What prime number should I use for a QRD diffuser?

N=7 is the most common choice, giving a good balance of diffusion quality and construction complexity. N=5 is simpler but less effective. N=11 or N=13 provide better diffusion but require more wells and deeper maximum depth. Higher primes give wider bandwidth diffusion.

What design frequency should I choose?

The design frequency determines the centre of the diffusion bandwidth. For rear wall studio diffusers, 1000-2000 Hz is common. The diffuser is effective from roughly half the design frequency up to c/(2w) where w is the well width. Lower design frequencies require deeper wells.

How wide should each well be?

Well width determines the upper frequency limit of diffusion. Narrower wells (30-40 mm) diffuse higher frequencies. The upper limit is approximately the speed of sound divided by twice the well width. Keep wells narrow enough that the highest frequency of interest is covered.

Can I build a QRD diffuser from wood?

Yes. QRD diffusers are commonly built from MDF or plywood dividers with a solid back panel. The wells must be rigid and non-absorptive — hard, smooth surfaces reflect sound into the diffusion pattern. Paint or varnish the interior surfaces. Ensure accurate depth cutting for each well.

AudioCalcs

Diffuser Calculator — QRD Well Depths

Calculate QRD diffuser well depths from prime number sequence for a target frequency range.

Prime Number (N)Higher primes give better diffusion but more wells

Design Frequency

Centre frequency for the diffuser design

Well Width

Internal width of each well opening

Panel WidthRepeating the sequence widens the diffuser panel

How We Calculate This

QRD Sequence

s(n) = n squared mod N (where N is the chosen prime number)

Well Depths

Depth(n) = s(n) x wavelength / (2 x N)

The wavelength is calculated from the design frequency and speed of sound. Each well depth creates a different path length, scattering reflected sound evenly across a wide angle.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.