A vibrating tuning fork, held over an air column of a given length with its one end closed, produces a loud audible sound. Name the phenomenon responsible for it and explain the observation.

In figure given below, A, B, C and D represent the test tubes each of height 20 cm which are filled with water up to heights of 12 cm, 14 cm, 16 cm and 18 cm respectively. If a vibrating tuning fork is placed over the mouth of test tube D, a loud sound is heard.

(a) Describe the observations with the tubes A, B and C when the vibrating tuning fork is placed over the mouth of these tubes.

(b) Give the reason for your observation in each tube.

(c) State the principle illustrated by the above experiment.

For normal ears, sensitivity is maximum at the frequency :

1. 5 kHz
2. 10 kHz
3. 1 kHz
4. 500 Hz

By reducing the amplitude of the sound wave, its :

1. pitch decreases
2. loudness decreases
3. loudness increases
4. pitch increases