FDN delay-line lengths are generally chosen to be mutually prime, which maximizes the psuedo-random behavior of the system.
A rough guide to the average delay-line length is the “mean free path” of the desired reverberant environment, which is defined as the average distance a ray of sound travels before it encounters a reflecting obstacle.
The mean free path can be approximated as
, where is the total volume and is the total surface area enclosing the space.
The desired modal density can guide the determination of the total sum of the delay line lengths . Schroeder suggests a modal density of 0.15 modes per Hz for a 1 second . This can be generalized to
.