# Differences measuring levels Root mean square (RMS) –For long (continuous) signals –Average power delivered Peak-to-peak (pp) –Extremely short signals.

## Presentation on theme: "Differences measuring levels Root mean square (RMS) –For long (continuous) signals –Average power delivered Peak-to-peak (pp) –Extremely short signals."— Presentation transcript:

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 p rms = 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 h 1 before h 2 : t 21 = (d 2 -d 1 )/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 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 Fam ily Diame ter Minimum Size: Lengt h (mm), Weig ht in Water (g) Maximum Size: Length (mm), Weight in Water (g) Power Output (dB) Sensors: T-Temp P- Pressure (depth) Battery Life V7 7 mm 17.5 mm, 0.7 g 20.5 mm, 0.8 g 136 None 200 days V9 9 mm 20 mm, 2 g 46 mm, 3.1 g 139-147T,P,TP400 days V13 13 mm 36 mm, 6 g 44 mm, 6.6 g 147-155T,P,TP700 days V16 16 mm 52 mm, 9 g 96 mm, 16 g 149-159T,P,TP10 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) t AB = t(A) - t(B) t CB = t(C) - t(B) dhdh dhdh C = 1533 m/s S(x, y) Methodology

Example of pair of signalers Note effective space Behavioral 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)

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