The video Ear rumbling happens below the range of human hearing demonstrates that the "ear rumbling" sound is actual sound and can be recorded by a sensitive microphone near the ear.

What is it exactly that produces this vibration and how is it radiated as sound waves into the air?

If it is subsonic, then why does it sounds like noise rather than being inaudible?

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I'm able to create rumbling in my ears at will. Unlike the poster of the video, I don't need to yawn to do so; I can do it without my face appearing to move. I actually discovered this alongside another ability of mine when I was little: the ability to create warmth flowing from the base of my neck outwards into the rest of my body. After some research I figured out that the warmth I'm able to create is actually part of voluntary piloerection (its use for warmth is mentioned here), while the ability to create a rumbling sound is simply the contraction of the tensor tympani, a muscle in the ear. Voluntary control over either one of these things is supposed to be rare, but I've found claims of several other people online who can do both (example1, example2).

Anyways, Wikipedia gives a brief explanation of how the ear rumbling works, which also specifically mentions "yawning deeply" as a trigger:

Contracting muscles produce vibration and sound. Slow twitch fibers produce 10 to 30 contractions per second (equivalent to 10 to 30 Hz sound frequency). Fast twitch fibers produce 30 to 70 contractions per second (equivalent to 30 to 70 Hz sound frequency). The vibration can be witnessed and felt by highly tensing one's muscles, as when making a firm fist. The sound can be heard by pressing a highly tensed muscle against the ear, again a firm fist is a good example. The sound is usually described as a rumbling sound.

Some individuals can voluntarily produce this rumbling sound by contracting the tensor tympani muscle of the middle ear. The rumbling sound can also be heard when the neck or jaw muscles are highly tensed as when yawning deeply. This phenomenon has been known since (at least) 1884.

It also seems like this type of noise can happen with conditions such as Tonic Tensor Tympani Syndrome.

The tensor tympani connects directly to the handle of the malleus, so I'd assume that's why it can be heard by the person doing it. Apparently this also causes the eardrum to move. This means it's classified as objective tinnitus.

Here are some relevant studies I found:

  • $\begingroup$ Thank you very much for taking the time to write such a well-sourced answer. This is great! $\endgroup$ – uhoh Jan 10 '19 at 6:16
  • $\begingroup$ @uhoh I didn't find it publicly, which is why I was using my school access to read it. Using that link was a mistake; I have edited it to be a link to the abstract. $\endgroup$ – Laurel Jan 10 '19 at 6:20
  • $\begingroup$ That looks great, thank you! I'll stop by a library soon and give all of these a good read. $\endgroup$ – uhoh Jan 10 '19 at 6:22
  • $\begingroup$ half-way through discussion Lise et. al. says "Voluntary control of the tensor tympani muscle is an extremely rare event." I can make this loud rumbling sound in both ears any time I want just as you describe in your introduction, and yes after 10-20 seconds of continuously doing it it gets difficult just like a muscle is getting fatigued. $\endgroup$ – uhoh Jan 10 '19 at 6:38
  • $\begingroup$ I wonder if this is really as rare of an ability as this paper suggests, or if it's simply rarely-reported or rarely-published? $\endgroup$ – uhoh Jan 10 '19 at 6:38

The rumbling is caused by two middle ear muscles. These muscles are connected to ear drum and staples, if they are activated, they make it harder for incoming air soundwave to push the eardrum, they increase eardrum acoustic impedance so less sound gets to cochlea. This is triggered by loud sounds and it serves to prevent hearing damage, its called acoustic reflex.

All muscles when activated and under tension shiver slightly, this vibration is the rumbling you hear. The reason you hear this so loud is becose the muscles are directly connected to your middle ear bones so the vibration goes directly to cochlea. You may also hear it if you tense up your jaw muscles but it will be alot quieter becose its further away from cochlea. You can also try pressing clenched fist against ear to hear this muscle rumble.

The term subsonic is confusing and vague, its generaly considered to be sound below 20 Hz but the problem is that humans can hear down to about 5 Hz if the sound is 110db loud. Also, while its true this muscle rumble does contain sub 20 Hz sound, it extends above it too, roughly to 70Hz.

  • $\begingroup$ Thanks! The vibration is an artifact of some collective behavior of nerve and muscle. If the result is not a pure sine wave but instead was anharmonic and had other noise and random effects, would those end up above ~20Hz? $\endgroup$ – uhoh Jan 10 '19 at 2:40
  • $\begingroup$ I am sorry I dont understand what you mean. $\endgroup$ – wav scientist Jan 10 '19 at 2:46
  • $\begingroup$ I'm thinking that the audible part is not a ~12 Hz (or similar) sine wave, but the random variations or noise in the contractions. Muscles have a bit of mechanical noise when they contract, in addition to the periodic vibration. I'll see if I can find a better word for it. I'm proposing that it's not the very low frequency sine wave we hear, but the random, noisy component on top of it. $\endgroup$ – uhoh Jan 10 '19 at 2:53
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    $\begingroup$ Noise is just word for bunch of chaotic sinewaves. You are right, you mostly hear the 40 to 70Hz part, the 10 to 20 Hz part doesnt contribute much to audibility. $\endgroup$ – wav scientist Jan 10 '19 at 2:56
  • $\begingroup$ newscientist.com/article/… links to aip.scitation.org/doi/10.1063/1.2737358 but I don't have access right now $\endgroup$ – uhoh Jan 10 '19 at 2:59

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