The sound-isolating headphone and voice

Recently, a reader asked me, why she would be feeling a lack of movement in her temporomandibular joint when speaking while wearing headphones.
I was surprised to hear this and “yes, that is by many, very common!”
Generally, head phones are made for listening to music.
The headphone manufacturer is not mindful of the consumer’s speaking ability while using the product,
I investigated the condition of phonation by using several headphones, and applied several tests.
And found there were two major factors which could cause this effect and discomfort.
The first factor is, the headphone’s style or type, most commonly being a tight head band with pads or firm ear buds.
The second factor is the placement of the product on the ear. It should be located near the temporomandibular joint or cover it.
Those 2 things can obstruct the joint, when opening and closing the mouth.
In fact, I recorded the phonation results, both with head phones and without head phones. .
1. The zone of audibility dropped.
[The wave form of the power spectrum:Left…w/o   headphone Right…w headphone]
2.The articulation rate decreased.
3.Made a boxy sound.
4.Aperture rating and speed slowed down.
Which makes it difficult to use the voice properly and facilitate its full ability.
When recoding in a studio, you cannot move and take your headphone off because it will create a leaking sound from headphones, and possibly throw the artists vocals off, or distract the artist with background noises.
That’s why, sound-isolating headphones are a must!
Use these tips as guidelines before singing while wearing any headphones on your ears:
1:Use a soft ear pad.
2:Choose a headphone that fits your specific build
3:Choose an ear cup that will not obstruct the movement of your temporomandibular joint.
4:Stretch your jaw before wearing headphones.
5:Massage the cheek muscles, digastric muscles, and lateral pterygoid muscles.
This may be possible to loose your great voice, and please be careful for singer or voice actor too.

Please see the following figure displays a skull while wearing sound-isolating headphones. As you can see, the headphones clearly effect aperture and phonation.

[the orange circle is the ear cup and the red circle is the porous acoustic]