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Acoustics and Biology

Acoustics• loudness (intensity) and pitch (frequency)• How to read a spectrogram

Use of sound by marine animals• Predation and defense• Echolocation• Communication and social interaction

Signal-to-noise ratio

Man-made sounds and their effects on animals

T

a

Anatomy of a sound wave

a = amplitude of waveT = period of wavef = frequency = 1/Tλ = wavelength (= cT = c/f, where c is sound speed)

Amplitude determines sound level pressure, this determines loudness

These waves have the same frequency and wavelength but different amplitude

aa

a = amplitude

Loudness (Amplitude, sound level)

SL(dB)=20 log10(P/Pref)SL(dB)=10 log10(I/Iref)

Chart shows loudness in dB of some familiar sounds

Sound levels in air and water have different reference levels, so 0 dB (air) ≈ 26 dB (water)

Frequency determines “pitch”

T

T

These waves have the same amplitude but different period, frequency, and wavelength

Frequency f = 1/T, wavelength λ= c/f

Larger instruments produce lower frequencies

Pitch (frequency)

Instrument dBBass drum 35-115Piano 60-100Trumpet 55-95Violin 42-95Voice 40-90

Marine animal sounds are made up of multiple frequencies

The sound spectrum gives the pressure level at each frequency

Intensity pressure2

SL [dB] = 10 Log10(I/I0)SL [dB] = 20 Log10(P/P0)

Worcester & Spindel 2005

Spectrogram shows how sound spectrum changes over time

Some fish use sound for courting and as a fright response

A fish example of sound use: Atlantic Croaker

Snapping shrimp make noise to stun their prey.They create a cavitation bubble that “snaps” as it collapses.

http://stilton.tnw.utwente.nl/shrimp/

claw crab

An invertebrate example: snapping shrimp

Toothed (odonticete) whales• Smaller (1.5 to 17 m long)• Social• Most are not migratory • Chase and capture individual

fish, squid, crabs• Echolocate, communicate

Baleen (mysticete) whales• Larger (15 to 30 m long)• Often solitary• Long annual migrations• Feed on aggregations of

krill, copepods, small fish• Communicate long-distance

Toothed (odonticete)

whales

Baleen(mysticete)

whales

Toothed whales Baleen whales

Baleen whales

Toothed whales

Dolphins live in social groups that stay together 5-10 years. They have “signature whistles” that can be used to recognize individuals at distances of >500 m.

Time (s)

Fre

quen

cy (

Hz)

Social calls - sound for communication

Whale can determine distance, angle, size, shape, etc. from sound echoes

Echolocation using echoes from sound pulses or clicks

Mellinger 2007

Echolocation frequencies

Toothed whales

Baleen whales do not echolocate. Why not?

1. They don’t produce high enough frequenciesBaleen whales produce low-frequency sounds with longwavelengths. Wavelength determines the minimumecho detection distance.

Frequency f (Hz) Wavelength λ (m)

10 150

100 15

1,000 1.5

10,000 0.15

100,000 0.015

Minimumecholocation

frequency

Food too far away

OK

Toothed whale prey:•Squid and large fish•More likely to be solitary•Good acoustic targets(squid pens and fish swim bladders have density different from water)

Baleen whale prey:•Plankton and small fish•More likely to aggregate•Poorer acoustic targets (density similar to water)

Baleen whale prey

Toothed whale prey

2. Baleen whale prey (krill, copepods) are poor acoustic targets

-Hydrophones and 3D accelerometers in a waterproof, pressure-resistant case with suction cups-Sneak up on whale, attach D-Tag-Record audio, pitch, roll, heading, depth-Tag pops off, floats to surface 18 h laterMark Johnson with D-Tag

A cool invention for listening to whales: acoustic whale tag

Toothed whale foraging:Beaked whales dive deep to find prey

Natacha Aguilar de Soto

Peter Tyack et al.Yellow indicates echolocation

Baumgartner and Mate 2003

Colors: copepod concentration (#/m3)

—: whale trajectory

--: bottom of mixed layer

: Times of visual contacts

: Times of CTD+OPC cast

(OPC = Optical Plankton Counter)

Baleen whale foraging: Right whales dive to bottom of the mixed layer where plankton are most concentrated

Blue whales migrate and need to communicate over long distances

High-frequency sounds are absorbed more quickly

Absorption of sound in SOFAR channel

Because baleen whales have long, solitary migrations, they need to use low frequencies to stay in communication.

Because toothed whales move in groups, they can use high frequencies without losing communication.

Transmission loss: Sound signal loss of intensity due to cylindrical spreading, spherical spreading, and absorption

Bluewhale

Dolphins

Signal-to-noise ratio (SNR)

SNR in decibels indicates how much of the signal can actually be heard over the background noise level.

For communication, need a minimum SNR of 3 to 5 dB.A good SNR is 20 to 30 dB.

A negative SNR(dB) indicates no signal gets through.

Baleen whales

Toothed whales

Seals, sea lions, and walruses

Manatees and dugongs

Echolocation (toothed whales)

earthquake rainfall

Marine mammal sound levels are generally between 100 and 200 dB

Airgun10 to 500 HzUp to 232 dB

Outboard engine6,300 Hz

Commercial Ship10 to 20,000 Hz

Low-Frequency Active Sonar100 to 500 Hz230 to 240 dB

These are loud enough to damage tissues and cause hearing loss

These add constant background noise

Man-made noise in the ocean

Before motors~30 dB

After motors~75 dB

Since the invention of propeller-driven motors (~150 years ago),• Background noise level in the ocean has increased by ~45 dB• Lowest background noise f has dropped from ~100 Hz to ~7 Hz

Before motors~100 Hz

After motors~7 Hz

Blue whale song20 Hz, ~155 dB

Pre-motor noise level30 dBWhale song stays above ambient noise level for ~2,000 kme.g. San Diego to Seattle(area ≈10,000,000 km2 )

Current noise level75 dBWhale song stays above ambient noise level for ~60 kme.g. New Brunswick to NYC(area ≈10,000 km2)

Bluewhale

Can use transmission-loss curves to calculate the effective communication range

Range of effective communication for blue whale singing at 20 Hz and 155 dB

Range before mid-1800s

Current range(yes, that tiny speck)

Potential effects of man-made sounds on marine mammals

• Disruption of feeding, breeding, nursing, acoustic communication and sensing

• Psychological and physiological stress

• Temporary or permanent hearing loss or impairment

• Death from lung hemorrhage or other tissue trauma

Noise-induced mass strandings

Mass strandings associated with Navy sonar activity The Bahamas (2000):14 beaked whales, 1 spotted dolphin, 2 minke whalesBleeding in ears

The Canary Islands (2002):14 beaked whalesGas bubbles and bleeding in multiple organs

Mass strandings associated with air gunsTasmania and New Zealand (2004): 208 whales and dolphins

Senegal and Madagascar (2008): > 200 pilot whales and melon-head whales

Humans use acoustics to understand whales. Are the whales doing the same to us?

Captive beluga imitates human voice!

A great source of information on sound in the ocean:http://www.dosits.org/