Bafang BBSHD is about 68 dB and BBS02 about 64 dB, while Bosch Performance CX is about 56 dB thanks to helical gears and tight tolerances.
A mid-drive motor sits close to the feet of the rider, so its noise is more noticeable than that of a hub motor. Gear whine is a frequent complaint, and acoustic performance differs clearly between brands.
Measured data: under typical load the Bafang BBSHD is about 68 dB, the BBS02 about 64 dB, and the Bosch Performance CX about 56 dB. A gap near 12 dB is, in subjective terms, roughly the line between "noisy" and "quiet".
The gap comes from gear design and manufacturing. Bosch uses helical gears with tightly controlled tolerances, giving smoother meshing and less impact. Some low-cost kits use straight-cut gears with loose tolerances, which tend to whine at high speed.
QERYU engineering focuses on "quiet". We choose helical transmission with high-precision gear pairs, add a damped housing and damping materials, and balance the assembly dynamically. This keeps operating noise close to the high-end benchmark and improves comfort reputation in commuting and fleet use.