I'm not educated in physics, but I learned that a bell struck in a small vacuum chamber will not be heard by people around it (in a school science lab). If it had been surrounded by air, there would have been energy propagating outward from the bell in a sound wave. Because this can't happen, does the bell vibrate harder from the impact? Will it break sooner?

  • 4
    $\begingroup$ I believe in all mediums sound vibrations dissipate over time to some degree and become heat energy. So the bell would probably not vibrate any harder, but it would probably vibrate much longer because the air isn't there to dampen the vibrations at all. $\endgroup$ – MonkeysUncle Mar 7 '15 at 21:08
  • 4
    $\begingroup$ There's also the matter of what's suspending the bell in the vacuum chamber. Whatever the mechanism (unless it's somehow magnetically suspended), the structure will convey some of that vibrational energy to the point of attachment and dissipate either into the wall or floor of the chamber wherever its attached. Rigid material will transmit the energy to a greater extent than softer material, and with softer material some of the energy will be dissipated within the material itself as heat. $\endgroup$ – docscience Mar 7 '15 at 22:32
  • 1
    $\begingroup$ @docscience Even if it is "somehow" magnetically suspended, that means there are going to be electrical effects as it vibrates and moves relative to the suspending device. $\endgroup$ – Random832 Mar 9 '15 at 1:30
  • $\begingroup$ @Random832 - Very true, I agree $\endgroup$ – docscience Mar 9 '15 at 14:36

When a bell vibrates in air, it pushes air molecules out of the way which will make the vibrations "decay". If you strike a bell in vacuum, this loss mechanism will not be there so the bell will "ring" for longer (but nobody can hear it). This doesn't mean the initial amplitude is significantly greater - just that it persists longer. Obviously if you rang the bell repeatedly there is a small chance that the moments of impact would amplify the motion of the bell (resonance) but that's not really very likely. I would not expect the bell to break sooner. On the other hand, for an electrical bell the coils used in the mechanism will heat up, and they would normally be air cooled (not terribly efficient but still - it's a cooling mechanism). It is conceivable that an electrical bell operated continuously in a vacuum would burn out because the cooling is less than in air - but that's pure speculation.

Back to the question of damped vibration, a sketch of what this might look like (arbitrary units of time along the horizontal, and amplitude along the vertical axis):

enter image description here

I am assuming there are "some" loss mechanisms for both - but at least one fewer for the bell in vacuum. The initial amplitude after being struck ought to be (very nearly) the same.


The bell will not vibrate harder, but will take much longer to decay. The chief dissipation mechanisms are air, bell suspension mount, and internal friction in the metal.

A well made bell's mount will be such that the fundamental vibrational mode of the bell does not produce much vibration or energy loss in the suspension point.

The internal friction of a metal bell will limit the quality factor to ~1000 and so the ringdown time will be approximately 3*freq_bell*Q.


Your Answer

By clicking "Post Your Answer", you acknowledge that you have read our updated terms of service, privacy policy and cookie policy, and that your continued use of the website is subject to these policies.

Not the answer you're looking for? Browse other questions tagged or ask your own question.