# Problem on Sound for Class 9

1. A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of sound. What is the time interval between successive compressions from the source?
2. A sound wave travels at a speed of 339 m/s. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
3. A person has a hearing range from 20 Hz to 20 kHz. What is the range of wavelengths of sound waves in air that a person can hear? Take the speed of sound in air as 344 m/s.
4. A sound wave has a frequency of 2 kHz and wavelength of 35 cm. How long will it take to travel 1.5 km?
5. Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child? Take the speed of sound in air as 344 m/s and that in steel as 6420 m/s.
6. To hear a distinct echo the time interval between the original sound and the reflected one must be at least 0.1 s. What is the minimum distance between the source of sound and the reflecting surface? Take speed of sound as 344 m/s.
7. A ship sends out ultrasound that returns from the seabed and is detected after 3.42 s. If the speed of ultrasound through seawater is 1531 m/s, what is the distance of the seabed from the ship?
8. A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
9. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given $g=10$ m/s2 and speed of sound is 340 m/s.
1. 0.002 s
2. 22600 Hz, not audible
3. 17.2 m, 1.72 cm
4. 2.1 s
5. 86:1605
6. 17.2 m
7. 2618 m
8. 1450 m/s
9. 11.4 s