Question

Why can’t we hear the scream of bats?
A) Because it's scream consists of infrasonic sound
B) Because its scream consists of ultrasonic sound
C) Because its scream consists of subsonic sound
D) None of these

Answer 1

The audible range of frequency of a human ear lies between $20Hz$ and $20000Hz$. Ultrasonic sounds have frequencies higher than $20000Hz$ whereas infrasonic sounds have frequencies lower than $20Hz$. Several birds and animals communicate using sounds of frequencies other than the frequencies lying between $20Hz$ and $20000Hz$.

Complete answer:
The audible range of frequency with which a human ear can perceive the sensation of sound, lies between $20Hz$ and $20000Hz$. This suggests that humans cannot hear sounds other than the mentioned range of frequencies, with the help of their ears. Bats communicate using sounds which have frequencies above $20000Hz$. These sounds are called ultrasonic sounds. Clearly, these sounds cannot be perceived by human ears. On the other hand, elephants and whales communicate using sounds that have frequencies below $20Hz$. These sounds are called as infrasonic sounds and clearly, these sounds too, cannot be perceived by humans.
Ultrasonic sound finds application in many fields such as medicine and engineering. In engineering, ultrasonic sound is used in measurement of distances while in the medical field, ultrasound is used in sonography or ultrasonic imaging of inner parts of the human body. In nature, bats and porpoises scream with ultrasonic frequencies to search for prey and obstacles. Many insects are also said to have the capability of detecting ultrasound. Ultrasonic sound ranges from $20000Hz$ to several $GHz$.

Therefore, the correct option to be marked is B.

Additional information:
Infrasonic sound finds application in geography as well as medicine. In medicine, these low frequency sounds are used to study the functioning of the heart and in geography, infrasonic sounds are used to detect earthquakes and volcanoes. Many animals are said to have the capability to perceive infrasonic sound. As an example, most of us would have heard nearby animals like dogs barking and making sounds just before we experience an earthquake. Infrasonic sound ranges from $20Hz$ to several $mHz$.

Note:
Students should not get confused with the names of sounds of different frequency ranges. Ultrasound and infrasound can be judged using the audible range of frequency of a human ear. Supersonic and subsonic sound depends upon the speed of sound. Supersonic sounds have speeds higher than the normal speed of sound whereas subsonic sounds have speeds lower than the normal speed of sound.