Sound in Space?

Can sound travel through empty space? Put a ringing bell inside a glass jar, switch on the vacuum pump and listen as the air disappears.

Pressure
101.3 kPa
Air left
100%
Loudness
70 dB
Quieter by
0 dB

The maths, live

Air left = p ÷ 101 kPa101.3 kPa ÷ 101 kPa = 1.00
Sound energy ∝ amount of air−10 dB for every 10× less air
Change ΔL = 10 × log₁₀(p ÷ p_air + leak)ΔL = 10 × log₁₀(1.00 + 0.00003) = 0 dB

Turn the sound on (quietly!) and pump. The last faint ringing comes through the bell's stand, not the air. In space there is no air at all, so there is no sound.

How it works

Sound is a wave of air particles bumping into each other. The bell shakes the air next to it, that air pushes the next bit, and so on until the push reaches your ear. Sound needs a material (a medium) to travel through.

As the pump takes air out, there are fewer particles to carry the vibrations, so less sound energy gets through: about 10 dB quieter for every ten times less air. A real jar never goes completely silent, because a little sound sneaks out through the bell's stand and the base. Space is an almost perfect vacuum, so there is no sound at all out there.

Try this

  1. Pump until only 10% of the air is left. How many decibels quieter is the bell?
  2. Keep pumping. Why can you still hear a tiny bit of ringing?
  3. Let the air back in. How quickly does the sound come back?