Bass Trap Depth Calculator

Free tool to find the depth and air gap of a porous absorber for a target frequency. No sign-up

Enter the lowest frequency you want to absorb, for example the lowest axial mode of your room.
Set the air temperature and the thickness of your panel, in centimetres or inches, or load a common thickness.
Read the quarter and tenth wavelength, and the air gap to leave behind the panel.
Check the drawing and the table to see how the gap changes with frequency.
Download the results as CSV or copy them as text.

Use the lowest frequency you want the absorber to work on, for example the lowest axial mode of your room. The thickness is that of the porous material itself (mineral wool, glass wool). The air gap is the empty space between the wall and the back of the panel.

Nothing is saved or uploaded: everything stays in your browser. Download or copy your results before closing the page. This is a rule of thumb, not a prediction: how much a real absorber absorbs depends on the density and flow resistivity of the material (mineral wool of about 40 to 100 kg/m³ is typical), on how it is mounted and on the room. A panel across a corner has deeper air behind it, which helps at low frequencies. Use the result as a starting point and measure the room when you need accuracy.

How deep should a bass trap be to absorb low frequencies?

A porous absorber such as mineral wool works best where the air moves fastest, about a quarter of a wavelength from the wall. At 100 Hz the wavelength is 3.43 m, so a quarter is 86 cm: much deeper than a typical panel, which is why panels are usually spaced from the wall with an air gap, or placed across a corner. As a rule of thumb, a panel flat on the wall starts to absorb when its thickness is about a tenth of the wavelength (34 cm at 100 Hz) and gets close to full absorption at a quarter. This free calculator gives you the quarter and tenth wavelength for your target frequency, the air gap needed behind a panel of your thickness, and the frequency from which that panel works.

What is a porous bass trap, and why does its depth matter?

Porous materials like mineral wool absorb sound by slowing down the air as it moves through their fibres. Right at a wall the air barely moves, so a material glued to it works poorly; the air moves most at a distance of a quarter of the wavelength. Bass has very long wavelengths, from one to several metres, so a thin panel on the wall hardly touches it. There are two ways to improve this: make the panel thicker, or leave an air gap between the panel and the wall so the material sits where the air moves. This tool tells you how deep the whole absorber should be and how big the gap should be, as a starting point for your build.

Made by Luca Martone, sound designer and live sound engineer based in Turin.
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