Loose belts cause layer shifts, ghosting, and dimensional accuracy problems. Over-tight belts wear out bearings and motors. Getting the tension right is a 5-minute job once a quarter, with the biggest single impact on print quality of any maintenance task.
Pluck the belt like a guitar string. A properly-tensioned 6 mm GT2 belt makes a clear note between 85 and 110 Hz on most consumer printers. Use a free phone tuner app (Spirit Level Belt Tension, GuitarTuna) to measure. After any tension change, re-run input shaping calibration (Bambu auto / Prusa Calibration menu / Klipper SHAPER_CALIBRATE ).
The fastest tension check. Works on every printer with exposed belts.
Your ear can't reliably tell 90 from 110 Hz. Use a tuner app:
Tensioning the belt does no good if the motor pulley is slipping on the shaft.