1. An airtight bell-jar is inverted on a platform connected to a vacuum pump. An electric bell fitted into the jar is operated but its sound cannot be heard. This is because sound cannot travel through _______.   glass   vacuum   air   Hint Half-n-half Clue   2. The process by which sound vibrations are produced in pipes is known as _______.   resonance   rarefaction   reflection   refraction   Half-n-half Clue   3. The approximate speed of sound in air is _______ m/s.   1600   330   5000   3 × 108   Half-n-half Clue   4. Out of the following materials, the best conductor of sound is _______.   Steel   Water   Air   Half-n-half Clue   5. A _______ note is the lowest note obtained on blowing gently across the top of a test-tube.   fundamental   harmonic   overtone   Half-n-half Clue   6. The _______ keeps the pressure in the middle ear the same as the outside air pressure.   eardrum   Eustachian tube   semicircular canal   Half-n-half Clue   7. The number of sound vibrations per second is called _______.   amplitude   time period   frequency   Half-n-half Clue