What is a QRD diffuser?

A Quadratic Residue Diffuser (QRD) uses wells of varying depths based on a quadratic residue sequence to scatter sound evenly across a wide angle. Designed by Manfred Schroeder, QRD diffusers are the most common type used in studios and concert halls.

What prime number should I use?

Common choices are N=7, N=11, N=13 or N=17. Higher prime numbers give more wells and wider bandwidth but require more construction precision. N=7 is a good starting point and produces a compact, effective diffuser.

Where should diffusers be placed?

Diffusers work best on the rear wall of a studio control room (behind the listening position), on the ceiling between the monitors and listener, and on side walls where absorption alone would make the room too dead. They preserve acoustic energy while reducing flutter echoes.

AudioCalcs

Diffusion Calculator — QRD & PRD Diffuser Design

Calculate diffuser well dimensions for target frequencies using QRD and PRD designs.

Design Frequency

Lower limit of diffusion range

Prime Number (N)

Use 5, 7, 11, 13 or 17

Diffuser Type

How We Calculate This

The QRD well depth sequence is calculated using:

depth_n = (n² mod N) × v / (2 × N × f)

Where n is the well number (0 to N-1), N is a prime number, v is 343 m/s, and f is the design frequency.

Well width = v / (2 × N × f)

The effective bandwidth runs from the design frequency up to N times the design frequency.

Frequently Asked Questions

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Last updated: February 2026

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