OpenAL- A binding to the OpenAL cross-platform 3D audio APISource codeContentsIndex
Handling the Distance Model
Evaluation of Gain/Attenuation Related State
No Culling By Distance
This module corresponds to section 3.4 (Attenuation By Distance) of the OpenAL Specification and Reference (version 1.1).
data DistanceModel
= NoAttenuation
| InverseDistance
| InverseDistanceClamped
| LinearDistance
| LinearDistanceClamped
| ExponentDistance
| ExponentDistanceClamped
distanceModel :: StateVar DistanceModel

Samples usually use the entire dynamic range of the chosen format/encoding, independent of their real world intensity. In other words, a jet engine and a clockwork both will have samples with full amplitude. The application will then have to adjust source gain accordingly to account for relative differences.

Source gain is then attenuated by distance. The effective attenuation of a source depends on many factors, among which distance attenuation and source and listener gain are only some of the contributing factors. Even if the source and listener gain exceed 1 (amplification beyond the guaranteed dynamic range), distance and other attenuation might ultimately limit the overall gain to a value below 1.

Handling the Distance Model
data DistanceModel Source

OpenAL currently supports six modes of operation with respect to distance attenuation, including one that is similar to the IASIG I3DL2 model. The application chooses one of these models (or chooses to disable distance-dependent attenuation) on a per-context basis.

The distance used in the formulas for the "clamped" modes below is clamped to be in the range between referenceDistance and maxDistance:

clamped distance = max(referenceDistance, min(distance, maxDistance))

The linear models are not physically realistic, but do allow full attenuation of a source beyond a specified distance. The OpenAL implementation is still free to apply any range clamping as necessary.

With all the distance models, if the formula can not be evaluated then the source will not be attenuated. For example, if a linear model is being used with referenceDistance equal to maxDistance, then the gain equation will have a divide-by-zero error in it. In this case, there is no attenuation for that source.

NoAttenuationBypass all distance attenuation calculation for all sources. The implementation is expected to optimize this situation.

Inverse distance rolloff model, which is equivalent to the IASIG I3DL2 model with the exception that referenceDistance does not imply any clamping.

gain = referenceDistance / (referenceDistance + rolloffFactor * (distance - referenceDistance))

The referenceDistance parameter used here is a per-source attribute which is the distance at which the listener will experience gain (unless the implementation had to clamp effective gain to the available dynamic range). rolloffFactor is per-source parameter the application can use to increase or decrease the range of a source by decreasing or increasing the attenuation, respectively. The default value is 1. The implementation is free to optimize for a rolloffFactor value of 0, which indicates that the application does not wish any distance attenuation on the respective source.

InverseDistanceClampedInverse Distance clamped model, which is essentially the inverse distance rolloff model, extended to guarantee that for distances below referenceDistance, gain is clamped. This mode is equivalent to the IASIG I3DL2 distance model.

Linear distance rolloff model, modeling a linear dropoff in gain as distance increases between the source and listener.

gain = (1 - rolloffFactor * (distance - referenceDistance) / (maxDistance - referenceDistance))

LinearDistanceClampedLinear Distance clamped model, which is the linear model, extended to guarantee that for distances below referenceDistance, gain is clamped.

Exponential distance rolloff model, modeling an exponential dropoff in gain as distance increases between the source and listener.

gain = (distance / referenceDistance) ** (- rolloffFactor)

ExponentDistanceClampedExponential Distance clamped model, which is the exponential model, extended to guarantee that for distances below referenceDistance, gain is clamped.
show/hide Instances
distanceModel :: StateVar DistanceModelSource
Contains the current per-context distance model.
Evaluation of Gain/Attenuation Related State

While amplification/attenuation commute (multiplication of scaling factors), clamping operations do not. The order in which various gain related operations are applied is:

1. Distance attenuation is calculated first, including minimum (referenceDistance) and maximum (maxDistance) thresholds.

2. The result is then multiplied by source gain.

3. If the source is directional (the inner cone angle is less than the outer cone angle, see coneAngles), an angle-dependent attenuation is calculated depending on coneOuterGain, and multiplied with the distance-dependent attenuation. The resulting attenuation factor for the given angle and distance between listener and source is multiplied with sourceGain.

4. The effective gain computed this way is compared against gainBounds.

5. The result is guaranteed to be clamped to gainBounds, and subsequently multiplied by listener gain which serves as an overall volume control.

The implementation is free to clamp listener gain if necessary due to hardware or implementation constraints.

No Culling By Distance

With the DS3D compatible inverse clamped distance model, OpenAL provides a per-source maxDistance attribute that can be used to define a distance beyond which the source will not be further attenuated by distance. The DS3D distance attenuation model and its clamping of volume is also extended by a mechanism to cull (mute) sources from processing, based on distance. However, the OpenAL does not support culling a source from processing based on a distance threshold.

At this time OpenAL is not meant to support culling at all. Culling based on distance, or bounding volumes, or other criteria, is best left to the application. For example, the application might employ sophisticated techniques to determine whether sources are audible that are beyond the scope of OpenAL. In particular, rule based culling inevitably introduces acoustic artifacts. E.g. if the listener-source distance is nearly equal to the culling threshold distance, but varies above and below, there will be popping artifacts in the absence of hysteresis.

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