Article: Analysis of bending wave fields: making the invisible visible
Lars Hörchens, Laboratory of Acoustical Imaging and Sound Control, Delft University of Technology, Lorentzweg 1, 2628 CJ Delft

Bending waves are the most important type of waves for the description of vibrations in plates and plate-like structures. They also play a key role with respect to the sound radiated from these structures. The analysis of bending wave fields can provide information on the position of vibration sources, reflecting and diffracting inhomogeneities, and transmission paths. Bending waves are dispersive: the speed of propagation is different for each frequency. As a consequence, wave fronts are spread out when propagating away from a source. This happens already at rather small distances from the source, such that the wave field cannot be analysed directly because of its low resolution. However, the effects of dispersion can be removed from the measurements by means of signal processing. Dispersive time signals are thereby transformed to non-dispersive signals containing information on source-to-receiver distance. Using this approach, it is possible to visualise and analyse wave propagation with high resolution as if it was observed in a non-dispersive medium. An acoustical image of sources and reflectors can then be generated. An interesting application of this method can be formed in combination with near-field acoustic holography. The presented approach can also be applied to constructions made of coupled plates in order to look around the corner. The complete article is available to members of the NAG

Article: Sound radiation of a non-rigid piston and pole cap compared with loudspeakers
Ronald M. Aarts, Philips Research Europe HTC 36 (WO-02), NL-5656 AE Eindhoven, The Netherlands,

Loudspeakers are often modelled as a rigid piston in an infinite baffle. As a model for real loudspeakers, this approach is limited in two ways. One issue is that a loudspeaker cone is not rigid and a second issue is that a loudspeaker is mostly used in a cabinet. Both issues are addressed here by developing the velocity of the radiator in terms of orthogonal polynomials known from optical diffraction theory as Zernike circle polynomials. Using these polynomials we develop semi-analytic expressions for the sound pressure from the radiator in two different cases: as a flexible flat radiator mounted in an infinite baffle, and as the cap of a rigid sphere. In the latter case the comparison is done not only for the pressure but also for other quantities viz. the baffle-step response, sound power and directivity, and the acoustic center of the radiator. These quantities are compared with those from a real loudspeaker. Finally, in the case of the baffled-piston radiation the spatial impulse response is presented. The complete article is available to members of the NAG

2011-09-28 Luchtvaartlawaai

Ontwikkelingen Europese rekenmethode vliegtuiggeluid
Dick Bergmans
Annette Kruger-Dokter 
Nationaal Lucht- en Ruimtevaartlaboratorium
The complete presentation is available to members of the NAG