Engineering and the Environment, University of Southampton

Ultrasonic and underwater acoustics

Sound travels better in water than any other form of signal, and research in the Centre for Ultrasonics and Underwater Acoustics (UAUA) brings together acousticians, oceanographers, archaeologists, zoologists, geophysical surveyors and chemists, who explore questions such as:

• What would a waterfall sound like on Titan, Saturn's largest moon? How could this have enhanced the Cassini-Huygens probe mission?

• How do dolphins think and how can this be exploited to protect shipping?

• Can we produce 3D pictures of shipwrecks, hidden for hundreds of years under the mud at the bottom of the sea? Can we undertake archaeology without disturbing the wreck?

• Do humpback whales create 'walls of sound' to trap prey and how can we exploit this phenomenon to protect seals from the noise of piling in harbours?

• How can we use sound to monitor the annual transfer of billions of tonnes of atmospheric gases into the ocean and back again – a process of key importance to our climate and its stability?

The themes of exploration and discovery underpin UAUA's research, and key activities range from climate studies to the protection of marine mammals. Using an interdisciplinary approach, UAUA takes projects from fundamental science to real-world applications.

Biomedical and high-power ultrasonics

In addition to our oceanographic activities, we conduct research which addresses a range of biomedical issues, such as how ultrasound can be exploited to change chemical reactions and therefore help industries become cleaner and more efficient.

Our research has led to developments in many areas, including:

• a 'smart stethoscope' to assess the effectiveness of ultrasound in destroying kidney stones

• an ultrasound system to detect osteoporosis and the general health of bone

• a method for assessing muscle quality using ultrasound

• techniques that enable industry to assess the effectiveness of ultrasonic devices designed
  to clean surgical instruments, circuit boards and other tools

• techniques to detect erosion in pipelines

• a discovery that could help electroplating  companies become cleaner and more efficient through the use of acoustics.

We have also contributed to a national study looking at the use of ultrasound to treat tumours and have advised on the safe use of ultrasound (eg for foetal scanning). These studies are carried out in collaboration with a range of hospitals, including Guy's and St Thomas', London; the Institute of Cancer Research; Churchill Hospital, Oxford; Southampton General Hospital; and Chemistry at the University of Southampton.

Aeroacoustics and nonlinear acoustics

Aeroacoustics is the study of aerodynamically generated sound. The Fluid Dynamics and Acoustics Group supports a substantial programme of research in aeroacoustics which focuses on reducing aircraft noise. This is a particularly challenging problem since aircraft noise is generated by multiple sources, many of which are associated with turbulent, unsteady motion. All must be reduced to achieve a significant reduction in overall noise. Aircraft noise is a major nuisance for residents who live close to airports, and a significant environmental constraint on the growth of commercial aviation. Our Group is home to the Rolls-Royce University Technology Centre (UTC) in Gas Turbine Noise which forms part of the global Rolls-Royce research network and undertakes research on all aspects of aircraft noise. The activities of the UTC include theoretical, computational and experimental studies of aircraft noise sources and the development of robust noise prediction tools. Areas of particular interest are the design of acoustic liners to reduce noise radiated from intake and bypass ducts, the development of improved models for fan broadband noise and jet noise, and their integration within whole aircraft noise prediction schemes. We are also responsible for developing and exploiting advanced measurement techniques for rig and full-scale engine noise tests, and for appraising noise data acquired in industrial test facilities by Rolls-Royce and other industrial partners.

Electroacoustics, virtual acoustics, imaging and inverse methods

In ISVR, we are actively researching the use of inverse methods in acoustics. For example, one project is developing visual head-tracking methods that enable digital filters in virtual audio to be updated in real time in response to listeners' head movements. Further work is under way on optimal source distribution, which provides effective loudspeaker design for 3D sound and signal processing principle which enables lossless crosstalk cancellation process.

Inverse methods are also being applied to microphone array technology in two areas: the development of circular microphone arrays for speech and the use of microphone arrays to assess the noise radiated from aircraft jet engines, both in the outdoor environment and in an indoor test cell. Work is also continuing to assess and improve the low-frequency performance of loudspeakers and listening rooms.

Semiclassical methods applied to acoustics and vibration

This research addresses the fundamental question of how the shape of a resonator governs natural frequencies and modes of vibration. While numerical methods enable them to be calculated on a case-by-case basis, we lack a direct link that would, for example, allow us to efficiently design a cavity with a particular set of resonances.

We can gain further understanding from the periodic orbit theory of semiclassical physics, which was originally developed to examine quantum problems. This enables us to classify shapes according to the stability of internal ray paths and to explore the implications of the presence or absence of 'quantum chaos'. Random matrix theory can then be used to obtain statistical descriptions of the modal structure. Of particular interest is Vergini's method of short periodic orbits which uses 'scar functions' constructed around the periodic orbits as a set of basis functions for the mode shapes. This theory suggests that periodic orbits may relate to modes in the same way that modes are linked to arbitrary vibrations.

Periodic orbits in sound and vibration
Theoretical tools developed in quantum physics are being applied to acoustical problems, particularly at mid to high frequencies, where the usual methods of statistical energy analysis may not be valid. The use of the semiclassical trace formula has led to analytic expressions for mode count deviations in ducts and membranes, which can be extended to thin plates.