Add an option to ignore the app's speed of sound for reverb decay

Alc/ALc.c

@@ -885,6 +885,10 @@ static void alc_init(void)
     if(str && (strcasecmp(str, "true") == 0 || strtol(str, NULL, 0) == 1))
         ZScale *= -1.0f;
 
+    str = getenv("__ALSOFT_REVERB_IGNORES_SOUND_SPEED");
+    if(str && (strcasecmp(str, "true") == 0 || strtol(str, NULL, 0) == 1))
+        OverrideReverbSpeedOfSound = AL_TRUE;
+
     ret = altss_create(&LocalContext, ReleaseThreadCtx);
     assert(ret == althrd_success);
 
@@ -2540,6 +2544,7 @@ static ALvoid InitContext(ALCcontext *Context)
     listener->Params.MetersPerUnit = listener->MetersPerUnit;
     listener->Params.DopplerFactor = 1.0f;
     listener->Params.SpeedOfSound = SPEEDOFSOUNDMETRESPERSEC;
+    listener->Params.ReverbSpeedOfSound = SPEEDOFSOUNDMETRESPERSEC;
 
     ATOMIC_INIT(&listener->Update, NULL);
     ATOMIC_INIT(&listener->FreeList, NULL);

Alc/ALu.c

@@ -89,6 +89,9 @@ ALfloat ConeScale = 1.0f;
 /* Localized Z scalar for mono sources */
 ALfloat ZScale = 1.0f;
 
+/* Force default speed of sound for distance-related reverb decay. */
+ALboolean OverrideReverbSpeedOfSound = AL_FALSE;
+
 const aluMatrixf IdentityMatrixf = {{
     { 1.0f, 0.0f, 0.0f, 0.0f },
     { 0.0f, 1.0f, 0.0f, 0.0f },
@@ -312,6 +315,11 @@ static ALboolean CalcListenerParams(ALCcontext *Context)
 
     Listener->Params.DopplerFactor = props->DopplerFactor;
     Listener->Params.SpeedOfSound = props->SpeedOfSound * props->DopplerVelocity;
+    if(OverrideReverbSpeedOfSound)
+        Listener->Params.ReverbSpeedOfSound = SPEEDOFSOUNDMETRESPERSEC;
+    else
+        Listener->Params.ReverbSpeedOfSound = Listener->Params.SpeedOfSound *
+                                              Listener->Params.MetersPerUnit;
 
     Listener->Params.SourceDistanceModel = props->SourceDistanceModel;
     Listener->Params.DistanceModel = props->DistanceModel;
@@ -1105,8 +1113,8 @@ static void CalcAttnSourceParams(ALvoice *voice, const struct ALvoiceProps *prop
         else if(SendSlots[i]->Params.AuxSendAuto)
         {
             RoomRolloff[i] = SendSlots[i]->Params.RoomRolloff + props->RoomRolloffFactor;
-            DecayDistance[i] = SendSlots[i]->Params.DecayTime * Listener->Params.SpeedOfSound *
-                               Listener->Params.MetersPerUnit;
+            DecayDistance[i] = SendSlots[i]->Params.DecayTime *
+                               Listener->Params.ReverbSpeedOfSound;
             DecayHFDistance[i] = DecayDistance[i] * SendSlots[i]->Params.DecayHFRatio;
             if(SendSlots[i]->Params.DecayHFLimit)
             {

Alc/effects/reverb.c

@@ -1362,8 +1362,8 @@ static ALvoid ALreverbState_update(ALreverbState *State, const ALCcontext *Conte
     hfRatio = props->Reverb.DecayHFRatio;
     if(props->Reverb.DecayHFLimit && props->Reverb.AirAbsorptionGainHF < 1.0f)
         hfRatio = CalcLimitedHfRatio(hfRatio, props->Reverb.AirAbsorptionGainHF,
-                                     props->Reverb.DecayTime, Listener->Params.SpeedOfSound *
-                                     Listener->Params.MetersPerUnit);
+            props->Reverb.DecayTime, Listener->Params.ReverbSpeedOfSound
+        );
 
     /* Calculate the LF/HF decay times. */
     lfDecayTime = clampf(props->Reverb.DecayTime * props->Reverb.DecayLFRatio,

OpenAL32/Include/alListener.h

@@ -50,7 +50,8 @@ typedef struct ALlistener {
         ALfloat MetersPerUnit;
 
         ALfloat DopplerFactor;
-        ALfloat SpeedOfSound;
+        ALfloat SpeedOfSound; /* in units per sec! */
+        ALfloat ReverbSpeedOfSound; /* in meters per sec! */
 
         ALboolean SourceDistanceModel;
         enum DistanceModel DistanceModel;

OpenAL32/Include/alu.h

@@ -516,6 +516,7 @@ void aluHandleDisconnect(ALCdevice *device);
 
 extern ALfloat ConeScale;
 extern ALfloat ZScale;
+extern ALboolean OverrideReverbSpeedOfSound;
 
 #ifdef __cplusplus
 }

docs/env-vars.txt

@@ -79,3 +79,13 @@ which some applications make use of to protect against partial updates. In an
 attempt to standardize on that behavior, OpenAL Soft has changed those methods
 accordingly. Setting this to "ignore" restores the previous no-op behavior for
 applications that interact poorly with the new behavior.
+
+__ALSOFT_REVERB_IGNORES_SOUND_SPEED
+Older versions of OpenAL Soft ignored the app-specified speed of sound when
+calculating distance-related reverb decays and always assumed the default
+343.3m/s. Now, both of the AL_SPEED_OF_SOUND and AL_METERS_PER_UNIT properties
+are taken into account for speed of sound adjustments to have an appropriate
+affect on the reverb decay. Consequently, applications that use reverb but
+don't set these properties properly may find the reverb decay too strong.
+Setting this to "true" or "1" will revert to the old behavior for those apps
+and assume the default speed of sound for reverb.