Flutter Echo Distance Calculator
A flutter echo is the rapid train of reflections you hear between two parallel hard walls. The reflections repeat at a rate set by how far apart the walls are and how fast sound travels. This calculator takes the wall spacing and the speed of sound and returns the round-trip distance, the time delay of each repetition, and the repetition rate in hertz, which is the pitch you perceive in the flutter. All values are user-editable so you can match your room dimensions and air conditions.
Flutter echo formula
Round-trip distance = 2 * d
Time per repetition = (2 * d) / c
Repetition rate = c / (2 * d)
Here d is the spacing between the two parallel walls and c is the speed of sound. The repetition rate in hertz is the inverse of the round-trip time and corresponds to the buzzing pitch of the flutter.
Worked example
For walls 5 m apart in air at 343 m/s: round-trip distance = 2 * 5 = 10 m. Time per repetition = 10 / 343 = 0.02915 s = 29.15 ms. Repetition rate = 343 / 10 = 34.30 Hz.
Flutter echo: frequently asked questions
What is a flutter echo?
A flutter echo is a rapid series of repeating reflections that occurs when sound bounces back and forth between two parallel, hard, untreated surfaces such as facing walls. The repetitions arrive so close together that the ear hears a buzzing, ringing or zipping quality rather than distinct echoes.
How do I work out the flutter echo repetition rate?
Sound travels from one wall to the other and back, a round trip of twice the wall spacing. The repetition rate in hertz equals the speed of sound divided by twice the distance: f = c / (2d). For walls 5 metres apart in air at about 343 m/s, the rate is roughly 34.3 reflections per second.
Why does the speed of sound change?
The speed of sound in air rises with temperature. A common reference value is 343 m/s at 20 degrees Celsius. This calculator keeps the speed of sound as a user-editable input so you can enter the value for your conditions or medium.
How do I stop a flutter echo?
Break up one of the parallel surfaces with absorption (acoustic panels), diffusion, or by angling a surface a few degrees so the reflections no longer return along the same path. This calculator only describes the timing of the flutter, not the treatment.
Official sources
- U.S. National Institute of Standards and Technology: Physical Measurement Laboratory (speed of sound and acoustic quantities).
- The repetition relationship f = c / (2d) follows directly from the definition of wave speed (distance divided by time).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.