block 5-bat echolataion Flashcards
microchrioptera bats
- are a group of small bats that:
Use echolocation (they send out high-pitched sounds and listen for the echoes to “see” in the dark)
how do bats naviagate in the dark?
-scientists found that bats navigating in his lab emit ultrasonic calls in the 12- 2000 kHz range (inaudible frequencies for humans)
-spectrograms visualise this
-also found that plugged ears or taping the moth closed prevents navigation ,showing bats need to use this
why do bats use ultrasonic calls?
-High frequency sounds are better than bouncing off (reflecting off) small things and getti g good return back. than low frequency sounds
-bats need to navigate small objects such as branches and detect small prey
-wavelength(m)= wave speed(m/s)/ frequency (Hz)
What has now been determined about the echolocation of bats?
- biological sonar (biosonar) is a major image-forming
system using reflected sound instead of light - active sense: animal emits pulse of sound
- sound is distorted by the environment and bounces
back producing an echo - echo is detected and interpreted
- used for navigation and forage(hunting)
information that bats can derive from their calls?
-presence of objects
-location of objects (direction and distance)
-Types of objects (e.g. moth or tree)
How do bats derive distance from calls?
-make a sound and the amount of time it takes for the sounds to return to you gives you an indication of how far the object is (time delay of echo)
-angular size(things appear to be smaller the further the way they are hences what the anguar size tells you)= detected via amplitude(loudness)
-Target (absolute) size ((of the object))= combines distance and amplitude
- small amplitude + very short delay = small, close object
- small amplitude + very long delay = larger, distant object
How do bats derive information about the direction of an object through their calls?
-derive information elevation by moving their ears independently = compare echo amplitude (like Owls)
- detect Azimuth = (where the object is in the horizontal plane) -auditory sensitivity 60 degrees cone in front.
* possibly use intensity difference
how do bats derive information about velcoty through their calls?
- detect how fast something is moving use doppler shift analysis of echoes frequencies
What types of echolocation calls that bats make?
- frequency modulated (FM)= a call that sweeps from high to low frequency
-constant frequency(CF)= is a call at constant frequency
-Different species of bats generate either FM or mixed CF/FM calls
-see lecture for image
frequency modulated calls
-frequency modulated calls comprise vocalisations of
varying frequency. E.g. call sweeps from higher to lower frequencies.
- FM calls are short bursts of sound (<5 msec).
-For any given frequency FM calls carry little energy: do not propagate far so not good for
detection at distance.
-But, precise localisation of close objects: Short duration and ability to analyse multiple time delays
returned from a single broadband call provides high
-accuracy information about object location.
-FM calls useful for hunting in foliage or near to ground.
continuous frequency calls
-Frequency is constant during call: frequency bandwidth is
narrow. Calls often contain harmonics.
* CF calls are longer duration (>10’s to 100’s of msec).
* CF calls focus a lot of energy into small frequency band which
facilitates signal detection against background noises lying in
the same bandwidth.
* Bats possess an ‘acoustic fovea’: have a large proportion of
auditory cells tuned to respond to specific frequencies,
including the CF bandwidth. This helps detection of echos from
CF calls with minimal interference from environmental noises
(that fall outside the CF frequency).
* CF calls useful for localisation of distant objects in open
spaces
* Doppler shift in CF calls is also used to determine the target
velocity
Doppler shift
-It’s the change in sound frequency due to the movement of the source or listener.
-Happens when either the source or observer is moving.
-Moving toward = waves compress → higher frequency (higher pitch)
-Moving away = waves stretch → lower frequency (lower pitch)
-Common example: a passing train or ambulance siren sounds higher as it approaches and lower as it moves away.
how does bats use the doppler shift?
-Bats use Doppler Shift to track prey while flying.
-Bats emit high-frequency sound pulses (echolocation calls).
-Echo Return and Motion Detection:
Higher frequency echo than the emitted call → prey is approaching.
-Lower frequency echo than the emitted call → prey is moving away.
-What Doppler Shift Helps Bats Do: Determine relative velocity of prey (how fast it’s moving toward or away).
-Track prey’s motion with high accuracy.
why is the dopller shift important in bats?
-It allows the bat to figure relative velocity
-Flutter analysis: Detection of wing beat. Produce tiny Doppler shifts as wings move forward and back
-hus frequency and amplitude of echo are modulated as angle of
wings change = acoustic glints
* Relative oscillation of wings specific to species
what is problems with the CF?
-the CF component of the echo
doesn’t match the acoustic fovea
doppler shift compensation
-* Mechanism to keep echo within acoustic fovea
* The bat adjusts the frequency of its call in order to allow the
echo to return within its acoustic fovea
what are other features of bat calls?
-Calls become shorter and more
frequent when near to object
-As bats approach an object they increase repetition rate and
decrease duration.
