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Sound is the chapter that pays twice. It appears in the general science section of SSC CGL, CHSL and Railway NTPC and Group D, and it appears again in BPSC TRE-4 and CTET science, where the same echo and SONAR sums are set almost word for word. Two or three marks a paper, from a chapter with exactly one formula.
That formula is v = fλ. Everything else here is a consequence of it, or of one fact: sound is a mechanical wave, so it needs a medium and cannot travel through vacuum. A bell ringing inside a jar as the air is pumped out goes silent while still visibly ringing — that single demonstration answers more questions than any derivation.
The rest of the chapter is speed in different media, the three characteristics a listener notices, and reflection, which gives you echo, SONAR and ultrasound. Ten minutes here is worth more than an hour on optics.
1. Sound travels fastest through which of the following?
2. A person shouts and hears the echo after 3 seconds. If the speed of sound in air is 340 m/s, how far away is the reflecting surface?
3. The pitch of a sound depends on which property of the sound wave?
4. What is the audible range of frequency for a normal human ear?
5. A SONAR pulse sent from a ship returns after 4 seconds. If the speed of sound in sea water is 1500 m/s, what is the depth of the sea bed?
6. Taking the speed of sound in air as 344 m/s, what is the minimum distance from a reflecting surface at which a distinct echo can be heard?
7. A tuning fork of frequency 256 Hz produces sound waves of wavelength 1.32 m in air. What is the speed of sound in that air?
8. Ultrasonic waves are those whose frequency is: