Lenovo X3 Wireless Bluetooth Earphone Air Conduction Sport Headset IPX5 Waterproof Neckband with Mic Noise Cancelling Earbuds

Why can see a Earpiece hole on the earphone?

Air conduction earphone also have a converter similar to the dynamic Moving coil structure, so air conduction headphones can transmit sound to the human ear through air conduction, and can also open a hole to transmit the sound of the speaker as a supplement to overcome air conduction Bandwidth and sensitivity are inherently insufficient, Lenovo X3 headset adopts this design, so you can see a Earpiece hole.
Thank you for your understanding.