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Spring Reverb

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Often followed by + Amp simulator + Tremolo + Limiter

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Settings are written to the URL as you change them. Nothing differs from the defaults yet.

Add a spring tank to audio

Spring reverb sends the signal down a coiled spring and picks it up at the other end. What comes back is not a room. It is metallic, focused in the mids, and it chirps.

Four controls shape it: decay, tension, damping, and mix.

Why a spring is not a small room

In a room, a reflection reaches the microphone with every frequency it contains arriving together. Geometry decides the timing, and the whole wavefront travels at one speed.

A spring does not work that way. The coil is dispersive: high frequencies travel along it faster than low ones. A single drum hit entering one end leaves the other spread out in time, high frequencies first, low frequencies trailing behind, which the ear hears as a falling whistle. The wave then reflects off the transducer at each end and does it again, so the chirps repeat as the tail decays.

That dispersion is modelled here as a long cascade of allpass filters inside the feedback loop. An allpass passes every frequency at full level and delays each one by a different amount, which is exactly the behaviour of the coil. Stack enough of them and a transient comes back as a boing.

Tension

Tension sets the length of the modelled coil. Higher tension is a shorter, tighter spring, so the round trip is quicker: reflections arrive closer together and the boing rises in pitch. It also deepens the dispersion, making each chirp more pronounced.

  • 0 to 30%: long and slack, a slow low boing with reflections far enough apart to count.
  • 40 to 60%: the range most amp tanks sit in.
  • 70 to 100%: short and tight, a fast metallic chatter with an obvious ring.

Decay and damping

Decay is the feedback around the loop, which sets how many times the wave travels the coil before it dies. Damping is a filter inside that loop, so it compounds pass by pass and the tail darkens as it fades rather than dropping its top end all at once.

Because damping sits inside the loop rather than on the output, raising it also shortens the tail. Energy the filter removes never comes back round.

Bandwidth

The step models the band a real tank passes, and it does it inside the feedback loop rather than on the way in, so the restriction compounds on every pass the way the physical one does. This is why spring reverb sounds the way it does even when it is fed something bright, and why it sits so well under a guitar without covering the top end of a mix.

Stereo

The two channels use unequal coil lengths, so the tanks do not ring in lockstep and the tail stays wide instead of collapsing to the centre. Real two-spring tanks are built with mismatched coils for the same reason.

In a chain

Put it after the amp simulator, distortion, and tape saturation, matching the order in a real amp where the tank sits after the preamp. Reverb before distortion gives the distortion a wash of decaying tail to clip, which sounds like mud rather than like a big sound.

Tremolo after spring reverb is the classic pairing, since that is the order in the amps this models. Level control comes last.

Frequently Asked Questions

Metallic, mid focused, and chirpy. A transient entering a spring comes out smeared into a descending whistle, and that chirp repeats each time the wave reflects off the transducer at either end. It is the reverb in a guitar amp, in dub records, and in most surf guitar.

The room reverb models a space, where every frequency in a reflection arrives together. A spring carries high frequencies faster than low ones, so one hit leaves the coil spread out in time as a falling chirp. That dispersion is the sound, and no combination of size and damping on a room reverb produces it.

The length and tightness of the modelled coil. Higher tension shortens the round trip, so the boing rises in pitch and the reflections come faster. It also increases the dispersion, which is how pronounced the chirp on each reflection is.

A real spring tank passes a narrow band, roughly a few hundred Hz to a few kHz, because the coil has little mechanical response below that and the transducers roll off hard above it. That band limit is modelled inside the feedback loop, so it compounds on every pass. Spring reverb is never bright.

After the amp simulator and distortion, the way a real tank sits after the preamp in a guitar amp. Putting reverb before distortion gives the distortion a wash of tail to work on, which turns to mud. Compression and limiting come after.

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