Transcript Ocean acoustics: biology

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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
Anatomy of a sound wave
a
T
a = amplitude of wave
T = period of wave
f = frequency = 1/T
λ = wavelength (= cT = c/f, where c is sound speed)
Amplitude determines sound level pressure,
this determines loudness
a = amplitude
a
a
These waves have the same frequency and
wavelength but different 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”
Frequency f = 1/T, wavelength λ= c/f
T
T
These waves have the same amplitude but
different period, frequency, and wavelength
Pitch (frequency)
Larger instruments produce
lower frequencies
Instrument
Bass drum
Piano
Trumpet
Violin
Voice
dB
35-115
60-100
55-95
42-95
40-90
Marine animal sounds are
made up of multiple
frequencies
The sound spectrum gives
the pressure level at each
frequency
Intensity
µpressure
2
SL [dB] = 10 Log10(I/I0)
SL [dB] = 20 Log10(P/P0)
Spectrogram shows how sound spectrum
changes over time
Worcester & Spindel 2005
A fish example of sound use: Atlantic Croaker
Some fish use sound for
courting and as a fright
response
An invertebrate example: snapping shrimp
claw crab
Snapping shrimp make noise to stun their prey.
They create a cavitation bubble that “snaps” as it collapses.
http://stilton.tnw.utwente.nl/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
Baleen
(mysticete)
whales
Toothed
(odonticete)
whales
Baleen whales
Toothed
whales
 Toothed whales
 Baleen 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.
Frequency (Hz)
Social calls - sound for communication
Time (s)
Echolocation using echoes from sound pulses or
clicks
Whale can determine distance, angle,
size, shape, etc. from sound echoes
Echolocation frequencies
Toothed whales
Baleen whales do not echolocate. Why not?
Mellinger 2007
1. They don’t produce high enough frequencies
Baleen whales produce low-frequency sounds with long
wavelengths. Wavelength determines the minimum
echo detection distance.
c
l = , c » 1,500 m/s speed of sound
f
Minimum
echolocation
frequency
Frequency f (Hz)
Wavelength λ (m)
10
100
1,000
150
15
1.5
10,000
100,000
0.15
0.015
Food
too far
away
OK
2. Baleen whale prey (krill, copepods) are poor
acoustic targets
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)
Toothed whale
prey
Baleen whale
prey
Baleen whale prey:
•Plankton and small fish
•More likely to aggregate
•Poorer acoustic targets
(density similar to water)
A cool invention for listening to whales:
acoustic whale tag
Mark Johnson with D-Tag
-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 later
Toothed whale foraging:
Beaked whales dive deep to find prey
Natacha Aguilar de Soto
Yellow indicates echolocation
Peter Tyack et al.
Baleen whale foraging: Right whales dive to bottom of
the mixed layer where plankton are most concentrated
Colors: copepod concentration (#/m3)
—: whale trajectory
--: bottom of mixed layer
: Times of visual contacts
: Times of CTD+OPC cast
(OPC = Optical Plankton Counter)
Baumgartner and Mate 2003
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
Blue
whale
Dolphins
Signal-to-noise ratio (SNR)
SNR =
I signal
SNR in decibels indicates
how much of the signal can
actually be heard over the
background noise level.
I noise
æ I signal
ö
ç
I ref ÷
÷
SNR(dB) = 10 log10 ç
ç I noise
÷
çè
I ref ÷ø
SNR(dB) = 10 log10
I signal
I ref
- 10 log10
For communication, need a
minimum SNR of 3 to 5 dB.
A good SNR is 20 to 30 dB.
I noise
I ref
SNR(dB) = SL signal (dB) - SL noise (dB)
A negative SNR(dB)
indicates no signal gets
through.
Marine mammal sound levels are generally
between 100 and 200 dB
Baleen whales
Toothed whales
Seals, sea lions, and walruses
Manatees and dugongs
Echolocation (toothed whales)
earthquake
rainfall
Man-made noise in the ocean
Outboard engine
6,300 Hz
These add constant
background noise
Commercial Ship
10 to 20,000 Hz
Low-Frequency
Active Sonar
100 to 500 Hz
230 to 240 dB
Airgun
10 to 500 Hz
Up to 232 dB
These are loud enough
to damage tissues and
cause hearing loss
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
After motors
~7 Hz
Before motors
~100 Hz
After motors
~75 dB
Before motors
~30 dB
Can use transmission-loss curves to calculate the
effective communication range
Blue whale song
20 Hz, ~155 dB
Pre-motor noise level
30 dB
Whale song stays
above ambient noise
level for ~2,000 km
e.g. San Diego to Seattle
(area ≈10,000,000 km2 )
Current noise level
75 dB
Whale song stays
above ambient noise
level for ~60 km
e.g. New Brunswick to NYC
(area ≈10,000 km2)
Blue
whale
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
whales
Bleeding in ears
The Canary Islands (2002):
14 beaked whales
Gas bubbles and bleeding in multiple organs
Mass strandings associated with air guns
Tasmania 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/