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 differences4 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 arrayUnique solution – sound can be localized
3D localizationSource 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 techniqueDirect signal and surface reflectionCan 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
ResolutionSimulations
90,000 pixels
Breaking wave noiseSteel sphere target
900
100
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
Tag Family
Diameter
Minimum Size:Lengt
h (mm),Weight in
Water (g)
Maximum Size:
Length (mm),Weight
inWater
(g)
Power Output
(dB)
Sensors:T-Temp
P-Pressure (depth)
Battery Life
V7 7 mm17.5 mm,
0.7 g20.5 mm,
0.8 g136 None 200 days
V9 9 mm20 mm,
2 g46 mm,
3.1 g139-147 T,P,TP 400 days
V13 13 mm36 mm,
6 g44 mm,
6.6 g147-155 T,P,TP 700 days
V16 16 mm52 mm,
9 g96 mm,
16 g149-159 T,P,TP 10 years
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
• 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
t(C) t(A)t(B)
tAB = t(A) - t(B)
tCB = t(C) - t(B)
dh dh
C = 1533 m/s
)t(t2c
)t(tc2dt
ABCB2
2CB
2AB
22h
1
1t
S(x, y)
)t(t2d
)t(td)ttt(tcs
CBABh
CBAB2h
2CBABCB
2AB
2
x
2x
21
2y stcs
Methodology
Example of pair of signalers
Note effective spaceBehavioral observations remove L/R ambiguity
Lammers et al 2006
Whistles occur between animals spaced far (median 23 m) apart
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)