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Reflection of Mechanical Waves

When mechanical waves encounter a new material, some of the wave energy bounces back into the old material.

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Echolocation

Echolocation

Credit: Petteri Aimonen
Source: http://en.wikipedia.org/wiki/File:Animal_echolocation.svg

How do bats locate their prey? A bat uses a signal sent out to determine the location of its next meal. When the signal hits a solid object, the signal's frequency is shifted and reflected back towards the bat. Through this process known as echolocation, bats are able to fly in the dark and determine where even the smallest objects are located in front of them.

Amazing But True

• Echolocation is the ability used by some animals, where a signal is sent out and the animal listens for the reflection of the signal to identify objects. Bats for instance are able to change the frequency, intensity, the length of the emitted signal and the pulse interval. The signal sent by bats can be composed of two different types of frequencies: those that are frequency modulated and those that are constant frequency.
• A constant frequency signal allows a bat to detect the velocity of a target via the Doppler shift of the frequencies. This is a very complex technique because the bat must take into account the possible shift in the frequency so the reflected wave does not return at a frequency the bat can't hear. A frequency modulated signal is one where the frequency of the signal is varied. Recent studies have shown that this allows for extremely precise determination of targets. By using frequency modulated signals it has been discovered that bats are able to tell the difference between to objects that are separated by half a millimeter!
• Watch the video below of bats using echolocation:

Show What You Learned?

Using the information provided above, answer the following questions.

1. If a bat sends out a signal at a frequency $f$, would the reflected sound wave come back at a higher or lower frequency if the object that reflected it sound was standing still while the bat was moving towards it?
2. If a stationary bat sends out a signal that reflects off of a stationary target, would the reflected frequency be perceived as high or lower than the original signal?
3. If the bat in the previous question was sitting still while the object moved toward it, would the reflected frequency be higher or lower?