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
