A huge amount of sound is generated in the world, which takes energy. As the sound dissipates, what happens to this energy?
Ron Dippold
San Diego, California, US
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Sound exists in the form of vibrations, or waves, in the air, which can be easily converted to heat. Heat is molecules vibrating; cold is molecules not vibrating as much. So, simply put, all the world鈥檚 sound turns into a little bit of heat.
Thankfully, this resultant heat is negligible in terms of global warming: just as you can鈥檛 usefully heat tea by stirring it, sound can鈥檛 appreciably heat things up.
Sound energy is weak. An annoyingly loud sound like a train engine or pneumatic drill is just a hundredth of a watt per square metre. Sunlight is 680 watts per square metre, so sunlight is a few hundred thousand times more powerful than the loudest sound.
Basically, all that sound just turns into a tiny, negligible amount of heat. This is little consolation when that dog barks all night long, but at least it isn鈥檛 contributing to global warming.
David Bortin
Whittier, California, US
If a tree falls in the forest and there is no one there to hear it, does it make a sound?
That old riddle pretends to the profundity of a puzzle for physicists or philosophers, but it is actually for semanticists to say whether a 鈥渟ound鈥� is only what is heard or all the pressure waves of molecules moving through the air. If a sound is only what is heard, then the answer to the question must acknowledge that some portion of the energy from the hearer鈥檚 vibrating eardrums is converted into the electrical energy of cognition.
If any pressure wave moving through the air counts as sound, those can also be converted into electrical energy, by employing electromagnetic induction or piezoelectric crystals. But in nature, most of this energy (which is already in the form of moving air molecules) becomes molecular vibration, aka heat.
And whence goes that heat in the cool stillness of the forest air? Short answer: every which way. Perhaps some thermal energy might promote the chemical bonding (a form of potential energy) needed to make another tree grow taller and more massive, thus making the sound that much louder when it, too, eventually falls. And we go round and round and round in the circle game.
Eric Kvaalen
Les Essarts le Roi, France
Sounds made indoors echo off the walls and probably mostly get absorbed by furniture and other 鈥渧iscoelastic鈥� materials, turning the energy into heat.
Sounds made outdoors mostly go into the atmosphere, either because the sound waves start off going somewhat upwards or because they reflect off the ground. Sound that goes downwind, up to a certain angle upwards, will bend (or refract) back down to the ground because of the gradient of wind speed, but sound going upwind will bend upwards, as will sound going downwind above a certain altitude, because of the fact that the speed of sound decreases with altitude. (Sound goes slower in colder air.) This sound won鈥檛 encounter anything solid.
But as they go higher, into more rarefied air, sound waves change. A sound wave consists of pressure variations and velocity variations. The points in the wave where the pressure is high are the points where the air is moving in the direction of propagation, and where the pressure is low, the air moves in the opposite direction.
The ratio between this pressure variation and the speed of the wave is equal to the ratio between the overall pressure and the speed of sound. As the waves go higher, this ratio goes down, because pressure decreases faster than temperature. (In fact, at some altitudes, temperature increases with height.) This means the speed of the air at the wave crests gets greater as the wave goes higher. It is like a wave of the sea coming to the shore 鈥� the water moves faster as it gets shallower.
Just as a wave coming to shore will eventually crash, the sound wave becomes a series of shock waves. Since the crests of the waves are moving forward, the speed of sound at the crests is higher than in the troughs, so the shape of the wave changes until it becomes sudden rises followed by gradual declines in both pressure and speed.
Each little shock wave comes upon the backward-moving air molecules in front of it and makes them start moving forward. It is similar to cars piling into a crash on a highway and suddenly changing velocity, while contributing energy to the pile-up. (Here, the shock is moving backwards up the highway.)
This means an increase in entropy and generation of heat. Through this process, a 400-hertz sound wave will become 400 tiny shock waves passing any given point per second, each one generating a small amount of heat.
Besides this effect, air has 鈥渂ulk viscosity鈥�, which means that even when there are no shock waves, some of the energy is constantly turning into heat.
Alan Dix
Aberaeron, Ceredigion, UK
As I am sure others will point out, the ultimate end of every word ever spoken is more hot air.
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