Add impact explosion sound.

This is the sound played when shots hit an enemy, but do not yet destroy it.
It's a small explosion sound combined with a metallic sound, simulating the
impact of the shot on the enemy's hull.

sound/Makefile

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

sound/explode2.csd

@@ -0,0 +1,42 @@
+<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 line 1, p3, 0
+	a1 noise k1, 0.1
+	a1 fofilter a1, k2, p6, p7
+	a1 limit a1, -1, 1
+	out a1 * p10 * k3
+endin
+
+; Metalic sound
+
+instr 2
+	k1 line 1, p3, 0
+	a1 barmodel 1, 1, p4, 0.001, 0.23, 5, p5, p6, p7
+	out a1 * k1
+endin
+</CsInstruments>
+<CsScore>
+i 2 0.0 0.1 40 0.1 5000 0.5
+i 1 0 0.7 200 100 0.01 0.0006 0.05 1.0 0.15
+e1
+</CsScore>
+</CsoundSynthesizer>