[sdiy] Looking for cheap lowpass filter algorithm

[Cornutt, David K]

-----Original Message-----
> Just to elaborate on this .. most sounds have a whole series 
> of harmonics diminishing in amplitude as they get higher.  
> When you put a sound through a very steep low pass filter you 
> suddenly remove all the harmonics above the filter and leave 
> the ones below it as they are.  
> With a more gentle filter you get a gradual transition from 
> normal harmonics to no harmonics.  Maybe the ear/brain prefers this?

Interesting observation, and it got me thinking about something.
A few years ago, I borrowed a calibrated Tek digital scope with
a frequency counter to calibrate my Juno-106 (which hadn't
been calibrated since 1989!).  After I finished that, just for
funsies, I used the synth and the scope to give myself a
hearing test.  Setting up the VCA to self-oscillate to
get an approximate sine wave, I found that I could hear fine
up to 17.9 KHz.  Then, it faded out very rapidly until it
was totally gone to my ear (stiil visible on the scope)
at 18.3 KHz.  When you think about what fraction of an octave
that is, that's one hell of a steep filter.  I don't know if
everyone's ears respond like this, but I'm guessing that
most people with normal hearing have a fairly steep roll-off
between 16 and 20 KHz.  Otherwise, we wouldn't design sound
systems to the parameters that we use.

So, if you use a brick-wall filter to cut off at say 12 KHz,
does that sound to your brain like your ears have gone bad?
Maybe that's what makes it not sound good.