Yawning is one activity that is inevitable. Yawning sometimes in the public can be embarrassing, maybe finding yourself yawning during important meeting or Other related functions. Yawing not being contagious only but sometimes passes a message like feeling sleepy, being hungry, fatigue and so on. This is one thing we wished to control, but we just can’t do that most times.
Now why do we go deaf for a moment when we yawn?

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    Firstly, let’s see the functions of the ear and how it receives sound.

Basically, there are three parts of the ear analysis which are the outer ear or eardrum(tympanic membrane), the middle ear and the inner ear(cochlea). During the transmission of sound waves into the ear canal, the ear drum vibrates. And during this transmission process, there is a movement of the tiny chain of bones e.g ossicles, malleus, incus, stapes in the middle ear. These bones works together making things easy by transmitting the sound to the cochlea, from where it is transmitted to the brain for interpretation.

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Now why do we go deaf partially?

There’s what is called Eustachian tube which is related to the reason we go deaf for the period of yawning. Eustachian tube has two ends which is linked between the eardrum and the uppermost part of the pharynx, nasal cavity (Nasopharynx). Therefore linking the back throat and the middle ear. So whenever there’s pressure change in your ear, this tube opens to stabilize the pressure and in this process of opening, the genesis of the sound now split into two The ear canal and the Eustachian tube. Then the tensor tympani muscle and the the stapedius muscles which controls the amount of sound entering your ear, gets consolidated, therefore making the transmission of vibration from the eardrum to the inner ear difficult.
This is why that brief ‘partial deafness’ occurs when you yawn.

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Reference:
Eustachian tube- Wikipedia