Physical properties characterisation
Latest research results for 2008
Research results for 2007
- Development of broadband transducers [PDF]
- Physical properties characterisation rig [PDF]
- Transducer characterisation and bandwidth enhancement [PDF]
The Physical Characterisation Group brings together scientists from Leicester University and the British Geological Survey with expertise ranging from signal processing, through materials and process characterisation to the geological sciences. The research objectives of the Physical Characterisation Group include the development of new technologies in acoustic sources, signals and signal processing that will lead to a greater understanding of the shallow and deep marine environment and processes. Bats and dolphins both use multi-spectral signals, but they employ very different strategies to deliver energy over a range of frequencies in order to interrogate their respective air and water environments. In both cases, the signal echo reflection must contain sufficient energy over a wide spectrum such that a target can be characterised. Understanding the way in which bats and dolphins have adapted their signals is providing the basis for bio-inspiration in our research, in which our signals development has driven associated improvements in wideband transducers.
Signals, such as our ladder chirps have provided the breakthrough required to investigate if bats employ ‘Vernier’ approaches to compare the phase information contained within chirp harmonics to achieve ranging resolutions of 1/20th the wavelength. Indeed, our experimentation has led to the development of a non-correlation based ranging method using Vernier signals.
Vernier approaches for the comparison of phase information
Vernier signals can be easily designed that contain multiple frequencies. These frequencies can be selected such that the Vernier differences increase on a logarithmic, ‘slide rule’ scale. Our example Vernier signal comprises 70kHz, 71kHz, 80kHz and 170kHz. The phase differences between these frequencies are used to provide very high resolution time delay estimation. Our experiments have achieved a resolution equivalent to 1/50th wavelength.
Vernier signal comprising frequencies of 70kHz, 71kHz, 80kHz and 170kHz.
Transducers capable of generating these signals have been developed using matched-backed, 1-3 piezo-composite ceramics with a single matching layer into a water medium. The 1-3 composite was used to achieve highest possible sensitivities and also to reduce extraneous frequencies from parasitic surface layer resonances. The front and rear face matching was used to achieve the widest possible fractional bandwidths. We are also currently investigating extending the fractional bandwidth via use of inversion layer, and graded matching design approaches.
Transducer with matched-backed, 1-3 piezo-composite ceramics.
These bio-inspired signals and transducer technologies have been incorporated into a precision transducer moving system to form the Low Frequency Ultrasonics Research Laboratory. This is a waterborne acoustics laboratory operating between 40 kHz to 250 kHz with a 116 micron reposition repeatability. This facility forms the basis of our physical characterisation experimental programme where we have investigated single layer target properties using echo-reflected and through-transmitted waves. We have identified the loss and standing wave characteristics though solid materials and are now investigating the acoustic properties of porous media.
Precision transducer moving system within the Low Frequency Ultrasonics Research Laboratory.