Sennheiser CX 400BT True Wireless review: Good sound at a better price | Android Central
Sennheiser CX400 II Stereo Earphones 3.5mm Wired Deep Bass Headset Sport Earbuds Precision HIFI Headphone for iPhone Android|Earphones| - AliExpress
User manual Sennheiser CX 400 (English - 3 pages)
Stereo Magazine: High Performance for 200 Euros
Amazon.com: Sennheiser CX400-II-B In-Ear Headphone with Dynamic Speaker System and Integrated Vol Controls (Discontinued by Manufacturer) : Electronics