[Openal-devel] Room Rolloff
dpeacock at creativelabs.com
Mon Mar 31 14:44:17 PDT 2008
No problems - the questions keep us on our toes.
1) The Reverb Room Roll-off parameter combines additively with a Source's
Room Roll-Off Factor to determine that Source's reverb send level
2) The Default values of the Reverb Roll-Off Factor are 0 and 0 for Reverb
and Source. These factors can be used to apply *additional* attenuation
of the reverb based on the source to listener distance. The equations
used for this extra attenuation are based on the same formula used to do
attenuation of the dry path of a sound Source over distance (and, like the
Source's dry path, the formula changes based on the Distance Model
selected). At their default values of 0 these terms do not attenuate the
The "mysterious" attenuation of the reverb is what we call the "Reverb
Statistical Model" that simulates the natural rolloff of reflections and
reverberation over distance in typical rooms. It takes into account lots
of parameters; source-listener distance, room size, decay time, and source
directivity. The exact formulas are Creative Intellectual Property, but
the idea is that the reverb levels sound natural as a source moves away
from a listener.
3) If you have a directive source it contributes less energy into a room
compared to an omni-directional source, so it does have an impact on the
All DirectSound3D / OpenAL cards should support directional sound sources.
Unfortunately not many games use directional sound sources - I'm not sure
why. It does need a sound designer to design appropriate cone angle
settings, and a programmer to calculate and set the orientation of the
sound source. When it is done well it does help - especially when
combined with the EAX & EFX Cone Outer HF parameter that adds a low-pass
filtering effect to the sound when it is not facing the listener.
Creative Labs, Inc.
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[Openal-devel] Room Rolloff
Apols about the endless spec questions - I think I'm nearly done.
For the EFX reverbs, there seem to be two room rolloff settings, one on the
source and one on the reverb. The spec doesn't seem to say how the two
should be combined - I'm guessing this is just a multiplication?
Also, the spec becomes kinda mysterious around what behaviour is expected
when the room rolloff works out as zero. EAX "naturally manages … natural
rolloff of reflected sound vs distance in typical rooms". Anyone know what
is expected here? At the moment I'm just using the zero rolloff setting,
which means the reverb is the same regardless of source location, which
sounds alright to me.
Incidentally, I don't really understand why OpenAL recommends that the
reverb send gain and gainHF should be modified by source direction. I'm
guessing it would sound better without… Out of interest, does anyone know
which games & cards support directional sources? Maybe the point is moot...
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