If soundwaves are compression waves in air molecules, and thermal energy is vibrations in air molecules, why aren’t loud things hot?

287 views

Like if I have a really bright flashlight I can feel heat from the beam, but I can blast a speaker at max volume and I might even feel the vibrations in my hand, but there’s no change in temperature.

Is this a glitch in the matrix?

In: Physics

Why do you believe that ‘louder’ isn’t hotter then the corresponding wave?

Being hot is a molecule vibrating chemical bonds but not translating in space (a little but not much). A compression wave is a whole bunch of molecules translating back and forth together.

Hope that helps!

Sound energy will cause temperature rise but it will be hard to measure outside a controlled experiment in a lab. One reason is that most of the time, it isn’t a lot of energy converted to heat in the air and source of sound isn’t high energy to begin with relatively speaking.

A 50 W sound system might drive a speaker to produce a few watts of actual sound energy and, in a regular room, that is pretty loud. Loud enough that most people won’t tolerate it. It would be hard to detect a temperature difference even if these few watts converted to heat. Most of the heat would come from the electronic equipment, the lights in the room, the body heat of people inside – all of which release much more heat into the room.

Edit: I misread your question. But I’ll keep my answer below, for why “hot things create sound”…my apologies!

You need a constantly changing temperature to create sound by rapidly changing the density of air in the immediate vicinity, but it is very well possible!

Check out [this carbon nanotube speaker ](https://youtu.be/EA2exNkoq8A), which is essentially just a thin sheet of carbon ntubes changing temperature so rapidly that it is creating sound (and literal music)! There is no moving components here, it is just the black sheet that is playing music.

There are 3 factors here.

* The sound wave itself doesn’t add much energy. The individual molecules are already bouncing around at the speed of sound. The fact that a wave is moving through them doesn’t change their average velocity by an appreciable amount. (But in extreme cases it can. Look up [acoustic refrigeration](https://en.wikipedia.org/wiki/Thermoacoustic_heat_engine).)

* Sound isn’t just compression. It is also rarefaction (low pressure) between the high pressure peaks. The transfer of heat (ignoring radiation) is based on the number of collisions between air and the object. While the high pressure regions have more collisions the low pressure regions will have less. This averages away much of the effect that you might expect.

* If you can feel the vibrations then it is producing heat in your hand. (Any time something deforms there will be heat generated.) However it is an extremely small amount of heat and is too small to notice.

Actually ultrasound can heat tissue slightly and you can produce high heat with high intensity ultrasound [https://en.wikipedia.org/wiki/High-intensity_focused_ultrasound#Temperature](https://en.wikipedia.org/wiki/High-intensity_focused_ultrasound#Temperature). It is due to absorption of sound waves in tissue but I am not sure that is the mechanism you are talking about

The air will be heated slightly by the sound, but not much. The speaker vibrations are giving energy to the air molecules, which are colliding with other air molecules. So the average kinetic energy (thermal energy) per molecule increases slightly.

By ‘thermal energy’, i mean average kinetic energy of molecules. The heat is related to the speed at which the molecules are moving on average.