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diff --git a/alc/filters/biquad.h b/alc/filters/biquad.h
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+#ifndef FILTERS_BIQUAD_H
+#define FILTERS_BIQUAD_H
+
+#include <cmath>
+#include <utility>
+
+#include "math_defs.h"
+
+
+/* Filters implementation is based on the "Cookbook formulae for audio
+ * EQ biquad filter coefficients" by Robert Bristow-Johnson
+ * http://www.musicdsp.org/files/Audio-EQ-Cookbook.txt
+ */
+/* Implementation note: For the shelf filters, the specified gain is for the
+ * reference frequency, which is the centerpoint of the transition band. This
+ * better matches EFX filter design. To set the gain for the shelf itself, use
+ * the square root of the desired linear gain (or halve the dB gain).
+ */
+
+enum class BiquadType {
+    /** EFX-style low-pass filter, specifying a gain and reference frequency. */
+    HighShelf,
+    /** EFX-style high-pass filter, specifying a gain and reference frequency. */
+    LowShelf,
+    /** Peaking filter, specifying a gain and reference frequency. */
+    Peaking,
+
+    /** Low-pass cut-off filter, specifying a cut-off frequency. */
+    LowPass,
+    /** High-pass cut-off filter, specifying a cut-off frequency. */
+    HighPass,
+    /** Band-pass filter, specifying a center frequency. */
+    BandPass,
+};
+
+template<typename Real>
+class BiquadFilterR {
+    /* Last two delayed components for direct form II. */
+    Real z1{0.0f}, z2{0.0f};
+    /* Transfer function coefficients "b" (numerator) */
+    Real b0{1.0f}, b1{0.0f}, b2{0.0f};
+    /* Transfer function coefficients "a" (denominator; a0 is pre-applied). */
+    Real a1{0.0f}, a2{0.0f};
+
+public:
+    void clear() noexcept { z1 = z2 = 0.0f; }
+
+    /**
+     * Sets the filter state for the specified filter type and its parameters.
+     *
+     * \param type The type of filter to apply.
+     * \param gain The gain for the reference frequency response. Only used by
+     *             the Shelf and Peaking filter types.
+     * \param f0norm The reference frequency normal (ref_freq / sample_rate).
+     *               This is the center point for the Shelf, Peaking, and
+     *               BandPass filter types, or the cutoff frequency for the
+     *               LowPass and HighPass filter types.
+     * \param rcpQ The reciprocal of the Q coefficient for the filter's
+     *             transition band. Can be generated from rcpQFromSlope or
+     *             rcpQFromBandwidth as needed.
+     */
+    void setParams(BiquadType type, Real gain, Real f0norm, Real rcpQ);
+
+    void copyParamsFrom(const BiquadFilterR &other)
+    {
+        b0 = other.b0;
+        b1 = other.b1;
+        b2 = other.b2;
+        a1 = other.a1;
+        a2 = other.a2;
+    }
+
+
+    void process(Real *dst, const Real *src, int numsamples);
+
+    /* Rather hacky. It's just here to support "manual" processing. */
+    std::pair<Real,Real> getComponents() const noexcept
+    { return {z1, z2}; }
+    void setComponents(Real z1_, Real z2_) noexcept
+    { z1 = z1_; z2 = z2_; }
+    Real processOne(const Real in, Real &z1_, Real &z2_) const noexcept
+    {
+        Real out{in*b0 + z1_};
+        z1_ = in*b1 - out*a1 + z2_;
+        z2_ = in*b2 - out*a2;
+        return out;
+    }
+
+    /**
+     * Calculates the rcpQ (i.e. 1/Q) coefficient for shelving filters, using
+     * the reference gain and shelf slope parameter.
+     * \param gain 0 < gain
+     * \param slope 0 < slope <= 1
+     */
+    static Real rcpQFromSlope(Real gain, Real slope)
+    { return std::sqrt((gain + 1.0f/gain)*(1.0f/slope - 1.0f) + 2.0f); }
+
+    /**
+     * Calculates the rcpQ (i.e. 1/Q) coefficient for filters, using the
+     * normalized reference frequency and bandwidth.
+     * \param f0norm 0 < f0norm < 0.5.
+     * \param bandwidth 0 < bandwidth
+     */
+    static Real rcpQFromBandwidth(Real f0norm, Real bandwidth)
+    {
+        const Real w0{al::MathDefs<Real>::Tau() * f0norm};
+        return 2.0f*std::sinh(std::log(Real{2.0f})/2.0f*bandwidth*w0/std::sin(w0));
+    }
+};
+
+using BiquadFilter = BiquadFilterR<float>;
+
+#endif /* FILTERS_BIQUAD_H */