diff --git a/daemon/src/audio/sound/tone.cpp b/daemon/src/audio/sound/tone.cpp
index 02c3903d5b10c0e56fd4e16facd40371dc6baf74..8a3eee76bf0eee2245c7ba899d721209abc15146 100644
--- a/daemon/src/audio/sound/tone.cpp
+++ b/daemon/src/audio/sound/tone.cpp
@@ -140,7 +140,7 @@ Tone::fillWavetable()
 }
 
 double
-Tone::interpolate(double x)
+Tone::interpolate(double x) const
 {
     int xi_0 = x;
     int xi_1 = xi_0 + 1;
diff --git a/daemon/src/audio/sound/tone.h b/daemon/src/audio/sound/tone.h
index bf6eeee85302759d711150a94007853eacefe3b9..55422bd0f508a76d26f209a639f5a785abf54614 100644
--- a/daemon/src/audio/sound/tone.h
+++ b/daemon/src/audio/sound/tone.h
@@ -73,8 +73,7 @@ class Tone : public AudioLoop {
          */
         void fillWavetable();
 
-        double interpolate(double x);
-
+        double interpolate(double x) const;
 
     private: