Flutter Echo Calculator

The Flutter Echo detector identifies whether flutter echo can occur between parallel reflective surfaces in a room. Flutter echo is a rapid series of reflections perceived as a metallic "ringing" or "pinging" after a hand clap. The tool computes the repetition rate in hertz, the time interval between reflections in milliseconds, and the critical distance at which the effect becomes audible to the human ear. Why it matters: flutter echo is a common defect in rooms with smooth parallel walls — typical of untreated rooms, hallways, and stairwells. In a studio it colors recordings and creates a "boxy" or "metallic" sound character noticeable even on quiet passages. The problem is often confused with excessive reverberation, even though the phenomenon and the remedies are fundamentally different. How to read results: if the distance between parallel surfaces exceeds 3 m and the absorption coefficient of both surfaces is below 0.4, the risk of flutter echo is high. The calculator shows the repetition rate: values in the 20 – 100 repetitions per second range are most audible and objectionable. Common mistakes: treating only one of the parallel walls with thin 10 – 20 mm foam that is effective only above 2000 Hz; completely ignoring the floor-ceiling pair, which also produces flutter echo; trying to solve the issue with furniture alone when the surfaces remain largely smooth. Next steps: consider installing diffusers on one of the parallel surfaces, or use the Porous Absorber calculator to determine sufficient material thickness.