# How is heat different from sound?

They’re both vibrations in solids? Are the frequencies vastly different?

Solids, liquids, and gases all have heat and can propagate sound. I am not sure about the sound qualities of plasma. My understanding is that heat is a representation of the energy in any arbitrary material object with hotter objects having more energy and sound is caused by variations in pressure. But I am not a physicist so you’ll have to wait for someone who knows science to answer definitively.

Sound is a wave of pressure differentials - the medium is alternatively compressed and rarified. It’s analogous to wave in a once with high density areas correspond to the crest of an ocean wave. And the medium can be in any state, including plasma.

Heat, technically, is the energy transferred between two substances at two different temperatures. I think you’re thinking more of temperature, which as related to the average energy of the particles in a substance. The energy is of a particle is stored mostly as the motion of the particle in space - thus the hotter a substance is, the faster it’s particles are moving.

Sound is a wave and has an actual frequency. Temperature is the average of random movement and doesn’t have a frequency as such.

Excellent question excellent!

“Heat” inside of solids and liquids is sound of extreme frequency and short wavelength. But heat is also longwave infrared light.

Objects are opaque, so the light waves (photons) keep being emitted and absorbed, wandering very slowly through the material. The same thing happens to the sound waves. Rather than acting like a wave in a medium, instead one vibrating atom causes its neighbors to progressively start vibrating too. The vibration “infects” the material rather than propagating as a wave.

Or in other words, a heated atom can heats its neighbors either by exchanging a photon, or exchanging a phonon.

In an old physics encyclopedia I saw the frequencies of heat-sound defined as “hypersonic” sound.

The above is largely true, but the real difference is the random nature of vibrations of the molecules or atoms in heat. At any temperature above absolute zero, they are bouncing around every which way. Add sound, and a mass of them move back and forth in waves. At any given time, any given molecule may have both a sound movement and a heat movement. It is like you moving around on the earth while it spins and orbits the sun.

So would it fair to analogize that sound is the frequency of waves in an ocean (which can occur and propagate with little movement of the individual molecules) and heat (or more precisely temperature) is the aggregate effect of the chaotic movement of the individual water molecules?

I think so. You might even be able to call the ocean waves very low frequency sound.

Not really. Heat is any form of energy that’s not well organized. You can have vibrations of molecules that aren’t well-organized, and that can be called heat, but you can also have organized vibrations of molecules, and that’s sound or other kinds of mechanical waves.

Likewise, you can have disorganized infrared light, but you can also have disorganized light of other wavelengths (in fact, it’ll always be a mixture of wavelengths), and that could be called heat, but you can also have organized light waves of any wavelength, which isn’t heat. The only reason people associate heat with infrared specifically is because that’s the kind of light mostly produced by things at temperatures we’re familiar with, like an animal body or a campfire. But the Sun mostly produces heat-light in the visible light range, and a welding torch produces it mostly in the ultraviolet.

Ocean waves, incidentally, aren’t sound waves at all, for two reasons. First, the restoring force for ocean waves is gravity, not the properties of the water itself, and second, ocean waves are transverse, not longitudinal.

By the same token, waves on a guitar string are not sound waves.

I guess you’re right, it’s more temperature than heat. Actually temperature is what you measure, I think the best term is “thermal energy”.

Just like the waves and planet analogy, I think sound is more coherent vibrations - there is synchronisation on a macro scale, while thermal vibrations are different for each atom. Sort of like why isn’t moving air hot? The air molecules have kinetic energy, just like high temperature air molecules. The interesting thing is, why doesn’t heat travel as fast as sound?

What I really want to know from this question is - is there any chance of making an anti-heat? Is thermal energy a wave? If noise, which is random vibrations, can be canceled out, why can’t heat be canceled out too? Is it because heat is coherent on an atomistic scale? In that case, if you make very very “fine” noise, can it be unable to be canceled out too?

While ocean waves was used as an analogy, I give … please explain what you are talking about when you refer to sound as longitudinal vs transverse waves and why the vibrations of a guitar string are not sound (avoiding the whole issue of sound actually being the the quale, the what we hear, and not the frequency of vibration itself … which can get as done as airplanes on treadmills).

As to the questions immediately above - noise is not random vibrations - it is a mix of a wide variety of distinct frequencies. “White noise” is a mix across the audible spectrum, hence the analogy to white light. Noise canceling headphones work by quickly analyzing that exact mix moment by moment and sending out the exact opposite set of waveforms to cancel those frequencies out. Your heat cancellation idea doesn’t work precisely because heat is not coherent.

a cross-section of transverse waves kinda look like this:

``````
Longitudinal waves are pressure waves, and would looks like this:

)).)..)...)..).)).)..)...)..).)).)..)...)..).))
Another way to visualize it is to move your hand in a swooping rollercoaster motion of ups and downs forming ocean-like waves. That's transverse. Now, bend your hand at the wrist, so it's perpendicular to the floor, and push it back and forth really fast... that's how sound waves work (and I believe light as well).

Next time you see an exposed subwoofer (or any kind of speaker), watch closely. The cone of the speaker moves in and out of the housing (its frequency), making longitudinal waves in the air.

The guitar string, when plucked makes a transverse wave, but I'm assuming its energy is then converted to sound throughout the body of the guitar and finally out the center hole (or rosette).``````

At first this really confused me because when discussing sound and then ocean waves I immediately pictured the waves within the medium - inside the ocean, not the waves on top of the ocean.

This might be causing others confusion also, not sure.

Sound is louder and heat is hotter.

You can demonstrate transverse and longitudinal waves using a Slinky. Stretch one out, and wave one end back and forth: That’s a transverse wave. The individual bits of the Slinky are moving (mostly) at a right angle to the direction the wave is moving. Now take the same stretched Slinky and bunch up several coils at one end, and then let the bunch go: That’s a longitudinal wave. The individual bits of the Slinky are moving in the same (or opposite) direction as the motion of the wave. Sound is a longitudinal wave, while light is a transverse wave.

I am afraid that I still don’t get it.

Oh I get that Slinky example - in one there is a wave that is the amplitude above and below an axis which would also correlate to a coherent frequency variation of force in a positive or negative direction, and in the other the wave is the change in density of the loops … but so? How is one analogous to sound and the other not?

For the ocean waves there is a variation of amplitude, which correlates with force and energy and pressure, that varies with a distinct frequency as the waves reach any specific point. The change in amplitude is a transverse wave, but the various other correlates are longitudinal.

The guitar string vibration is certainly a wave that is analogous to sound. Even if the string was unattached to the guitar but was somehow attached to my temporal bone, I would, through bone conduction, hear it as sound. True, sound waves traveling through air are longitudinal (as they are very analogous to the Slinky, the frequency is in changes in pressure), but that does not mean that such is true for the nature of sound in all media.

If it’s not longitudinal, it’s not sound, but some other kind of mechanical wave. Vibrations of one kind can lead to vibrations of another kind, though, so a guitar string does (obviously) produce sound.

Some would disagree.

I also found this interesting tidbit.

Agreed however that ocean waves and not sound waves.

You have a basic conceptual problem here, and that is one of thinking that heat is a “thing” that is physically identifiable, discrete, and (presumably) conserved. Heat, in fact, is the property of the state of an overall system, and can only be defined within a particular boundary as a statistical measure of randomized kinetic (thermal) energy. Individual atoms, or even small (countable) collections of atoms do not have definable heat; heat can only be defined for a statistical population of atoms. As an example, if you have a handful of atoms that have a lot of kinetic energy, but most of that energy is in the form of a definable momentum going toward a particular target, then they have low heat. The same amount of kinetic energy with random or unmeasurable momentum will have high heat. The amount of heat energy is relative to a ground state–hence, why we have scales of absolute temperature (Rankine or Kelvin)–but the heat is only a measurable property of the overall system, and the ability to transfer heat energy from one system to another is determined strictly by the difference in temperature and properties of the media (absorptivity, emissivity, conductivity, et cetera).

The point can be made that sound, which depends upon a media for conduction, is also a state property, but it is a conceptually discrete property with direction and definition (frequency, amplitude) within that system. Because of this, sound has “real” properties in and of themselves, while heat is just a measured amount of energy within a box (boundary) that can’t be allocated to momentum or some other form of directional energy. The distinction may appear subtle, but it is significant in understanding thermodynamics.

In what possible way do you believe that this contributed anything to the discussion at hand?

Stranger