If you don’t have any visible or detectable problems at the otological hospital when you have a boxy voice, you might be LDP because
Exhalation decreases in depth due to the larynx being positioned deep. The air exhaled also decreases in velocity when coming out of the body, and looses resonance in the nasal cavity. People have five resonance chambers.
Note1:If you treat LDP, you will get a clear and rich voice.
Note2:The nasal cavity plays a very important role in vocalization, and the space is really small.