AL_INVERSE_DISTANCE_CLAMPED versus AL_INVERSE_DISTANCE for deterministic audio culling

Direct Answer

Use AL_INVERSE_DISTANCE_CLAMPED with AL_MAX_DISTANCE set to your gameplay culling radius. This is the only model that guarantees a hard gain of 0.0 at a predictable, finite distance—exactly what deterministic culling requires. The unclamped AL_INVERSE_DISTANCE model never reaches zero gain at any finite distance and exhibits implementation-dependent floating-point behavior at extreme ranges, making it unsuitable for hard culling boundaries.

Gain Formulas and rolloffFactor Interaction

Both models share the same core inverse-distance formula; the clamped variant simply clamps the effective distance to [AL_REFERENCE_DISTANCE, AL_MAX_DISTANCE] before evaluation:

// Clamped model (spec §3.5.1)
effectiveDistance = clamp(distance, AL_REFERENCE_DISTANCE, AL_MAX_DISTANCE)
gain = clamp(AL_REFERENCE_DISTANCE / (AL_REFERENCE_DISTANCE + AL_ROLLOFF_FACTOR * (effectiveDistance - AL_REFERENCE_DISTANCE)), 0, 1)

// Unclamped model (spec §3.5.1)
// AL_MAX_DISTANCE is NOT used
gain = clamp(AL_REFERENCE_DISTANCE / (AL_REFERENCE_DISTANCE + AL_ROLLOFF_FACTOR * (distance - AL_REFERENCE_DISTANCE)), 0, 1)

AL_ROLLOFF_FACTOR scales the distance term linearly in both models. For the clamped model, the effective attenuation range is (AL_MAX_DISTANCE - AL_REFERENCE_DISTANCE) / AL_ROLLOFF_FACTOR. For the unclamped model, the distance at which gain reaches threshold T is AL_REFERENCE_DISTANCE + (AL_REFERENCE_DISTANCE/T - AL_REFERENCE_DISTANCE) / AL_ROLLOFF_FACTOR. You can predict gain exactly by evaluating the formula—no reimplementation needed if the driver follows the spec (OpenAL Soft, Creative, and Apple implementations do).

Precision Safeguards for Unclamped Model

If you must use the unclamped model (e.g., for smooth falloff without a hard boundary), apply these safeguards:

• Set AL_MAX_DISTANCE to a finite gameplay radius (1,000–10,000 units) even though the spec says it's unused—some older drivers incorrectly clamp gain at FLT_MAX.
• Perform CPU-side culling checks in double precision; compute the expected gain before submitting distance updates.
• Clamp gain manually in the audio thread if the source approaches subnormal values (< 1e-38) where hardware may flush to zero inconsistently.
• Avoid AL_ROLLOFF_FACTOR values that make rolloffFactor * distance approach ~1e38 (single-precision overflow to INF).

Perceptual Acceptability of the Hard Cutoff

The clamped model produces a discontinuity in the first derivative of gain at AL_MAX_DISTANCE. Whether this is audible depends on the gain slope at the cutoff:

• If gain at AL_MAX_DISTANCE - ε is above ~0.001 (-60 dB), the step to 0.0 may be heard as a click or sudden silence.
• Mitigation: choose AL_MAX_DISTANCE where the calculated gain is already below perceptual threshold, or apply a short game-logic fade-out (e.g., 50–100 ms) before the source reaches the culling radius.
• Verify empirically: generate a sine wave, automate distance crossing AL_MAX_DISTANCE, record output, and inspect the waveform for discontinuities.

Required Setup for Deterministic Culling

1. Call alDistanceModel(AL_INVERSE_DISTANCE_CLAMPED) once at initialization.
2. For each source: set AL_REFERENCE_DISTANCE (typically 1.0 unit), AL_MAX_DISTANCE to your culling radius, and AL_ROLLOFF_FACTOR to tune falloff steepness.
3. Verify with alGetError() after each call; query AL_DISTANCE_MODEL to confirm the enum was accepted.
4. Disable or synchronize other gain-affecting state: listener gain (AL_GAIN), cone outer gain (AL_CONE_OUTER_GAIN), and EAX/reverb send levels can reintroduce signal from a "culled" source.

One Missing Diagnostic Detail

What is your gameplay culling radius in world units and the reference distance you use for sources? The perceptual risk of the hard cutoff depends entirely on the gain value at that specific radius given your chosen rolloffFactor. If you provide those two numbers, I can tell you whether the step to zero will be audible or safely inaudible.