Bass Trap Thickness Calculator
Porous absorbers such as mineral wool soak up sound by turning air motion into heat, so they work best where air particle velocity peaks, which is a quarter wavelength from a rigid wall. This calculator finds that quarter-wavelength depth for a target frequency, the full wavelength, and a practical half-quarter depth you can reach with an air gap. Enter the lowest frequency you want to control and the speed of sound, which varies with temperature and is therefore editable.
Quarter-wavelength formula
wavelength = speed of sound / frequency
quarter-wavelength depth = wavelength / 4
depth (cm) = quarter depth * 100
useful minimum = wavelength / 8
Air particle velocity peaks a quarter wavelength from a hard wall, where a porous absorber dissipates the most energy. Reaching that depth, with material or an air gap, gives efficient absorption at the target frequency.
Bass trapping facts
- Lower target frequencies need much greater depth because wavelength grows.
- An air gap behind a thinner panel raises low-frequency absorption.
- Room corners hold the highest pressure, making them effective trap locations.
- An eighth-wavelength depth still gives partial absorption when space is tight.
- Required depth scales directly with the speed of sound you enter.
Bass trap depth: frequently asked questions
How thick does a bass trap need to be?
A porous absorber works best when its thickness reaches about a quarter of the wavelength of the target frequency, where particle velocity is highest. For a target of 100 Hz at 343 m/s, a quarter wavelength is 343 / (4 times 100) = 0.858 metres of effective depth.
What is the quarter-wavelength rule?
Porous absorbers convert air motion into heat, so they absorb most where air particle velocity is greatest. That maximum sits a quarter wavelength from a rigid wall, so an absorber roughly a quarter wavelength thick (or with an equivalent air gap) is efficient at the target frequency.
Can I use an air gap instead of solid material?
Yes. Mounting a thinner panel with an air gap behind it shifts the effective depth toward the velocity maximum and improves low-frequency absorption without filling the whole quarter wavelength with material. The total cavity depth is what counts.
What speed of sound should I enter?
About 343 metres per second at 20 degrees Celsius. It is a user input because it varies with air temperature. The wavelength, and therefore the required depth, scales directly with the speed you enter.
Why are bass traps so much bigger than treble absorbers?
Wavelength is inversely proportional to frequency, so low notes have very long wavelengths. A 100 Hz wave is about 3.4 metres long, making its quarter wavelength roughly 0.86 metres, far thicker than the few centimetres needed for high frequencies.
Official sources
- National Institute of Standards and Technology: SI units and length.
- Acoustical Society of America: sound absorption references.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.