Delay, Reverb and Space

Chorus, flanger, and phaser all combined a signal with one modulated copy of itself. This week asks what happens once that delayed copy is fed back into its own delay line, round and round, rather than summed just once. By the end of this week you should be able to explain a delay line and its feedback loop; explain why decaying repeats — not runaway repeats — are the normal case; connect feedback delay to the comb filtering met in Week 8; describe the structure of a natural reverb (direct sound, early reflections, dense decaying tail); and distinguish algorithmic from convolution reverb.

1. Delay line→
2. Feedback loop (gain < 1)→
3. Decaying repeats→
4. Wet/dry mix

A single feedback delay produces a sparse, evenly spaced set of repeats — recognisable as discrete echoes rather than a room. Natural reverberation is what happens once a sound reflects off every surface in an enclosed space, producing far more reflections than one delay line can, arriving from every direction at once. That structure has three parts in sequence: the direct sound (arriving first, straight from source to listener), the early reflections (a handful of distinct, separately audible bounces off nearby surfaces), and a dense reverberant tail (thousands of reflections arriving so closely together they blur into one continuous decaying wash). Two properties describe that tail: room size, which sets how far apart the early reflections are spaced, and decay time, which sets how long the tail takes to die away.

Two families of reverb reproduce that structure differently. Algorithmic reverb never records a real space at all — it builds the direct/early/tail structure out of networks of delay lines and feedback loops (an extension of exactly the feedback loop above), tuned to produce a plausible early-reflection pattern and a smoothly decaying tail. Convolution reverb instead measures a real room’s actual impulse response — its reflections, recorded directly — and applies that measured response to any input signal, reproducing that specific space’s reflections rather than an approximation of one.

1. 01

   A feedback delay recirculates a fraction g (g < 1) of its own delayed output, producing a series of repeats that decays rather than growing without bound.

2. 02

   Feedback delay is the same mechanism behind Week 8's comb filter, just recirculated: more feedback sharpens the same peak/notch spacing into taller peaks and deeper notches.

3. 03

   Natural reverberation has three parts: direct sound, a handful of distinct early reflections, and a dense reverberant tail of thousands of overlapping reflections.

4. 04

   Room size sets how far apart early reflections are spaced; decay time sets how long the reverberant tail takes to die away.

5. 05

   Algorithmic reverb builds that structure from delay/feedback networks; convolution reverb measures a real room's impulse response and applies it directly.