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 _______.   air   glass   vacuum   Hint Half-n-half Clue   2. The repetition of sound which is reflected from an object is referred to as _______.   the pitch   the amplitude   an echo   Half-n-half Clue   3. The apparatus used in a ship to find out the depth of the sea underneath is called _______.   tuning fork   sonar   bell jar   Half-n-half Clue   4. The approximate speed of sound in air is _______ m/s.   330   3 × 108   5000   1600   Half-n-half Clue   5. The number of sound vibrations per second is called _______.   time period   amplitude   frequency   Half-n-half Clue   6. Ultrasonic vibrations are used in medicine for imaging internal organs of the human body.   True   False   7. The lower limit of frequency that can be heard by the average human is about   2 Hz   20 Hz   200 Hz   2000 Hz   Half-n-half Clue