SMS 204: Physics for Marine Sciences

• Instructors: E. Boss.

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• Today’s topic: Sound and hearing

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• Sound propagation:

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• A pressure (density) wave.
• Cannot propagate in vacuum.
• Displacement in the same direction as propagation
• c = (dp/dρ)^1/2

• Intensity and its attenuation
• I = (dp_max)²/(ρ₀c) [W/m²]
• db=10log₁₀{I/I_ref}=20log₁₀{dp/dp_ref}

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Attenuation is frequency and salinity dependent:

• Frequency [Hz], f=c/λ. {c=λ/T=λf }
• C_water~1500m s^-1
• C_air~300m s^-1

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Frequencies emitted by

organisms:

Note frequency shift in marine organisms. Why?

• Differences in frequencies and source intensities result in differences in range of propagation of sound:

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Sound propagation in the oceans:

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Snell’s law (wave dynamics): n₁sin(θ₁)=n₂sin(θ₂)

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Some consequences:

1. θ₁=0 🡪θ₂=0

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• Total internal reflection only when moving from slow 🡪 fast.

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• Reciprocity principle: if light (sound) can travel from A to B it will take the same route from B to A.

n₁/n₂ = v₂/v₁

http://www.sasked.gov.sk.ca/docs/physics/images/u3c12_1.gif

θ₁

θ₂

http://blog.soton.ac.uk/soundwaves/files/2013/12/refracter.gif

Sound propagation in the oceans:

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• Refracted rays bend as the speed increases until total internal reflection occur.
• Sound channel is centered at the minimum of sound speed.
• Similar to fiber optics and other waveguides (equator for Kelvin waves).

• Fish hearing: Otolith
• Several pairs per fish
• Relative motion due to differential acceleration trigger nerve cells; can tell source direction.

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• How about mammals?

Wave phenomena -Doppler shift: used to estimate relative velocity of target

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f = c/l

Stationary source:

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f = c/λ

f’=(c±u_r)/λ

🡪 Δf =±fu_r/c

Stationary receiver:

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Δf=±fu_s/(c±u_s)~ ±fu_s/c

Both moving:

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Δf=f(±u_s±u_r)/c

Demo – Doppler

Wave phenomena : Resonance

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• Physical construct have natural frequencies based on their dimensions.

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• Forcing at these frequencies (among others) result in large response at the resonant frequency (ies).

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Demo – Resonance, beating

Use of sound and light to study marine organisms

• When the organism is of the same size as the wavelength we get the most scattering per mass.

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• Light: μm-size organisms

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• Sound: 100μm🡪cm size organisms (bubbles, 100 × smaller)

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High-resolution sonograms�(From: “High resolution acoustic structure of fish,” Nash, R., Sun, Y. & Clay, C., J. Cons. Int. Explor. Mer, 43: 23-31, 1987)

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Zooplankton backscattering�(From: Chu, D. & Stanton, T., J. Acoust. Soc. Am., 104 (1), July 1998, pp. 39-55)

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Calibration:

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SciFish 2000:�Broadband Fish ID Sonar

Layer of Jelly Fish

Layer of Euphasids Being Eaten By Pollock

Schools of�Small Pollock

Loose Layer of

Medium Sized�Pollock

Loose Layer of

Large Pollock

Zoom View

Class Distribution of

Zoom View

Scientific Fishery Systems, Inc.

16253 Agate Point Road NE

Bainbridge Island WA 98110

Ph. (206) 855-8678

Mobile (206) 660-6587

Scifish@ispchannel.com

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Additional ways sound is used to study the oceans and its bottom?

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Active:

• Depth sounding.
• Tomography of water.
• Tagging.
• Currents (Doppler).

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Passive:

• Whales
• earth-quakes
• Rain
• Reefs activity

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Wikimedia – acoustical tomography

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Want to know more?

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https://dosits.org/

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UNH – center for coastal and ocean mapping.