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cathy4208
#1 Posted : Monday, June 29, 2015 2:17:59 AM
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I had a few questions regarding this passage, as summarized below. I'm not sure how to arrive at the answer to this.


"By listening to the returning echoes of ultrasonic tones, the bat acquires information about the location and even the identity of insect prey. If a flying insect enters the echolocation beam, the time interval between call emission and arrival of the echo is used by the bat to determine the distance to background objects or prey, and Doppler shifts and changing reflection angles associated with the insect’s beating wings introduce oscillating frequency and amplitude shifts into the returning echo. The mustached bat can detect these variations, or “glints,” superimposed on the undistorted pure tone that is reflected from stationary obstacles or dense vegetation. Because each insect species has a unique wing shape, wing movement, and wing beat frequency, the returning echo signals form unique signatures from which the bats can identify their prey.

A flying mustached bat is inevitably faced with one particular problem: Because of the relative motion between the bat and background objects, the whole echo (CF plus acoustic glints) would be Doppler shifted out of its auditory fovea, resulting in a loss of information about the potential prey. The bat is able to precisely compensate for the frequency shift by appropriately lowering its calling frequency, so that the returning echo CF falls within the auditory fovea."

QUESTION:
Doppler effects will cause which of the following echo patterns from the wings of a flying insect as they beat first toward and then away from a bat approaching the insect at a speed much greater than the insect is flying?

ANSWER CHOICES:
• First lower, then higher frequency glints superimposed on a uniform downward frequency shift
• First higher, then lower frequency glints superimposed on a uniform downward frequency shift
• First lower, then higher frequency glints superimposed on a uniform upward frequency shift
• First higher, then lower frequency glints superimposed on a uniform upward frequency shift
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joelhutchinson
#2 Posted : Tuesday, June 30, 2015 1:30:41 AM
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Hi there,

I always like to remember the sounds of an ambulance passing by when answering doppler questions. If the source and observer have relative motion TOWARDS one another, then the frequency is shifted up (the observer hears a higher pitch) if there is relative motion AWAY from one another, then the frequency is shifted down (lower pitch). In this case, the bat is the observer, and the insect's wings and other background objects act as the source for the returning echo.

Now the first part of the answers deals with the doppler effect due to the movement of the insect's wings. As the wings beat towards the bat, there is relative motion between the bat and wing TOWARDS each other, yielding a higher frequency glint. As the wings beat away from the bat, there is relative motion between the bat and wing AWAY from each other, yielding a lower frequency glint. That rules out the first and third answers.

Next we just have to decide about the uniform frequency shift due to the echoes bouncing off the background objects. There is a hint in the passage:

"The bat is able to precisely compensate for the frequency shift by appropriately lowering its calling frequency"

This means that the background frequency shift must be upwards, since the bat needs to compensate for it by LOWERING the frequency it emits. That just leaves the last answer.
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