Transcript Sounds in the sea - Ocean Mixing Group

Differences measuring levels
• Root mean square (RMS)
– For long (continuous) signals
– Average power delivered
• Peak-to-peak (pp)
– Extremely short signals (pulses)
– Integral cannot be calculated
• prms = A/√2 = 0.707A
• Our hearing works similarly
Localizing a sound source
• Passive listening arrays
• Active sonar arrays (e.g. multibeams)
Hyperbola
Fixed focus points
Hyperbola - set of fixed points in a plane that the
difference in distance between any point on plane
and the two foci is a positive constant
Two hydrophone array
Source
Signal will arrive at h1 before h2 : t21 = (d2-d1)/c
From this one time difference, signal could be anywhere along hyperbola
Three hydrophone line array
3 time of arrival differences
4 hyperbolas – in the dotted pair, only one is applicable (see signs)
Is the signal above or below the x axis?
Left-right ambiguity
• Affects line arrays
– Typically those towed behind a vessel
• No matter how many hydrophones added
• Rearranging 3 hydrophones can eliminate
ambiguity
Three hydrophone triangle array
Unique solution – sound can be localized
3D localization
Source is not in same plane as hydrophones
4 hydrophones (not in a line) – 2 possible points (similar to line array)
5 hydrophones – unique solution (if not in a line)
3D localization exception
• 4 hydrophones in one plane (not in a line)
• Near surface or seafloor
• Ambiguity points occur below the surface
and above it
• One solution in invalid
4 hydrophone array
Single hydrophone technique
Direct signal and surface reflection
Can determine the depth of the source
If we also obtain a bottom bounce and can
measure its time delay, range can also be
determined
Only works for very short signals (reflections do not overlap in time)
Measuring time differences
• Precise measurements of small differences
• Cross-correlation of one hydrophone (reference)
to others
– Good for complex signals (animal sounds)
• Problems
– Reverberation (shallow areas)
• Multipath propagation
– Ray bending
– Noise
• Rule of thumb
– Accurate localization restricted to distances ~5 times
the maximum size of array
Applications of arrays
Acoustic daylight
• Passive sonar
• Proposed by Buckingham 1992
• Noise sources
– Passing ships, breaking waves, popping of
bubbles, snapping shrimp
• ‘Image’ objects
ADONIS
• Dish focus on slight variations in the
ocean's ambient noise field (lens)
• 3 meters in diameter, 8-80 kHz
• Reflects the collected sound
• A series of 126 hydrophones
• 1m resolution
Cross target
Data analysis
• Noise has broad frequency range
• Higher frequencies only – higher spatial
resolution
• Adding lower frequencies increases information
– acoustic ‘color’
– Spectral shape may indicate surface properties,
material properties, etc.
• Produce images continuously in real time at 25
Hz
• Show movement
• Currently only 130 pixels
Resolution
Simulations
90,000 pixels
Breaking wave noise
Steel sphere target
100
900
Tracking with tags
• Single frequency coding (~50-100 kHz)
– Repetition rate
– Pulse intervals
• Tags emit a series of pings in a pulse train which
contains ID and error checking information (up
to 192,000)
• Individually track multiple fish
• Time between pulse trains is varied randomly
about a mean to ensure that other transmitters
have a chance to be detected by the receivers
Acoustic tracking
(pingers)
Tag characteristics
Diame
ter
Minimum
Size:
Lengt
h
(mm)
,
Weig
ht in
Water
(g)
Maximum
Size:
Length
(mm),
Weight
in
Water
(g)
V7
7 mm
17.5 mm,
0.7 g
20.5 mm,
0.8 g
136
None
200 days
V9
9 mm
20 mm,
2g
46 mm,
3.1 g
139-147
T,P,TP
400 days
V13
13 mm
36 mm,
6g
44 mm,
6.6 g
147-155
T,P,TP
700 days
V16
16 mm
52 mm,
9g
96 mm,
16 g
149-159
T,P,TP
10 years
Tag
Fam
ily
Power
Output
(dB)
Sensors:
T-Temp
PPressure
(depth)
Battery Life
Tag ideas
• Incorporation into ocean observatories
• Archival tags with sensors that download data to
listening stations
• Tags that are also receivers, record contacts
with other tags
• Widely spaced ‘array’
– Presence/absence at various locations over time
– For example, at marine reserve boundary
• How often do fish emigrate or immigrate?
• Closely spaced array
– Tracking of individual fish over time
Determining source levels
Au and Benoit-Bird, Nature 2003
Source level and range
White curve is 20 log R + constant
Conclusions
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•
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As dolphins approach targets, sound gets louder
How to avoid hearing effects?
Bats constrict ears to hear less at close range
Human sonars apply gain function
Dolphins adapt the signal instead of the receiver
Receive constant echo from schools of fish
– Do not fatigue hearing system
– Reduce processing
Line array and dolphin behavior
• Clicks
– Pulsed, broadband signals
– Function: echolocation
• Interclick interval longer than
two-way travel time
– Function: communication
• Very short interclick interval
• Whistles
– Tonal signals
– Function: communication
Methodology
dh
t(C)
dh
t(B)
t(A)
t1
tAB = t(A) - t(B)
S(x, y)
tCB = t(C) - t(B)
C = 1533 m/s
2dh2  c 2 (t 2AB  t 2CB )
t1 
2c 2 (t CB  t AB )
c 2 (t 2AB t CB  t AB t 2CB )  dh2 (t AB  t CB )
sx 
2dh (t AB  t CB )
s y   c2 t 12  s2x
Example of pair of signalers
Whistles occur between animals
spaced far (median 23 m) apart
Note effective space
Behavioral observations remove L/R ambiguity
Lammers et al 2006
Burst pulsing pair
Burst pulsing occurs at closer range (median 14 m)
Dolphin signaling conclusions
• Whistles
– Maintain contact between group members
• Burst pulses
– More intimate communication
– (Consider propagation)
• Regular clicks
– Highly variable distances
– No paired signaling
– Vigilance (not feeding during study)