FDN-reverb Implementation and Delay Line Configuration : How do the Delay Lines Affect the Sound Output?

Abstract: The FDN reverb is based on a number of delay lines that each sample of the audio stream divides onto. The samples travel through a matrix of gain coefficients and once again divide onto all the delay lines. The results pops out to the output after every pass through the line and will therefore decay at a given rate depending on how the delay lines and matrix are configured.
This report means to examine how the number of delay lines affects the output in terms of reverb sound quality. To accomplish this, a program has been written for creation of the reverbs. This program will use six and twelve delay lines respectively. The actual reverb method was first written as a recursive function though this turned out to be highly ineffective due to the huge amount of function calls being made. The author revised the method as an iterative function with a vector buffer. But as it turns out this produces just as many function calls to the vector class. Finally the vector buffer was replaced with a circular array buffer with pointers keeping track of the read and write indexes. This turned out to be a lot faster.
The test files, in form of acoustically dry recordings of a flute and a guitar, was loaded and run through the reverb functions to produce a reverb. As the program’s delay lines have been written with a large number of different settings, it took some time to determine a good setup. It came down to an initial time delay gap of about 30ms and a separation of the delay lines of about 14ms (varies between lines). The delay times were slightly varied to prevent the lines becoming multiples of each other.
And so the test reverbs was created and a survey handed out to six participants of different background stretching from home user to audio educated experienced listeners. The participants got to listen to the original sound, the six line sound and the twelve line sound.
The outcome was surprisingly good. None of the participants found the reverb unrealistic or thin. Instead they found it quite good and realistic. The number of delay line didn’t seem to affect the sound output quality.
Ordinarily a greater amount than six lines is used. Either that’s not necessary with general FDN reverbs or this method makes it unnecessary. There might also be other reasons as to why one would choose a greater number of delay lines. This might be a good reason to try this experiment with even more delay lines, but according to the investigation at hand, less delay lines will not equal less quality.