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I found bone conduction headphone, which is an interesting technique to listen to the music.
Usually, we hear sounds from our ears.
However, when we hear the recorded voice of ourselves, we have a feeling of wrongness.
The reason is that we hear the voice of ourselves directly and indirectly at the same time.
In the indirectly way, the voice wave propagates through the air and reach at the tympanum (Komaku), and in the directly way, the voice wave propagates through the bone.
　
The headphone uses this bone conduction, and is expected to be a tool for listening without leakage sounds.
And another interesting point is that the headphone can hear ambient sound because the headphone is not equipped in ears.

But sound quality of the headphone is developing.