One of the earpieces is placed on the mastoid bone behind the ear.
From the sciencedaily.com
There are no openings in the mastoid bone.
From the en.wikipedia.org
I have been sick in bed with mastoiditis, a serious infection of the mastoid bone behind the ear that is connected to the skull.
From the thisisbristol.co.uk
The Aurex-3, developed by ADM Tronics in New Jersey, works by pressing a small vibrating probe onto the mastoid bone behind the ear.
From the newscientist.com
These are connected to a tiny electromagnetic vibrator which is positioned inside the mastoid bone behind the ear and attaches to the hearing bones.
From the sciencedaily.com
The mastoid portion of the human temporal bone, which can be felt as a bump in the skull behind the pinna, also contains air, which is ventilated through the middle ear.