diff --git a/libse/RealFFT.cs b/libse/RealFFT.cs
index 06ebc0ad5..b06437083 100644
--- a/libse/RealFFT.cs
+++ b/libse/RealFFT.cs
@@ -8,9 +8,9 @@ namespace Nikse.SubtitleEdit.Core
 {
     public class RealFFT
     {
-        private int _length;
-        private int[] _ip;
-        private double[] _w;
+        private readonly int _length;
+        private readonly int[] _ip;
+        private readonly double[] _w;
 
         public readonly double ForwardScaleFactor;
         public readonly double ReverseScaleFactor;
diff --git a/libse/WaveToVisualizer.cs b/libse/WaveToVisualizer.cs
index 4f2df823c..8b4645a41 100644
--- a/libse/WaveToVisualizer.cs
+++ b/libse/WaveToVisualizer.cs
@@ -505,13 +505,13 @@ namespace Nikse.SubtitleEdit.Core
         {
             private const double raisedCosineWindowScale = 0.5;
 
-            private int _nfft;
-            private MagnitudeToIndexMapper _mapper;
-            private RealFFT _fft;
-            private FastBitmap.PixelData[] _palette;
-            private double[] _segment;
-            private double[] _window;
-            private double[] _magnitude;
+            private readonly int _nfft;
+            private readonly MagnitudeToIndexMapper _mapper;
+            private readonly RealFFT _fft;
+            private readonly FastBitmap.PixelData[] _palette;
+            private readonly double[] _segment;
+            private readonly double[] _window;
+            private readonly double[] _magnitude;
 
             public SpectrogramDrawer(int nfft)
             {