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)