Fix and clarify the peaking biquad filter

alc/effects/equalizer.cpp

@@ -116,25 +116,26 @@ void EqualizerState::update(const ALCcontext *context, const ALeffectslot *slot,
     ALfloat gain, f0norm;
 
     /* Calculate coefficients for the each type of filter. Note that the shelf
-     * filters' gain is for the reference frequency, which is the centerpoint
-     * of the transition band.
+     * and peaking filters' gain is for the centerpoint of the transition band,
+     * meaning its dB needs to be doubled for the shelf or peak to reach the
+     * provided gain.
      */
-    gain = maxf(sqrtf(props->Equalizer.LowGain), 0.0625f); /* Limit -24dB */
+    gain = maxf(std::sqrt(props->Equalizer.LowGain), 0.0625f); /* Limit -24dB */
     f0norm = props->Equalizer.LowCutoff/frequency;
     mChans[0].filter[0].setParams(BiquadType::LowShelf, gain, f0norm,
         BiquadFilter::rcpQFromSlope(gain, 0.75f));
 
-    gain = maxf(props->Equalizer.Mid1Gain, 0.0625f);
+    gain = maxf(std::sqrt(props->Equalizer.Mid1Gain), 0.0625f);
     f0norm = props->Equalizer.Mid1Center/frequency;
     mChans[0].filter[1].setParams(BiquadType::Peaking, gain, f0norm,
         BiquadFilter::rcpQFromBandwidth(f0norm, props->Equalizer.Mid1Width));
 
-    gain = maxf(props->Equalizer.Mid2Gain, 0.0625f);
+    gain = maxf(std::sqrt(props->Equalizer.Mid2Gain), 0.0625f);
     f0norm = props->Equalizer.Mid2Center/frequency;
     mChans[0].filter[2].setParams(BiquadType::Peaking, gain, f0norm,
         BiquadFilter::rcpQFromBandwidth(f0norm, props->Equalizer.Mid2Width));
 
-    gain = maxf(sqrtf(props->Equalizer.HighGain), 0.0625f);
+    gain = maxf(std::sqrt(props->Equalizer.HighGain), 0.0625f);
     f0norm = props->Equalizer.HighCutoff/frequency;
     mChans[0].filter[3].setParams(BiquadType::HighShelf, gain, f0norm,
         BiquadFilter::rcpQFromSlope(gain, 0.75f));

alc/filters/biquad.cpp

@@ -47,7 +47,6 @@ void BiquadFilterR<Real>::setParams(BiquadType type, Real gain, Real f0norm, Rea
             a[2] =             (gain+1.0f) + (gain-1.0f)*cos_w0 - sqrtgain_alpha_2;
             break;
         case BiquadType::Peaking:
-            gain = std::sqrt(gain);
             b[0] =  1.0f + alpha * gain;
             b[1] = -2.0f * cos_w0;
             b[2] =  1.0f - alpha * gain;

alc/filters/biquad.h

@@ -11,10 +11,11 @@
  * 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).
+/* Implementation note: For the shelf and peaking filters, the specified gain
+ * is for the centerpoint of the transition band. This better fits EFX filter
+ * behavior, which expects the shelf's reference frequency to reach the given
+ * gain. To set the gain for the shelf or peak itself, use the square root of
+ * the desired linear gain (or halve the dB gain).
  */
 
 enum class BiquadType {