diff --git a/src/SpectralDifferenceAudioCurve.cpp b/src/SpectralDifferenceAudioCurve.cpp
new file mode 100644
index 0000000..fe26e3e
--- /dev/null
+++ b/src/SpectralDifferenceAudioCurve.cpp
@@ -0,0 +1,64 @@
+/* -*- c-basic-offset: 4 indent-tabs-mode: nil -*-  vi:set ts=8 sts=4 sw=4: */
+
+/*
+    Rubber Band
+    An audio time-stretching and pitch-shifting library.
+    Copyright 2007 Chris Cannam.
+    
+    This program is free software; you can redistribute it and/or
+    modify it under the terms of the GNU General Public License as
+    published by the Free Software Foundation; either version 2 of the
+    License, or (at your option) any later version.  See the file
+    COPYING included with this distribution for more information.
+*/
+
+#include "SpectralDifferenceAudioCurve.h"
+
+namespace RubberBand
+{
+
+SpectralDifferenceAudioCurve::SpectralDifferenceAudioCurve(size_t sampleRate, size_t windowSize) :
+    AudioCurve(sampleRate, windowSize)
+{
+    m_prevMag = new double[m_windowSize/2 + 1];
+
+    for (size_t i = 0; i <= m_windowSize/2; ++i) {
+        m_prevMag[i] = 0.f;
+    }
+}
+
+SpectralDifferenceAudioCurve::~SpectralDifferenceAudioCurve()
+{
+    delete[] m_prevMag;
+}
+
+void
+SpectralDifferenceAudioCurve::reset()
+{
+    for (size_t i = 0; i <= m_windowSize/2; ++i) {
+        m_prevMag[i] = 0;
+    }
+}
+
+void
+SpectralDifferenceAudioCurve::setWindowSize(size_t newSize)
+{
+    m_windowSize = newSize;
+}
+
+float
+SpectralDifferenceAudioCurve::process(float *mag, size_t increment)
+{
+    float result = 0.0;
+
+    for (size_t n = 0; n <= m_windowSize / 2; ++n) {
+        result += sqrtf(fabsf((mag[n] * mag[n]) -
+                              (m_prevMag[n] * m_prevMag[n])));
+        m_prevMag[n] = mag[n];
+    }
+
+    return result;
+}
+
+}
+
diff --git a/src/SpectralDifferenceAudioCurve.h b/src/SpectralDifferenceAudioCurve.h
new file mode 100644
index 0000000..c6f4484
--- /dev/null
+++ b/src/SpectralDifferenceAudioCurve.h
@@ -0,0 +1,42 @@
+/* -*- c-basic-offset: 4 indent-tabs-mode: nil -*-  vi:set ts=8 sts=4 sw=4: */
+
+/*
+    Rubber Band
+    An audio time-stretching and pitch-shifting library.
+    Copyright 2007 Chris Cannam.
+    
+    This program is free software; you can redistribute it and/or
+    modify it under the terms of the GNU General Public License as
+    published by the Free Software Foundation; either version 2 of the
+    License, or (at your option) any later version.  See the file
+    COPYING included with this distribution for more information.
+*/
+
+#ifndef _SPECTRALDIFFERENCE_AUDIO_CURVE_H_
+#define _SPECTRALDIFFERENCE_AUDIO_CURVE_H_
+
+#include "AudioCurve.h"
+#include "Window.h"
+
+namespace RubberBand
+{
+
+class SpectralDifferenceAudioCurve : public AudioCurve
+{
+public:
+    SpectralDifferenceAudioCurve(size_t sampleRate, size_t windowSize);
+
+    virtual ~SpectralDifferenceAudioCurve();
+
+    virtual void setWindowSize(size_t newSize);
+
+    virtual float process(float *mag, size_t increment);
+    virtual void reset();
+
+protected:
+    double *m_prevMag;
+};
+
+}
+
+#endif