9. How do the appearance and structure of the FD3/FD3 Pro improve its sound performance (the small hole on the front cavity, semi-open acoustic design, front acoustic prism)?

[The advantage of the small hole in the front cavity]
1. The small hole in the front cavity is used to relieve air pressure. It can help to moderate pressures that happen to the ear canal when putting the FD3 inside the ears. If the front hole is blocked, it would enhance the bass performance. However, hearing fatigue would also be caused easily due to increasing pressures in the ear canal.
2. The diaphragm of the FD3/FD3 Pro is made of Diamond-Like Carbon (DLC). Therefore, if the front pressure-relief hole is blocked, it will cause unneeded distortion due to slight  transformation of the diaphragm when it is vibrating.

[Semi-open acoustic design]
The semi-open acoustic design of the FD3/FD3 Pro based on FiiO's patented technology for balanced air pressure within brings several advantages. Not only does such a design lead to more natural sound, but it also effectively protects your hearing by balancing air pressure within the cavity, leading to less pressure on your eardrums. This makes your listening experience less tiring and the sound less forced.

[Front acoustic prism]
The acoustic prism so effectively used in the flagship FD5 for great sound quality is also featured in the FD3. Like with the FD5, the acoustic prism system brings numerous benefits such as eliminating high frequency standing waves, enhancing overall sound wave diffusion, and even effectively solving the problem of time delay caused by sound wave propagation path in the cavity. This all leads to marvelous sound quality that will surely take your breath away.