Underwater Acoustics Research
Laser Vibrometry Applications to
Underwater Sound Field
Measurements
Paul Lepper & Simon Dible
Senior Research Fellow
Applied Signal Processing Group
Loughborough University
([email protected])
Dept. Electronic and Electrical Engineering, Loughborough University
Laser Doppler Velocimeter
Fibre optical
Beam-splitter
Acoustic
Source
Pellicle
Nd:YAG Laser
(532 nm)
3 port
Bragg cell
Zero order
(unshifted)
Pig-tailing
mount/optics
RF
RF
Oscillator
Oscillator
80MHz
80MHz
Output
+
Fibre
optic
coupler
Bulk optics for
focusing and light
collection
Silicon
Photodetectors
1st order 80MHz
shifted
Output
Outputsignal
signal
Conditioning
Conditioning
Mixer
MixerRF
RF
Oscillator
Oscillator
Frequency
Frequencyshift
shift&&
Demodulation
Demodulation
Signal
Signal
PrePreconditioning
conditioning
Vibrometer calibration tests NPL
500 kHz pulsed tone
hydrophone / pellicle
x
Projector
LDV
Mid-frequency pulsed tone
Fibre Vibrometer Sensitivity
Acoustic projector ITC1042, distance 0.5 m
- 2 mm reflective pellicle
Reference hydrophone - TC4034
Interferometer
1.5
Acoustic projector Panamertics 0.25 MHz, distance 0.5 m
- 2 mm reflective pellicle
1.0
6.0
0.5
0.0
40
50
60
70
Acoustic velocity (mm/s)
Acoustic velocity (mm/s)
2.0
5.0
Reference hydrophone - TC4034
Interferometer
4.0
80
90
3.0
Frequency (kHz)
-219 dB re 1V/μPa @ 75 kHz
100
110
120
2.0
1.0
0.0
100
150
200
250
300
Frequency (kHz)
-212 dB re 1V/μPa @ 240 kHz
350
400
Focused transducer 800 kHz
hydrophone / pellicle
x
Projector
LDV
Focused transducer 400 kHz
400 kHz
Focused transducer
800 kHz
400 kHz
Instantaneous amplitude
Acousto-optical effects
Mirror
Rate of change of optical path
length
Acoustic tone-burst
Acoustic
source
z
⎛
d⎜ 2∫ n( z, t ) d z ⎞⎟
0
dL
⎠
= ⎝
dt
dt
Scanning
vibrometer
The refractive index change can be
related to the pressure by
Acousto-optical effects
Reflector
Transmission axis
Projector
LDV
Instantaneous Amplitude
(400 kHz tone)
Instantaneous Amplitude
(600 kHz tone)
mms-1
200 mm piston transducer 600 kHz tone
Magnitude
Magnitude and phase
Magnitude (400 kHz tone)
Magnitude (600 kHz tone)
Relative phase (400 kHz tone)
Reflection off a glass sheet
Projector
Glass sheet
500 kHz tone
Scattering from rod
600 kHz tone
50 mm piston transducer
300 kHz tone
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 195-220 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 10-12 m water
depth for various simulated mine types.
Bio-sonar systems in difficult environments
X-Ray CT scan of a Porpoise head
Vestibular sacs
MLDB
Melon
Cranford (1996)
Transmitter system
Sampled Emerging Waveforms at A, B & C
Relative peak amplitudes:
A = -3 dB, B= 0 dB, C=-2.1 d
Receiver system
Upper Skull
Acoustic Channel
Test Bone
Spongy
Tissue
Tursiops truncatus
Tooth acoustic properties: In air
The properties under investigation were:
ƒThe speed of sound through a tooth
ƒThe vibrational modes present within a tooth
Shear and compress ional
sound velocity
Input 100kHz Signal
Filtered between 70-130kHz
Transverse
Bending Mode
Breathing Mode
Tooth acoustic properties: Results
Numerical Results
Speed of Sound (Transverse) ~2200 m/s
Speed of Sound (Longitudinal) ~3380 m/s
Breathing
TransverseMode
Mode
Sound propagation through the lower jaw bone: In water
Position Speed From Point 1
2
1523m/s
3
1559 m/s
4
1560 m/s
5
1596 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
Sound Propagation Through The Lower Jaw Bone: In Air
Laser
•The bone was suspended by fishing wire
•Unimorph attached to the tip of the bone
•Laser was directed at the bone
Jawbone
Sound Propagation In The Lower Jaw Bone: In Air - Results
Results
Speed of sound (transverse):
Attenuation (transverse):
Jaw tip
Displacement
2607 m/s
1.2 dB/mm
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 60145kHz
Tooth Resonance Patten In The Bone: In Water - Results
18.0
Vibration Velocity in μm/s
Vibration Velocity μm/s
23.0
18.0
13.0
8.0
15.0
12.0
9.0
55
75
95
115
Frequency in kHz
Front Tooth
135
55
75
95
115
Frequency in kHz
Rear Tooth
135
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