Add second debris explosion sound.

This one combines the filtered noise and the random spline approach,
creating a more complex explosion sound.

sound/Makefile

@@ -1,4 +1,4 @@
-SFX = plasma explode explode2 explode3
+SFX = plasma explode explode2 explode3 explode4
 OGG = $(SFX:%=%.ogg)
 WAV = $(SFX:%=%.wav)
 

sound/explode4.csd

@@ -0,0 +1,46 @@
+<CsoundSynthesizer>
+<CsInstruments>
+sr = 48000
+ksmps = 32
+nchnls = 1
+0dbfs = 1
+
+; Explosions
+; Noise filtered by fofilter.
+; After p8 s attack time, linear decay to 0.
+; The filter frequency varies linearly between p4 and p5.
+; The fofilter impulse rise and decay times are given by p6 and p7.
+; p6 low: smoother sound, p6 high: more rumbling sound
+; p7 low: low deep sound, p7 high: tending towards white noise
+; p8: attack time (s)
+; p9: maximum amplitude
+
+instr 1
+	pset 0, 0, 1, 50, 30, 0.01, 0.01, 0.01, 1, 1
+	k1 linseg 0, p8, p9, p3 - p8, 0
+	k2 line p4, p3, p5
+	k3 expon 1, p3, 0.01
+	a1 noise k1, 0.1
+	a1 fofilter a1, k2, p6, p7
+	a1 limit a1, -1, 1
+	out a1 * p10 * k3
+endin
+
+instr 2
+	k1 linseg 0, p4, 1, p3 - p4, 0
+	k2 linseg p5, p4, p6, p3 - p4, p5
+	a1 rspline -k1, k1, p5, k2
+	a1 limit a1, -1, 1
+	out a1 * 0.5
+endin
+
+
+</CsInstruments>
+<CsScore>
+i 1 0.0 2.0  50 30 0.01 0.001  0.01 1.0 0.5
+i 1 0.5 2.0  50 30 0.01 0.002  0.01 1.0 0.2
+i 1 1.0 2.0  50 30 0.01 0.004  0.01 1.0 0.1
+i 2 0.0 3.0  1.5 10 1000
+e
+</CsScore>
+</CsoundSynthesizer>