Acoustic Black Holes for Control of Structural Vibration and Sound Radiation

Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment

Abstract

Acoustic black holes (ABHs), which are based on introducing carefully designed geometrical features to a structure, have been shown to achieve significant levels of structural damping in both beams and plates. The aim of this project is to develop the design, optimisation and practical implementation of ABHs and, therefore, facilitate their exploitation in complex practical structures. In particular, the project may investigate the interaction between multiple ABHs and also explore methods of optimising their locations on complex structures to minimize either structural vibration or sound radiation. Additionally, the integration of active technologies into the design of ABHs may also be explored. The specific focus of the research is likely to depend on the skills and interests of the successful candidate, but is will involve a mixture of theoretical, numerical and experimental investigation

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/P510646/1 01/10/2016 30/09/2021
1941775 Studentship EP/P510646/1 01/01/2017 30/06/2021 Kristian Hook
 
Description ABHs are structural features that are realised by introducing a taper, usually defined by a power law, into a beam or plate. The decrease in thickness along the taper causes the wave speed in the ABH to decrease, theoretically to zero at zero thickness. In this case, there is no reflection from the ABH, similarly to an astrological black hole, which is where the slightly misleading terminology originated. However, when the tip height is finite, there is significant reflection from the ABH. The decreasing wave speed along the taper results in a decreasing wavelength. A small amount of damping material can be added to the taper to significantly improve the performance, particularly at higher frequencies. The low frequency performance of ABHs is dependent on the local modes of the taper, at which frequencies bands of low reflection occur. The objective of this work is to overcome this performance limitation of Acoustic Black Holes (ABHs) through design and integration of active control technologies.

Initially, an investigation has been carried out to determine how three key design parameters can be used to design an ABH with practical dimensions. A finite element (FE) model has been used to perform a parametric study where the tip height, taper length and power law of the ABH termination have been varied over practical ranges. The reflection
coefficient has then been calculated for each configuration and frequency bands of low reflection have been observed, which are dependent on the parameters. A modal analysis
has also been carried out using the FE model to explain the spectral variation in the reflection coefficient. Local minima have been observed within the bands of low reflection,
which lead to an investigation into the trade-off between the design parameters. This investigation showed how the power law and tip height could be selected to minimise
the reflection from an ABH. An experimental validation has then been performed. This work is published in the Journal of the Acoustical Society of America.

Piezoelectric components have then been integrated into the ABH to address the performance limitations of the passive system. A wave-based feedforward active control strategy has been used to control the reflected wave, and hence the reflection coefficient of the AABH. The AABH termination has been shown to require less computational and electrical power to achieve a significantly better performance when compared to a constant thickness active termination. However it has also been shown that this control strategy produced a significant enhancement of the vibration in the AABH that is not present in the constant thickness active termination. An experimental validation is performed for each configuration. This work is published in the journal Smart Materials and Structures.

The enhanced vibration caused by controlling the reflection coefficient may cause structural fatigue when used for a long period. To further investigate the relationship between the reflection from the AABH and the local vibration in the AABH, a remote damping feedback active control strategy has been applied using the AABH. A selection of feedback compensators have been calculated using a geometric approach that allows a local feedback loop to be used to control both the taper vibration and the reflected wave component simultaneously. It has been shown that controlling the reflection enhances the taper vibration and that controlling the taper vibration generally enhances the reflection. It has also been shown, however, that a constraint can be put in place so that the reflection coefficient or taper vibration can be controlled without enhancing the other quantity.

Building upon the previous investigations, the AABH concept has been extended a plate. It has been shown that, when AABHs are used, broadband damping can be achieved with a hybrid of active control and the passive damping provided by the ABH effect. The resulting structure with AABHs is also significantly lighter than the standard plate. It has been shown that the computational requirements of implementing a global feedforward control strategy at low frequencies using AABHs are similar to those of a constant thickness active plate. In addition, the resulting performance of the plate with AABHs and the constant thickness active plate are also similar, however the energy required to implement the AABHs is lower. When combined with the passive performance of the AABHs at higher frequencies, it has been shown that the plate with AABHs significantly
outperforms the constant thickness active plate.

Overall, the work performed under this award has shown that active control can be integrated into ABHs to improve performance and that AABHs can be used as a lightweight and effective damping solution in both beams and plates.
Exploitation Route The results from the parameter study may be used as guidance for a similar investigation, where the performance criteria is changed from the reflection coefficient to the radiated sound power. The investigations into AABHs could also be extended in a similar way, by investigating control strategies that are designed to control the radiated sound power.
Sectors Aerospace, Defence and Marine,Construction,Transport
URL https://www.researchgate.net/profile/Kristian_Hook
 
Description BAE Systems AGM 2019 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Supporters
Results and Impact The BAE Systems AGM was an opportunity to present my work on acoustic black holes to an audience that included company investors, politicians and industrial partners.
Year(s) Of Engagement Activity 2019
URL https://investors.baesystems.com/agm
 
Description BAE Systems awards 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact These awards were presented to 5 UK-based PhD students from BAE Systems strategic partner universities that are funded by EPSRC Industrial Cooperative Awards in Science and Technology (iCASE) who have demonstrated excellence in their understanding of real-world problems and are working towards innovative solutions.

This award has opened a number of opportunities to present work at a variety of BAE Systems events.
Year(s) Of Engagement Activity 2018
URL https://www.baesystems.com/en/article/phd-students-at-strategic-universities-recognised-with-awards
 
Description New Scientist Live 2019 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact A huge public event where people from all backgrounds with an interest in cutting edge science and engineering come for a day of talks and demonstrations. I was one of two PhD students presenting work in association with the BAE Systems. It was a fantastic experience and I thoroughly enjoyed engaging with a diversity of people.
Year(s) Of Engagement Activity 2019
URL https://live.newscientist.com/#/