Bat Hearing Puzzle
Do you know that:
Bats do not respond to frequency below 10, 000 Hz but we find that their ears have the capability to do so.
Let us put our heads together and understand why?
Just as when we understood how they capture their prey, try explaining all of this to your friend or parent, after you think you have understood it yourself.
While reading about the waves , you learnt what is a wavelength.
Can you also recall from our talk on Waves that you can calculate the wavelength of a wave if you know its frequency! You just need one more thing : the speed of the wave.
How?
If you divide the speed by the frequency, you get the wavelength!
[ Ask your science teacher why this is so. This will help us to go on with bats. ]You can go back and read our Wave Page once again : Teacher’s Scribbles on Waves
So, what is the wavelength of a sound wave with frequency 10 , 000 Hz? You know that its speed is 340 m / sec.
By dividing speed by frequency i.e 340 / 10, 000 = 0.034 meters, which is 3.4 centimetres.
So, we get the wavelength to be 3.4 centimeters.
We are coming close to answering our question, but first :
Let us tell ourselves what we learnt from the way bat’s send sound waves to sense insects.
If you want to sense any object, you can send a wave towards it. This wave hits the object and comes back, looking somewhat different. As you have sent the wave, you know how it looked at first. You can see the reflected wave which comes after hitting back and point out how different it is from the one you sent. From the difference you can tell many things like :
How big is the object?
How far is it?
In which direction is it moving?
Just as the bats did it with sound, we can do it with all kinds of waves, specially light, sound and water waves . All of these have a frequency and a speed from which you can calculate wavelength.
Now comes the interesting fact:
If you want to know the location of an object 5 m across, you should bombard it with a wave of wavelength not more than 5 m. Else you won’t be able to sense the object with the help of the wave which comes back after hitting it. The wave with a larger wavelength just swooshes over the object which is smaller than its wavelength without getting affected at all!
It doesn’t even come back to you to tell : “ Look how different I am from the way you sent me. I hit an object.”
It doesn’t even come back to you to tell : “ Look how different I am from the way you sent me. I hit an object.”
No. Its wavelength which is roughly a few centimetres is much larger than the size of the dust particle (size is a few micrometres) and just passes over it without getting affected at all. But if you have a big rock, these waves will come back and tell you that they have hit something which was of a size similar to their wavelength.
If you want to sense an object 2 cm s across, you should send a wave with wavelength less than 2 cm so that you can sense it properly.
Now, the wavelength we calculated for our sound wave of 10 , 000 Hz is 3.4 cm. If you calculate the wavelength of a frequency smaller than this, it will be larger than 3.4 cm.
( Check for 9000 Hz ! )
Now let us understand the final answer to the puzzle :
The insects which are microbats look for are even smaller than them. A wavelength larger than this just “swooshes” over their prey without coming back. So, they don’t find such sound waves useful that have wavelengths larger than this and frequency smaller than this .
( You just saw how longer wavelengths mean lesser frequency ) .
All animals conserve their efforts so that they are used only where they are needed most. This helps them to survive with a body with simple functions and needing less energy from food which is difficult to get.
Therefore, their brains want to conserve their effort and do not process these frequencies as they are not of much use to the bats.
For you to know : A sound wave with a frequency of 80,000 Hz has a wavelength of around 4 millimeters, which is suitable for locating a small moth.
Also, the noise from the environment is found to be of a similar wavelength, which can interfere with their echo. They ignore all those sounds which do not help them in finding their prey.
So, bats get a clear sound of the echo produced by just their own sounds and are able to sense where their prey is, very accurately. Nothing can disturb them. Amazingly, even when thousands of bats are flying out of a cave they can still use their echolocation with all the noise around.
So, we learn that every sense organ and how a living being uses them is carefully “tuned to” the surrounding it lives in.
In the next page we learn how different animals have to produce sounds so that they can be heard.
Click : How living beings tune into the sound surrounding them ?
