Underwater Acoustics Research

Transcription

1 Underwater Acoustics Research Laser Vibrometry Applications to Underwater Sound Field Measurements Paul Lepper & Simon Dible Senior Research Fellow Applied Signal Processing Group Loughborough University Dept. Electronic and Electrical Engineering, Loughborough University

2 Laser Doppler Velocimeter Fibre optical Beam-splitter Nd:YAG Laser (532 nm) Pig-tailing mount/optics RF RF Oscillator Oscillator 80MHz 80MHz 3 port Bragg cell Zero order (unshifted) 1 st order 80MHz shifted Fibre optic coupler Mixer Mixer RF RF Oscillator Oscillator Pellicle + Acoustic Source Bulk optics for focusing and light collection Silicon Photodetectors - Signal Signal Preconditioning Preconditioning Output Output Output signal signal Conditioning Conditioning Frequency Frequency shift shift & Demodulation Demodulation

3 Vibrometer calibration tests NPL

4 500 khz pulsed tone Projector hydrophone / pellicle x LDV

5 Mid-frequency pulsed tone

6 Fibre Vibrometer Sensitivity Acoustic velocity (mm/s) Acoustic projector ITC1042, distance 0.5 m - 2 mm reflective pellicle Reference hydrophone - TC4034 Interferometer Frequency (khz) -219 db re 75 khz Acoustic velocity (mm/s) Acoustic projector Panamertics 0.25 MHz, distance 0.5 m - 2 mm reflective pellicle Reference hydrophone - TC4034 Interferometer Frequency (khz) -212 db re 240 khz

7 Focused transducer 800 khz Projector hydrophone / pellicle x LDV

8 Focused transducer 400 khz 400 khz

9 Focused transducer 800 khz 400 khz Instantaneous amplitude

10 Acousto-optical effects Mirror Acoustic tone-burst Rate of change of optical path length Acoustic source L dt d 2 d 0 = z n ( z, t) dt d z Scanning vibrometer The refractive index change can be related to the pressure by

11 Acousto-optical effects Reflector Transmission axis Projector Instantaneous Amplitude (400 khz tone) LDV Instantaneous Amplitude (600 khz tone) mms -1

12 200 mm piston transducer 600 khz tone Magnitude

13 Magnitude and phase Magnitude (400 khz tone) Relative phase (400 khz tone) Magnitude (600 khz tone)

14 Reflection off a glass sheet Projector Glass sheet 500 khz tone

15 Scattering from rod 600 khz tone

16 50 mm piston transducer 300 khz tone

17 Bio-Mimetics - the DBS The US Navy (SPAWAR BioSciences) have been developing a Dolphin Bio-mimetic Sonar to try to emulate the detection performance of the Mk7 (dolphin) mine hunting systems currently in fleet service. The equipment employs only 3 wideband transducers - 1 Tx & 2 Rx (Binaural). Transmits SLs db (equivalent to a bottlenose dolphin). The system exploits a neural net processor trained to recognise mine-like features and claims reliable detection ranges up to100m in m water depth for various simulated mine types.

18 Bio-sonar systems in difficult environments

19 X-Ray CT scan of a Porpoise head Vestibular sacs MLDB Melon Cranford (1996)

20 Transmitter system Sampled Emerging Waveforms at A, B & C Relative peak amplitudes: A = -3 db, B= 0 db, C=-2.1 d

21 Receiver system Upper Skull Acoustic Channel Test Bone Spongy Tissue Tursiops truncatus

22 Tooth acoustic properties: In air The properties under investigation were: The speed of sound through a tooth The vibrational modes present within a tooth Shear and compress ional sound velocity Input 100kHz Signal Filtered between kHz Transverse Bending Mode Breathing Mode

23 Tooth acoustic properties: Results Numerical Results Speed of Sound (Transverse) Speed of Sound (Longitudinal) ~2200 m/s ~3380 m/s Breathing Transverse Mode

24 Sound propagation through the lower jaw bone: In water Position Speed From Point m/s m/s m/s m/s Average 1560 m/s Aligned to avoid acousto-optic interference Painted and coated with retroreflective beading Mounted with an 11 degree tilt Sound transmitted: 1 ms pulsed 100 khz sine wave

25 Sound Propagation Through The Lower Jaw Bone: In Air Laser Jawbone The bone was suspended by fishing wire Unimorph attached to the tip of the bone Laser was directed at the bone

26 Sound Propagation In The Lower Jaw Bone: In Air - Results Results Speed of sound (transverse): Attenuation (transverse): 2607 m/s 1.2 db/mm Jaw tip Displacement

27 Tooth Resonance Patten In The Bone: In Water Bone aligned as in previous experiment Teeth were coated in retro-reflective beads Laser positioned on the front tooth and rear tooth in the bone Swept frequency source was performed kHz

28 Tooth Resonance Patten In The Bone: In Water - Results Vibration Velocity μm/s Vibration Velocity in μm/s Frequency in khz Frequency in khz Front Tooth Rear Tooth

29 Conclusions Laser interforometers already provide current primary standard in UK established by NPL for frequencies 500 khz - 20 MHz. A Laser Doppler Velocimeter has shown good agreement with both established Michelson Interferometers and conventional hydrophone techniques providing an alternative techniques to the measurement of underwater acoustic pressure. The LDV is capable of detecting an acoustic pressure field evolution both temporally and spatially without perturbing the field. LDV has been applied as a non invasive technique for measurement of remote sound propagation in solids both air and water Thank you