AudioEmitter

DirectXTK	Audio

AudioEmitter is a helper object derived from X3DAUDIO_EMITTER that can be used with the Apply3D method of SoundEffectInstance, SoundStreamInstance, and DynamicSoundEffectInstance.

See AudioListener.

Related tutorial: Using positional audio

classDiagram
direction LR
class AudioEmitter{
   +EmitterCone
   +EmitterAzimuths
   +SetPosition()
   +SetVelocity()
   +SetOrientation()
   +Update()
}
class X3DAUDIO_EMITTER
X3DAUDIO_EMITTER <|-- AudioEmitter

Header

#include <Audio.h>

Initialization

The default constructor creates a point omnidirectional emitter located at 0,0,0 facing -z and an up vector of +y for a mono (single-channel) source.

CurveDistanceScaler and DopplerScaler are set to 1 by default. InnerRadius is set to 0, and InnerRadiusAngle is set to X3DAUDIO_PI/4.0 (45 degrees).

When using left-handed coordinates, be sure to set OrientFront

Helpers

In addition to setting the members of X3DAUDIO_EMITTER directly, these helper functions are provided:

SetPosition (XMVECTOR) or (XMFLOAT3): Sets the Position of the emitter.
SetVelocity (XMVECTOR) or (XMFLOAT3): Sets the Velocity of the emitter.
SetOrientation (XMVECTOR forward, XMVECTOR up) or (XMFLOAT3 forward, XMFLOAT3 up)
SetOrientationFromQuaternion (XMVECTOR): Sets the OrientFront/OrientTop of the emitter.
Update (XMVECTOR newPos, XMVECTOR upDir, float dt): Computes a direction and velocity for the emitter based on the current Position value, the new position, and the provided delta time (Δt). This updates the OrientFront/OrientTop to match, and then sets the Position to the new position. If dt is 0, the update is skipped.
EnableDefaultCurves sets to no distance attenuation for the volume and LFE. LPF and reverb curves are set to nullptr. These are consistent with the default curves used by the legacy XACT engine.
EnableLinearCurves sets linear distance attenuation for the volume and LFE. LPF and reverb curves are set to nullptr. This is equivalent to using X3DAudioDefault_LinearCurve.
EnableInverseSquareCurves sets the volume and LFE to an inverse square fall-off with distance. LPF and reverb curves are set to nullptr. This is the behavior for X3DAUDIO_EMITTER if you pass nullptr for these curves. Note: This is the state the default ctor already sets.

You must use a distinct instance of AudioEmitter for each active 3D sound if using the Update method. Otherwise, if you reuse the emitter instance for multiple sounds you need to explicitly initialize both the position and velocity before each Apply3D call.

Multi-channel 3D Audio

X3DAudio supports multi-channel sound sources for 3D audio (i.e. stereo, quad, etc.). The default constructor for AudioEmitter sets the source up for mono (i.e. single-channel), so to use multi-channel sources, you should set the ChannelCount member to match the number of channels in your source, and adjust ChannelRadius and the EmitterAzimuths array as desired.

std::unique_ptr<SoundEffect> soundEffect;
AudioEmitter emitter;
emitter.EnableDefaultCurves();

soundEffect = std::make_unique<SoundEffect>( audEngine.get(), L"sound.wav" );

emitter.SetPosition(x, y, z);
emitter.ChannelCount = soundEffect->GetFormat()->nChannels;

You can also set the channel count from the sound instance:

effect = soundEffect->CreateInstance(SoundEffectInstance_Use3D);

emitter.SetPosition(x, y, z);
emitter.ChannelCount = effect->GetChannelCount();

For multi-channel emitters, OrientTop must be valid.

AudioEmitter includes an EmitterAzimuths array which is pointed to by pChannelAzimuths and defaults to all 0. This is because pChannelAzimuths cannot be a nullptr for multi-channel sound emitters.

The helper EnableDefaultMultiChannel can be used to set up a default set of emitter azimuths based on common speaker configurations for up to 8 channels, and sets the channel radius:

emitter.SetPosition(x, y, z);
emitter.EnableDefaultMultiChannel(effect->GetChannelCount(), 10.f);

Directional Emitters

AudioEmitter defaults to an omnidirectional emitter. To create a sound-cone, set the pCone member to point to a X3DAUDIO_CONE structure. The pointer must point to valid memory at the time Apply3D is called. The SetCone helper uses the EmitterCone variable to store the cone settings.

SetOmnidirectional sets the pCone value to nullptr indicating an omnidirectional emitter (the default).
SetCone (X3DAUDIO_CONE) sets the pCone value to point to EmitterCone and copies the passed in cone parameters.

Be aware that pCone is ignored for multi-channel emitters.

Custom distance curves

The AudioEmitter constructor sets pVolumeCurve, pLFECurve, pLPFDirectCurve, pLPFReverbCurve, and pReverbCurve to nullptr which uses X3DAudio default curves. Any custom curve definitions must have a pointer to valid memory at the time Apply3D is called.

Example

float dt = static_cast<float>(timer.GetElapsedSeconds());

emitter.Update(soundPosition, Vector3::Up, dt);
listener.Update(playerPosiiton, Vector3::Up, dt);
if (soundSource)
{
    soundSource->Apply3D(listener, emitter);
}