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