single pitch stiff string equation simulation

src/synth/Voice.hpp

@@ -4,6 +4,7 @@
 #include <stdint.h>
 
 #include "Instrument.hpp"
+#include "Instruments/PianoString.hpp"
 
 // a voice is a tone generator that the synth uses for polyphony
 // the synth mixes multiple voices together into a polyphonic audio. calculations for samples are handled in the instrument
@@ -37,6 +38,6 @@ private:
 
     ConfigService* config_;
     LoggerService* logger_;
-    Instrument instrument_;
+    PianoString instrument_;
 
 };