terms are most pronounced in the higher frequency region where isolators show resonant behavior due to internal wave effects. A subsystem of identical isolators is completely described by equal-order terms, therefore no error is introduced considering such cases. Moreover, errors inherent in the rigidly mounted case due to cross-order source or receiver mobilities are reduced significantly when isolator mobilities are large compared to those from source and receiver. Findings from theoretical studies were validated in practical settings considering structure-borne sound transmission from a ship diesel engine and a magnetic resonance imaging (MRI) device.

Interface Mobilities for Characterization of Structure-Borne Sound Sources with Multi-Point Coupling

The concept of source descriptor and coupling function is commonly recognized to form a rigorous basis for structure-borne sound source characterization. While this concept initially is valid for the single-point case only, it can be extended to sources with multi-point coupling by including the interface mobility approach. By considering a continuous interface that passes all contact points, the velocities, forces and mobilities are series expanded into interface orders by means of a spatial Fourier decomposition. The use of a continuous formulation for the multi-point case, however, can be problematic from a practical point of view. This paper discusses a reformulation of the interface mobility approach for a simplified calculation and clarified interpretation of the interface orders. With a discrete Fourier series as a basis for the interface mobility approach, the interface is reduced to a set of points and the interface orders are shown to describe the interplay of the data at the contact points. A discrete formulation furthermore yields simplified equations and a strict upper bound for the number of orders that have to be included, thus enhancing the practicability of interface mobilities for source characterization.

The influence of cross-order terms in interface mobilities for structure-borne sound source characterization: Force-order distribution

For the characterization of structure-borne sound sources with multi-point or continuous interfaces, the concept of source descriptor and coupling function can be reformulated by incorporating the interface mobilities. The applicability of this approach, however, depends on the admissibility of neglecting the so-called cross-order terms. Hence, the objective of the present work is to investigate the importance and significance of cross-order terms for the characterization of vibrational sources. Such cross-order terms consist of the previously studied cross-order interface mobilities and the force orders which are addressed herein. The distribution of force orders depends on both the complex data of the contact forces and their location along the interface. From measurements and numerical simulations it is found that the relative magnitudes of the force orders vary up to a factor 10. Only at low frequencies a larger range in magnitude can be observed. Combined with the knowledge of the cross-order interface mobilities, henceforth, a complete assessment of the influence of cross-order terms is possible.

The influence of cross-order terms in interface mobilities for structure-borne sound source characterization: Frame-like structures

The applicability of interface mobilities for structure-borne sound source characterization critically depends on the admissibility of neglecting the cross-order terms. Following on from a previous study [H.A. Bonhoff, B.A.T. Petersson, Journal of Sound and Vibration 311 (2008) 473–484], the influence of the cross-order terms is investigated for frame-like structures under the assumption of a uniform force-order distribution. Considering the complex power, the cross-order terms are significant from intermediate frequencies on upwards. At lower frequencies, the cross-order terms can come into play for cases where the in-phase motion of the structure along the interface is constrained. The frequency characteristics of the influence of cross-order terms for the zero-order source descriptor and coupling function are similar to those of the complex power. For non-zero source descriptor and coupling function orders, the quality of the equal-order approximation mainly depends on the presence of low-order cross-order interface mobilities. By analyzing the symmetry of an interface system, it is possible to predict which cross-order terms are equal to zero. The equal-order approximation manages to capture the main trends and overall characteristics and offers an acceptable estimate for engineering practice.

The significance of cross‐order terms in interface mobilities for structure‐borne sound source characterization

For the characterization of structure‐borne sound sources and the description of the associated transmission process, the source descriptor and coupling function were introduced. The concept of source descriptor and coupling function can be reformulated by incorporating the interface mobilities. The applicability for source‐receiver assemblies with a multi‐point or continuous connection is thereby granted. The accuracy of the results and consequently the validity of this approach, however, depends on the significance of the so‐called cross‐order terms. Such cross‐order terms consist of force orders and cross‐order interface mobilities. In recent theoretical and experimental work, the influence of cross‐order interface mobilities as well as the distribution of force‐orders have been investigated. Based on this knowledge, the significance of the cross‐order terms is assessed in the present contribution.

Towards a structure-borne sound source characterization

Characterization of mechanical sources is complicated since up to six forces and moments contribute at a contact and the response at one contact is the result of forces and moments at all contacts. For the transmission process, the quantity of prime interest is the complex power, and, therefore, a proper source characterization should relate to it. The source descriptor does so and offers a consistent base for studies of source characterization. Theoretical and experimental progress on the source descriptor is reviewed. From inspection of the source descriptors and the mobilities from which they are composed, it is observed that the uncoupled component source descriptors can establish the bandwidth and dominant components of excitation. The ‘signatures’ of the point mobilities required allows contacts to be classified as mass-, stiffness- or resonance-controlled component, establishing simplifications with respect to source characterization.

Structure-borne Sound Sources in Buildings

It is more difficult to solve structure-borne noise problems, including those in buildings, than airborne noise problems. One reason for this is that there is not an agreed approach; in particular, on how to describe machines as vibrational sources. A review is given of proposed approaches to the characterisation of machines as structure-borne sound sources. It is demonstrated that both a machine's activity and its dynamic characteristics are needed to properly describe its ability to emit structure-borne sound. It is also demonstrated that emission and source characterisation are distinctly different. In addition, at present, there does not appear to be a compromise possible between a proper characterisation and methods which are simple and practical and it is argued that present attempts to produce standards are premature. In the short term, a measure of source activity such as free velocity may be acceptable. A source descriptor is described which is a proper characterisation but requires the acquisition and processing of much data. It would appear worthy of development but the challenge remains to present product data in a simple and practical way.

Use Of The Source Descriptor Concept In Studies Of Multi-Point And Multi-Directional Vibrational Sources

A study of the influence of multi-point and multi-component coupling on structure-borne sound source characterization has been carried out. In particular, the effects of cross-coupling between different components of excitation and motion at a contact point have been examined. Theoretical results are supported by results of experiments on real machines, a family of fan units. The significance of the various components of excitation and motion is interpreted in terms of the source descriptor, a quantity defined on a power basis. It is shown that measured source descriptors associated with force and moment can be equally important and, accordingly, none can be excluded a priori. It is also shown that multi-point and multi-component coupling can have a profound influence. The ability of the source to impart vibrational power at global anti-resonances of the source structure is demonstrated. In addition, this ability is spectrally transposed to a higher frequency range than that predicted from a single point, single component analysis.

The Characterization of Structure-borne Emission of Building Services Machinery Using the Source Descriptor Concept

The analysis of a structure-borne sound problem is often made difficult because a universal characterization of the structure-borne emission of machines is not available. Most methods proposed characterize the machine, rather than the emission, and the problem of properly including the effect of receiver response is avoided by either choosing a particular class of machines, such as those resiliently mounted, or by classifying machines according to likely receiver configurations. Thee various mnethods proposed yield results which are physically incompatible and comparison of machines remains difficult. A source descriptor has been proposed which is calculated from source factors only, the free veiocity and component mobility. It has the dimension of power which allows comparisons of machines. Additional information on receiver mobility allows an estimnation of the structure—borne emission at the points of contact. The practicalities of measurement for a family of machines, building services, centrifugal fans, are described. Consideration is given to more than one degree of freedom of motion and for multiple supports. Simplifications in data presentation are suggested which might ensure ease of comparison of machines and a rapidly expanding and useful data base.

Characterization of structure-borne sound sources: The source descriptor and the coupling function

An introductory, theoretical study for characterization of structural acoustic sources has been carried out. The concept source denotes any active structure with unknown internal elementary source mechanisms and unknown internal transmission paths from generation to the contact points with the receiving structure. An alternative description of the power transmission from the source to the receiver is presented in the case of one component of motion and one contact point. A general source parameter called the source descriptor is introduced. This parameter is a function of source data only. It is proportional to power and involves both the internal vibration of the source and its mobility at the contact point. From experimental measurement series, it can be emphasized that the velocity of the free source is a suitable and manageable quantity for describing the internal vibration of many structure-borne sound sources. The active power transmitted from the source structure to the receiving one is found to be the product of the source descriptor and a dimensionless function called the coupling function. This function illustrates the dynamic properties of the source-receiver interface.