A battery-operated electric bell is suspended in air inside a thick glass bell-jar connected to a vacuum pump. The bell is set ringing continuously while air is gradually removed from the jar.
(a) Describe what an observer outside the jar will hear as the air is pumped out.
(b) Explain the reason for the change in loudness of the sound heard.
(c) State the conclusion that can be drawn from this experiment about the nature of sound waves.
(d) If, instead of removing air, water vapour were pumped out while some water remained at the bottom of the jar, how would the observation differ? Give a reason.