1. The apparatus used in a ship to find out the depth of the sea underneath is called _______.   tuning fork   sonar   bell jar   Hint Half-n-half Clue   2. The _______ keeps the pressure in the middle ear the same as the outside air pressure.   semicircular canal   Eustachian tube   eardrum   Half-n-half Clue   3. Ultrasonic vibrations are used in medicine for imaging internal organs of the human body.   True   False   4. Sounds are produced by musical wind-instruments when columns of air are made to vibrate.   True   False   5. The number of sound vibrations per second is called _______.   time period   amplitude   frequency   Half-n-half Clue   6. 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   7. Echoes are caused by _______ of sound.   refraction   rarefaction   reflection   Half-n-half Clue