Sounds in the sea

Snapping shrimp• Major source of biological noise in shallow

temperate and tropical waters• 20 dB above the noise level typical of sea state 6• Little diurnal and seasonal variations • Broad frequency extent • Extremely difficult to filter this noise• Can severely limit the use of underwater

acoustics • Interfere with the transmission and reception of

sounds by other animals

Shrimp dominated ambient noise

A single snap

Intensity 10 -20 dB higher than dolphin echolocation click

How sound is produced• Not claw hitting stationary mate • Cavitation

– Water moves above a critical speed and experiences a drop in pressure

– Allows tiny air bubbles in the fluid to swell– Fluid slows and the pressure again rises, the bubbles

implode– Generates a shock wave and an accompanying

sound• Tooth-shaped piece on the moving part of the

claw plunges through a hole in the stationary part, shooting out a jet of water fast enough to cause cavitation

Snapping shrimp

Shrimpoluminescence

High temperature and pressure in bubble as it collapsesToo brief to be seen with the naked eye

Rain• Major role in heat and water budgets• Accurate measures over ocean almost non-existent• Noise distinct from wind

Quiet

Heavy rainfall

How rain sound produced?

• Impact of drop on sea surface• Formation of bubble underwater

– Most often loudest source– Bubble not in equilibrium so it radiates sound

while reaching equilibrium• Changes in drop size change shape of

splash and bubble and thus, sound production

Small drop

High resonance (ringing) frequency

Large drop

Low resonance (ringing) frequency

Acoustic rain gauge

Humpback whale chorusing

Humpback whale song• “Most complex display in animal kingdom”• Singers lone, stationary males• Winter mating grounds• Structured

– Phrases organized into themes in sequences– All males sing same song in one area– Song evolves over season

• Function?– Sexual advertisement– Physical male-male competition– Territory defense

• Production mechanism?– Have larynx but no vocal chords– Do not exhale to produce sound

Au et al 2000

Humpback whale chorusing levels

Dominant source of noise

In Hawaii from ~Jan-AprilSong levels recorded on 1 hydrophone over 4 monthsChorus of many whales not in synchrony

Diel variability in chorusing level

Few whalesLevels below 110 dB

Peak whale abundance

Reasons for diel variability?

• Whales singing louder at night• More singers at night• Moving closer to hydrophone at night

(nearshore)• Cannot be separated with one hydrophone

Ships/propellers

on-axis source level spectra of cargo ship at 8 & 16 kts measured directly below ship B – propeller Blade rateF – diesel engine Firing rateG – ship’s service Generator rate

Application – Manatee collisions• Hearing peak 16-18 kHz• Dominant vessel <1 kHz

Gerstein and Gerstein 2004

Manatee management• Slow vessel down• Lowers intensity of sound and frequency• Large vessel

– 3 mph detectable 2 to 3 seconds (12 - 18 feet) away from the propellers (hull of the boat extends 24 feet ahead of the propellers)

– 24 mph detectable 16 seconds (650 feet) before propellers

• Small boat– 3-4 mph detectable 6 to 24 feet from the propellers– 24 mph detectable 600 feet from the propellers.

Speed effects on vessel noise

Ship speed and source level

Vessel shadowingEffect strongest close to the surface

Vessel shadowing

Measuring sounds in the seaSampling rules

• Convert analog (voltage) signal to digital– Nyquist frequency rule

• Sampling frequency must be at least twice that of the highest frequency component of the signal

• The signal can be fully recovered from the sampled signal

fs = 8 fa

fs =1.5 fa

fs=1/t

Aliasing fa=7/8 fs

Digitization

• Digital signals made up of bits• Each bit is a 0 or 1• At most, digitizer can represent 2n values

where n is the bit rate• Dynamic range

– Dynamic range (dB) = 20 log (2n) ≈ 6 n• 12 bit A-D converter

– 4096 values– 72 dB dynamic range

Hardware

• Pre-filter– Remove constant noise– Cut off above Nyquist frequency (Anti-

aliasing)• Pre-amplifier

– Improve analog signal/noise ratio– Improve dynamic range

Finite Fourier Transform

(FFT)• Represent signal

in time or frequency domain

• All signals can be described as the sum of a series of sin and cos waves of varying frequencies and amplitudes

FFT examples

Duration and bandwidth

Each signal is 100 kHz