Science Questions

If sound and heat are both vibrations, then why isn't sound hot and heat loud? Dylan via email

They are essentially the same thing. Basically heat is happening on a very very small scale: if you imagine the heat waves and the wavelengths of the vibrations are very small and about the same size as a normal atom. However with sound, the wavelengths of the vibrations are a few centimetres. So in a sound all the atoms are moving in the same direction close to each other. Eventually the vibrations of sound will start breaking up and moving off in random directions and it will convert into what we feel as heat.

Dr Hugh Hunt, who appeared as a guest on the show last week, provides this reply: 'Sound is vibration of the air and the ear is designed to detect air vibrations, but only over a small range of frequencies from 40Hz to 20kHz. Heat is vibration associated with the motion of photons and our skin is good at detecting a certain small range of frequencies of photons. Light is like heat, but our eyes are optimized to a different small range of frequencies. So our senses are all very specialized. All waves carry energy from A to B. The energy that our ears detect we call sound. The energy that a light bulb produces we call heat on our skin and light in our eyes. So it is all arguably down to language: hotness, loudness, brightness - they are all simply alternative names for the energy level.

May 2006