The diagram shows a glass bell jar connected to a vacuum pump. A ringing electric bell is inside the jar. What happens to the sound heard as air is pumped o...
Question 1 Report
The diagram shows a glass bell jar connected to a vacuum pump. A ringing electric bell is inside the jar.
What happens to the sound heard as air is pumped out?
Answer Details
Sound is a mechanical wave that requires a medium (material with particles) to travel through. As the vacuum pump removes air from the bell jar, there are fewer and fewer air particles inside to transmit the vibrations from the ringing bell to the glass walls and onward to the listener's ears.
As pumping continues, the sound gradually becomes quieter because fewer particles are available to carry the wave. Eventually, when enough air has been removed to create a near-vacuum, the sound cannot be heard at all, even though the bell is still ringing. The bell's hammer still strikes and the bell vibrates, but without air particles to transmit those vibrations, no sound energy reaches the observer.
The frequency of the bell does not change (the bell itself vibrates at the same rate regardless of the surrounding air pressure). The sound certainly does not get louder. The pitch does not increase. The correct observation is that the sound fades progressively and eventually disappears.
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