Acoustics and Echolocation Flashcards
What are some reasons why echolocation evolved?
- Sound propagation (sound travels faster and farther in water than in air)
- Low visibility (light is scarce in deep waters)
What are the characteristics of sound?
- Frequency (cycles per second (Hz))
- Wavelength (λ)
λ = c/f - C = speed of sound
- F = frequency
Describe amplitude
- Loudness
- The amplitude of a sound wave is characterized by its sound pressure level (SPL)
Sound pressure level is measured in decibels
Describe sound attenuation
- Decrease in sound intensity over space
- P / 4πr² = I
- P = Power
- 4πr² = Area of a sphere
- I = intensity
- As you move farther from a source, energy has to travel farther to reach the object. As the object gets bigger, the energy must spread over a larger area.
Why is sound attenuation important for marine mammals?
What two factors affect sound attenuation?
- Absorption: viscosity / thermal conductivity
- Scattering: sound strikes foreign bodies
High frequencies = more suseptible to absorption
Do higher frequencies attenuate faster or slower than low frequencies?
Faster
(Higher frequencies do not travel as far as low frequencies)
Describe the relationship between the sounds that marine mammals produce and the range in which they can hear
Marine mammals have a range of sounds that they can produce ad this matches the range of sounds that they can hear
What is the hearing range for humans?
20 to 20,000 Hz (20kHz)
- Ultrasonic > 20kHz
- Infrasonic < 20 Hz
What are some different types of sounds?
- Constant frequency (CF)
- Frequency modulated (FM)
- Broadband
- Narrow band
Constant Frequecny
- Pure tone
- Stable oscillation period (wavelengths all = the same size)
- Amplitude or loudness (hight of waves) can vary
Frequency Modulated
Frequency = changing with time (wavelengths vary in size)
Broadband
Short duration “click”
Wide range of frequency
Narrow band
- Tonal “whistle”
- High frequency
- Long duration
Small range of frequency
What is a spectrogram
- Plots frequency vs. time
- Reflects the tone that an animal is producing over time
Do high frequencies have a longer or shorter wavelength than low frequencies?
- High frequencies have a shorter wavelength
- Low frequencies have a longer wavelength
Echolocation
The ability to orient by transmitting sound and receiving echoes from objects in the environment
Similar to sonar sound navigation and ranging
What are the only marine mammals that utilize echolocation
Odontocetes
What are the three distinct processes that can characterize echolocation?
- Sound production: outgoing signal
- Sound reception: sound bounces off a target and comes back
- Signal processing: interpretation of sound information
Describe “foraging buzzes” in dolphins
Series of clicks that get closer together as the dolphin gets closer to its prey
What is the source of echolocation?
- Current hypothesis: Monkey Lips / Dorsal Bursae Complex
- Previous Hypothesis: Larynx
Describe the Monkey Lips / Dorsal Bursal Complex (phonic lips) and how they produce echolocation clicks
- Valves made up of ridges located in the walls of the nasal passage
- Air is pushed through the monkey lips at pressure that vibrates the air, lips, and surrounding tissue
- Sound is transferred into the melon for eventual transmission into the water column
What is believed to assist in odontocete echolocation capabilities?
- Cranial asymmetry
- Melon
- Nasal Sacs
Why is the melon important for echolocation
- Comprised of metabolically inactive fatty acids
- Core of the melon is composed of less dense fat than outer layer (slower rate of sound transmission)
- As sound passes through the core it is refracted (narrowing the sound beam) and reflected (focusing the sound beam forward)
The melon is different in different families of odontocetes
Where do incoming sounds come in for odontocetes?
Specialized fats in the lower jaw
Describe how Sperm Whales echolocate
- Spermaceti organ located in the enormous cranium secretes a waxy oily liquid
- Sounds are produced by the phonic lips at the front of the head
- The spermaceti organ sends the sound backwards first
- Sound travels through he spermaceti organ until it is reflected by air sacs at the back of the organ
- The sound then moves forward where it is refracted by the cranium
- This allows the whales to build a 3D image of their surroundings
What is an audiogram?
Displays the intensity of sounds required for an organism to hear
What portion of an audiogram represents the best hearing for an organism?
The lowest portion
Higher frequencies
What information can we learn from the audiograms of odontocetes ?
Odontocetes can hear better at higher frequencies
How do the number, source level, and time intervals between clicks vary in odontocetes?
- Size of object
- Distance to object (ex: foraging buzz)
- Background noise
How do odontocetes time echos and clicks?
- Outgoing clicks occur between returning echoes to reduce interference
What are the two steps in Bottlenose Dolphin echolocation?
- Target detection
- Target discrimination
How big does a target have to be in order for it to be detected by a Bottlenose Dolphin?
- Diameter of target must be > 1 wavelength (0.01m)
This is how big the object must be in order for the dolphin to detect it
What are some factors that have driven different odontocete species to have the echolocation clicks that they use today?
- Location
- Predation
- Size
- Prey types
What are some examples of high frequency clickers?
- Harbor porpoise
- Hector’s dolphin
Why do the Harbor Porpoise and Hector’s Dolphin have high frequency clicks?
- Live in shallow (nearshore) “cluttered” waters
- Small body size
High frequency = high resolution
Why do high frequencies have higher resolutions?
They have shorter wavelengths and narrower beam widths resulting in better axial and lateral resolution
Why don’t all odontocetes produce high frequency clicks?
Why is the echolocation technique of Porpoises unique?
Porpoises utilize a frequency range that no other organisms uses making them undetectable to predators and prey
What are some of the costs of echolocation?
- Inter-specific interception: alerting predators and prey
- Intra-specific interception: alerting competitors
The energetic costs are unknown
What is the passive listening hypothesis (Bottlenose Dolphins) and describe the experimental procedure done to test this hypothesis?
- Bottlenose dolphins prey on soniferous fish
- It is believed that dolphins passively listen rather than actively echolocate in order to detect their prey
…………………………………………………………………………………..
Experimental Procedure: - Control stimulus: Snapping shrimp (dolphins do not eat this)
- Experimental stimulus: Croaker (soniferous prey item for bottlenose dolphins)
- Dolphins stopped echolocating / started listening when they heard the croaker sounds
What are the differences in echolocation between transient and resident killer whales?
- Transient killer whales: feed on marine mammals - infrequent echolocation (irregular clicks)
- Resident killer whales: feed on salmon - frequent echolocation
What evidence suggests that Harbor Porpoises high frequency clicking is an adaptation used to avoid predation?
- Harbor Porpoises click above the hearing range of killer whales
- We would expect to see Harbor Porpoise whistle frequency above killer whales as well however, they DO NOT WHISTLE
