[sdiy] Envelope follower w/ control voltage proportional toperceived loudness?

[Steve Lenham]

>I am trying to use a guitar to generate a control voltage that is 
>proportional to the *perceived* loudness of the sound, not the actual 
>voltage envelope itself.  Would it make sense to try using a log converter 
>after the envelope follower to get an overall voltage response closer to 
>what the ear hears?  Has anyone tried using a linear-to-log converter after 
>their envelope followers to generate a control voltage for this purpose?  I 
>was thinking a circuit with matched transistors such as is used in analog 
>synthesizers to make 1V/Octave response.  Would this work?

Hi Dave,

I think the main things you need to model are:

a) perceived loudness is logarithmic, as you suggest.
b) perceived loudness varies with frequency.

To solve a), you could either put a log converter after your envelope 
detector or use a detector that is inherently logarithmic. I'd suggest the 
THAT 2252 (http://www.thatcorp.com/2252desc.html) as a single-chip solution.

For b), just add a bit of filtering prior to the envelope detector. The ear 
is less sensitive at low and high frequencies, so just roll those off a 
little. If you want to be precise, I'm sure the web is crawling with details 
and graphs of the exact curves and rolloff rates.

Cheers,

Steve L.