[Openal-devel] Doppler Velocity

[Jason Daly]

Alexandre Mah wrote:

> Ah.  Okay.  But I would have to disagree.  They both alter the speed 
> of sound (as their sole purpose).
>
> In fact, the speed of sound is not dependent on AL_DOPPLER_VELOCITY, 
> but rather on AL_DOPPLER_VELOCITY / AL_DOPPLER_FACTOR (so it seems 
> AL_DOPPLER_VELOCITY is somewhat unintuitively named).
>
> Knowing the speed of sound is important if you want to calculate 
> things like sonic boom.


Well, I've been arguing that AL_DOPPLER_VELOCITY is supposed to be set 
to the speed of sound.  When I've used OpenAL in the past, I've always 
called alDopplerVelocity(343.3) in my apps, because my database units 
are in meters, so the speed of sound is 343.3 meters/sec.  With this, I 
get a pretty good-sounding Doppler effect when I leave AL_DOPPLER_FACTOR 
set to 1.0.

The way I read things back then (when the 1.0 spec was brand new) was 
that for a realisitic effect, AL_DOPPLER_VELOCITY should be set to the 
speed of sound (the "propagation speed", as the spec calls it), and 
AL_DOPPLER_FACTOR should be left at 1.0.  The purpose of 
AL_DOPPLER_FACTOR was not to change the speed of sound, but to change 
the Doppler effect itself.  It was essentially an artistic control to be 
used after you have your realistic calculations working properly.  If 
the Doppler effect doesn't sound right for your particular application, 
you can always use AL_DOPPLER_FACTOR to exaggerate or reduce it and get 
the effect you want, while leaving the real speed of sound alone.  So, I 
don't consider them redundant.

Unfortunately, the original spec was a little ambiguous on exactly what 
DOPPLER_VELOCITY was supposed to be, so we have the dilemma now that 
there may be 1.0-compliant applications that depend on the default 
DOPPLER_VELOCITY being 1.0.  So essentially, what has happened is that 
DOPPLER_VELOCITY doesn't mean speed of sound anymore, and it in effect 
has become a different Doppler factor.  So, perhaps in a future version, 
we do need to scrap DOPPLER_VELOCITY in favor of a more explicit speed 
of sound parameter.

-- 

--"J"

"I'm a castaway stranded in a desolate land,
 I can see the footprints in the virtual sand."
	--Neil Peart