As mentioned in §3.4, an ``ideal'' late reverberation
impulse response should resemble exponentially decaying noise
[317]. It is therefore useful when designing a
reverberator to start with an infinite reverberation time (the
``lossless case'') and work on making the reverberator a good ``noise
generator''. Such a starting point is ofen referred to as a
*lossless prototype*
[154,434]. Once
smooth noise is heard in the impulse response of the lossless
prototype, one can then work on obtaining the desired reverberation
time in each frequency band (as will be discussed in §3.7.4
below).

In reverberators based on feedback delay networks (FDNs), the smoothness of the ``perceptually white noise'' generated by the impulse response of the lossless prototype is strongly affected by the choice of FDN feedback matrix as well as the (ideally mutually prime) delay-line lengths in the FDN (discussed further in §3.7.3 below). Following are some of the better known feedback-matrix choices.

- Hadamard Matrix
- Householder Feedback Matrix
- Householder Reflections
- Most General Lossless Feedback Matrices
- Triangular Feedback Matrices

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