From my understanding energy and matter are interchangeable ($E=mc^2$). Also, motion requires energy, but energy does not require motion.

Now, for my question, as an example, I will use the chain of creating sound with vocal cords.

Does the energy begin with the lungs then they move and transfer energy through the air being sucked in and then blown to the vocal cords, then from the vibration of the vocal cords on the air and lastly from the air to our eardrums?

I have no background in physics, I’ve only done a tad bit of research today as I’m very interested in finding a breakdown of how this process works.

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    $\begingroup$ I have a rule that in no case does including the human body make the physics clearer. $\endgroup$
    – JEB
    Mar 30 at 23:33
  • $\begingroup$ Can you say on what level you see matter and energy as interchangeable, and how that matters here? $\endgroup$ Apr 1 at 18:02
  • $\begingroup$ When you did that 'tad bit of research' what Answers did you find and what did they leave unclear about how this process works? If you use the 'chain of creating sound with vocal cords' what could matter before the simple vibration of those cords? Do you think energy beginning with the lungs then they move and transfer energy through the air being sucked in means something useful? If you must go that route, isn't the truth opposite? The lungs transfer energy through the air being pushed out? $\endgroup$ Apr 1 at 18:19

4 Answers 4


I have a rule that in no case does including the human body make the physics clearer. – JEB

First off, I would like to second JEB's wise comment. The way the human body works is astonishingly complicated and full of paradoxes which get in the way of understanding. It's a beautiful machine, really, but absolutely abysmal for helping with understanding basic physics.

Second off, I recommend ignoring $E=mc^2$ for a bit. While it is a true relationship, the interplay of mass and energy in relativity and the stress-energy tensor really only adds to confusion until you're about six years into physics and have the mathematics background to back it up. Instead, I encourage thinking two rules:

  1. Energy cannot be created nor destroyed
  2. Mass cannot be created nor destroyed

As $E=mc^2$ shows, those two statements are false, and you'll eventually have to reach beyond them. But it was good enough for several hundred years, and its certainly good enough of an approximation for this particular topic. Once you're interested in atomic decay or objects traveling faster than 1,000,000m/s, revisit those assumptions.

I'd like to start a little earlier than perhaps necessary. I'd like to start with the inhale. We could really start with the exhale, but I want to demonstrate the lack of potential energy in the air, and it's useful to start a little bit early.

Oh boy... organic chemistry. Our cells have a molecule called adenosine triphosphate. That's a mouthful, so it's usually just called ATP. This is a molecule that has a great deal of chemical potential energy built up in its bonds. Living creatures rely on it to do work. In a chemical reaction with water, ATP can transform into adenosine diphosphate, or ADP. This transformation releases some of that potential energy. When the reaction occurs in the right places in our diaphragm muscles, the diaphragm contracts. This does work (transfers energy) into the air inside the lungs. The diaphragm pulls down, reducing the pressure inside the lungs. This causes air to rush inward, providing kinetic energy to the air molecules going in. The diaphragm also does work stretching the tissues of the lungs, which are extremely elastic. This means potential energy is transferred into the elastic tissue, stretching it.

When the breath is finished, the diaphragm stops doing work. Any motion of the air that was into the lungs is quickly dissipated into random molecular motion: heat. There is still potential energy pent up in the elastic tissue of the lungs, those will be important soon. But at this point, the air has no special energy. It has some thermal energy, as the atoms bounce around, but that won't play a big part in the process of making noise, so we'll ignore it.

Now we're to the big moment. The muscles of the diaphragm stop contracting. The muscles have been spending some ATP just keeping the diaphragm where it is (the energy of that conversion eventually becomes heat). But now those muscles relax. The forces on the lungs go down, and the elastic tissue begins to release its potential energy. It releases it into the air in the lungs. It increases its pressure, which then causes the air to move out of the lungs towards the vocal box.

In the vocal box, the vocal chords flap in this wind. They turn a steady flow of air into an oscillating stream. Some of the energy turns into random motion here (heat), but a lot of it is now oscillating motion. This is less of transfer of energy and more of a change of the behavior of that energy. It becomes easier to think of that motion as sound. You could indeed track it as just air movement, but it turns out to be really hard to do so. Lots of calculations for very little gain.

That energy propagates through the air, causing motion in different parts of the air. Eventually it reaches the eardrum, where that oscillating energy transfers into the eardrum. This motion then pushes on the bones in the ear, which eventually transfer that energy into the cochlea. In there a tiny fraction of the energy gets transferred to the hairs inside of it. These are sensory organs. They magnify the energy. They do so by spending ATP of their own to put the structures in the cells around the hair into a highly energetic state, like a ball balancing at the top of a hill. The energy from the sound wave disturbs this balance, causing the cells to transfer a great deal of that energy into electrical energy which flows down a neuron. Eventually this becomes sound that you hear. The way that happens is not really all that clearly understood, so I can't go any further than that.

To close, I'd like to return to JEB's wise comment:

I have a rule that in no case does including the human body make the physics clearer. – JEB

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    $\begingroup$ ty for the props. A classic example is the twirling skater--to teach us about angular momentum, conserving $mr^2\omega$, but I was always, ..wait..what about energy, $mr^2\omega^2$...can't conserve both. So give her dumbbells and she needs to 'lift' them against centrifugal force...but at the point..keep the weights, lose the skater and add some springs (formerly arms) or a counter weight and get quantitative. $\endgroup$
    – JEB
    Mar 31 at 12:01
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    $\begingroup$ As a med student, I would like to add : There are muscles in between the ribs called intercostals. When they contract, they lift the ribs and sternum, thereby increasing the volume of the thoracic chamber in the dorso-ventral axis. Contraction of the diaphragm increases the volume in the antero-posterior axis. There are also additional set of muscles in the abdomen that help to increase the strength of inspiration/expiration when necessary. The human body is fascinating. $\endgroup$ Apr 1 at 14:18

I'll try to answer, although, depending on the level one wants to approach this question, there are many levels of complexity.

Forget for one moment the energy-matter equivalence (since we are dealing with non relativistic systems, that equivalence is not really accessible) and think of energy as motion (kinetic energy) or as something that can potentially cause motion (potential energy). More or less everything falls in these two categories. For example, thermal energy is actually due to the motion of particles.

This said, you actually got it already: the lungs expand, taking air in. This happens because the diaphragm contracts, which means it stores some potential energy. When the diaphragm relaxes, it releases its potential energy pushing the lungs that, in turn, push the air out. Thus, as usual, potential energy (from the diaphragm) transforms into kinetic energy (of the air). This air hits against the vocal chords, which start moving (from kinetic energy of a system to kinetic energy of another system). If the tension of the vocal chords is the correct one, they vibrate in such a way to produce pressure oscillations in the air (again, from kinetic energy to kinetic energy). These oscillations are the sound that we hear.

Hopefully, this makes the problem clear.


How does energy become sound?

I have a rule that in no case does including the human body make the physics clearer. – JEB

So, let's look at a loudspeaker...

The way a loudspeaker works is by using the kinetic energy of the speaker diaphragm to compress/expand the air at atomospheric pressure near the speaker diaphragm creating pressure waves.

The mass-energy equivalence does not apply since none of the mass of the loudspeaker is converted to energy: the weight of the speaker doesn't change.

  • $\begingroup$ Technically, most of the "kinetic energy of the speaker diaphragm" is actually the "kinetic energy of the speaker system" which is mostly "the kinetic energy of the air mass". The Electro Motive Force on the Coil acts on the Air through the Diaphragm without ever imparting much KE to the low-mass diaphragm. $\endgroup$
    – david
    Mar 31 at 23:33

The energy taken to produce sound in your example is usually mechanical energy which then gets dissipated throught the medium of travel. When you exhale you rush air out of your body by creating a pressure difference between the inside and outside of your lungs, as you tense up your vocal chords some of the molecules in the air hit them, causing them to vibrate.

If the energy produced by the change in volume of your lungs is $W=p\Delta V$ then a portion of it is absorbed by your vocal chords, call it $E_{abs}=\alpha W$ and a portion of gas did not interact with your vocal chords, call it $E_{breath}= (1-\alpha) W$ so then the total energy is conserved. When you speak the only portion of this net energy that is eligible to be transformed into sound is $E_{abs}=\alpha W$, but not all of it is, as some may be absorbed by the tissue and air, increasing their temperatures (lost as random motion of molecules, not coordinated sound waves)so then if we take the heat generated by this interaction to be $Q$, the energy translated into actual sound is $E_{sound}= E_{abs}-Q=\alpha W-Q= \alpha p\Delta V - Q$. Meaning that only a part of the energy that gets absorbed by your vocal chords passes through to the air as work, maintining the same vibration frequency as that of the vocal chords.
Hope this helps!


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