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When I play a sine wave together with another pitch shifted sine wave I hear the beating very clearly. This is the expected physical phenomenon.

When I use headphones to play the lower frequency sine wave in my left ear and the slightly higher frequency sine wave in my right (or vice versa), there should be no way for them to physically interfere with each other? But I still hear the beatings, just less strongly.

I tested it with a few different people and how strongly one perceives the beatings varies between people, but many do still hear it. Why?

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Short answer
Dependent on the auditory stimuli used, it sounds like binaural beats to me.

Background
Binaural beats are the resultant perception evoked by the interaction of two auditory tones with different frequencies when presented to both ears (Lane et al., 1998). The presented tones should be below 1000 to 1500 Hz and differ in frequency between 1 and 30 Hz. For example, if a pure tone of 400 Hz is presented to one ear and a 410 Hz tone in the other, an amplitude modulated standing wave with a frequency equal to the frequency difference (10 Hz in this case) is perceived. Note that these 1-30 Hz beat frequencies are below the dynamic range of acoustic hearing. Hence, the beatings are the result of neurophysiological interactions in the brain, and not the result of physical interactions of the auditory stimuli impinging on the inner ear. From an experimental point of view, limiting the sound levels to 60 to 70 dB should prevent gross cross-over stimulation to the other ear.

Binaural beats are thought to be elicited in the auditory brainstem, and specifically in the the superior olivary nuclei, when the two wave forms mesh in and out of phase within the brainstem.

Reference
- Lane et al., Physiol Behav (1998); 63(2): 249–52

Source
- Web US

Related post
- How can binaural beats change mental state?

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  • $\begingroup$ Thanks for your answer. However when playing a pure tone of 400Hz and a pure tone of 410Hz I hear the beating (meaning periodic varation in volume). I do not hear the frequency that is equal to the frequency difference. $\endgroup$ – macco Jan 21 '17 at 20:25
  • $\begingroup$ No. I perceive periodic varations in volume (aka beating) as you would perceive if you play the two pure tones on one channel, due to physical interference of the waves. $\endgroup$ – macco Jan 21 '17 at 20:31
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    $\begingroup$ Interesting. Although I would not rule out bone conduction, because bone conduction sounds very reasonable too. I will have to test with studio headphones and the little earplug headphones (since the little ones would have far less bone conduction hopefully?). I will also test with frequencies above 1000 Hz. (For example with 2000Hz and 2010Hz, since that would rule out binaural beats) From the papers presented it seems you can only perceive binaural beats with auditory stimuluses below 1000 Hz. $\endgroup$ – macco Jan 21 '17 at 20:57
  • $\begingroup$ The ear phones versus ear buds shouldn't make any difference. Make sure to limit the sound level to say 60 to 70 dB to prevent cross over between the ears. Let me know how you go. Try 8000 - 8020 Hz too, as an extreme example (exceeding the carrier frequency by a few octaves). And try 1000 and 1100 too (to exceed the difference limit). I'd love to hear more on your experiments. $\endgroup$ – AliceD Jan 21 '17 at 22:26
  • $\begingroup$ I used studio headphones at pretty low volume. I didn't find any absolute threshold frequency where I don't hear the beating anymore. It was progressively (though not totally uniform) harder to hear at higher frequencies. At about 150-1500Hz it was pretty easy to hear. Then at 2500Hz I could hear it too though weaker, and very very weakly (very hard to tell) at 5000Hz and above that I can't hear it. When you increase the difference between the frequencies you hear a faster beating (as expected) until you hear two separate tones, as you would if you played it on only one channel. $\endgroup$ – macco Jan 21 '17 at 23:21
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Actually there is a way for waves to interact with each other. Our bones conduct vibrations very well: bone conduction. Some of hearing aids are based on this effect.

I can think about another example: the Weber test. It is a quick test for hearing.

In the Weber test a vibrating tuning fork (Typically 256 Hz ...) is placed in the middle of the forehead, above the upper lip under the nose over the teeth, or on top of the head equi-distant from the patient's ears on top of thin skin in contact with the bone. The patient is asked to report in which ear the sound is heard louder. A normal weber test has a patient reporting the sound heard equally in both sides.

So the explanation could be the bone conduction of vibrations.

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  • $\begingroup$ Yes, I think that is what must happen. If someone knows could you please specify where exactly it cancels out physically (I think it is somewhere in the ear before it reaches the brain) and how exactly the signals get there. Maybe I will have to ask in Physics stackexchange... $\endgroup$ – macco Jan 15 '17 at 13:40
  • $\begingroup$ What do you mean by "cancels out physically"? $\endgroup$ – Istrel Jan 15 '17 at 13:50
  • $\begingroup$ Sorry, cancels out is an inaccurate word. The two soud waves have to interfere with each other which causes the beating. (See wiki page I linked in question if you dont understand) $\endgroup$ – macco Jan 15 '17 at 13:54
  • $\begingroup$ Bones vibrations affect middle ear (mostly). In normal situation sound waves (in air) conduct into osculation of middle ear bones (incus, malleus, stapes) through the ear drum. In case of bones conduction waves conduct to middle ear directly (without ear drum vibrations). Therefore, i think, waves interference occurs in middle ear. $\endgroup$ – Istrel Jan 15 '17 at 14:04
  • $\begingroup$ Bone conduction might well be the answer, but I'm quite interested in whether something neurological might also play a role. Perhaps there is interference within the neural pathway combining left and right sound input? $\endgroup$ – MBever Jan 18 '17 at 21:39

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