How to use DetectSilence in SuperCollider to start a new sound when another sound falls below a certain amplitude level. Here we use AllpassC to add a reverb on the tail of the sound that we are tracking.

SC_detect_silence

(
SynthDef("releaser", { | gate = 1 |
  var env;
	env = Env([0, 1, 0], [0.04, 6.0], releaseNode: 1);
	Out.ar(0, WhiteNoise.ar(1) * EnvGen.kr(env, gate, doneAction: 2))

}).add;
)

(
SynthDef("reverbreleaser_template", { | gate = 1 |
	var env, source, reverb;
	env = Env([0, 1, 0], [0.04, 6.0], releaseNode: 1);
	source = WhiteNoise.ar(1) * EnvGen.kr(env, gate, doneAction: 0);
	reverb = GrayNoise.ar(1) * DetectSilence.ar(source, 0.5);
	Out.ar(0, [source, reverb])

}).add;
)

(
SynthDef("reverbreleaser", { | gate = 1 |
	var env, source, reverb;
	env = Env([0, 1, 0], [0.04, 6.0], releaseNode: 1);
	source = SinOsc.ar(LFNoise2.kr(5).range(400, 4000))
	* EnvGen.kr(env, gate, doneAction: 0);
	reverb = Mix({
		AllpassC.ar(source, 0.3, 0.001.exprand(0.1) min: 0.3,  decaytime: 5);
	} ! 20
	)* DetectSilence.ar(source, 0.5);
	Out.ar(0, [source, reverb])

}).add;
)

a = Synth('reverbreleaser');

a.release(10);
a.release(3);

a.release(10);


a = Synth('releaser');


a.set(\gate, 0);