From bw@ashbysolutions.com Thu Jun 21 10:55:42 2012
Subject:Re: Expression pedal?

[Warning -- complicated technical explanations below!]

________
The simple answer: The human ear is more sensitive to midrange than
bass or treble, and an organ expression pedal tries to mimic this
effect. A volume pedal, OTOH, will change all frequencies equally.
Also, an expression pedal doesn't quite go to 'zero,' but drops to about
10-15% of the maximum level.

________
The tech-y answer: The human ear is more sensitive to midrange than
bass or treble. This is most likely do to genetic selection over the
years: humans who could hear the pounce of a predator or the whispers
of their hunting companions (midrange-heavy) over the background noise
survived to have children; those that couldn't, didn't last so long. ;-)
This phenomenon was studied at length, and a set of 'equal loudness
curves' was developed by Fletcher and Munson in the '30s --

http://en.wikipedia.org/wiki/Fletcher-Munson_curves

These curves show the amount of sound energy (SPL) that is needed to
seem 'the same volume' to the average human at different sound levels.
What's important to note is that while the curves are pretty flat at
high volumes, they are 'scooped' at lower levels. What does this mean
to the musician? At lower volumes, it takes more treble and bass to
keep up with the midrange. This explains why one can create a 'perfect'
digital piano or synth sound at practice volume, and be dissatisfied
with the results on the stage. The organ expression pedal attempts to
simulate this volume-sensitive effect, so that the bass/midrange/treble
balance remains in place as the music rises and falls in volume.

And, the expression pedal does something else ... If we look at the
*really* old-school expression pedal -- i.e., the one on a pipe organ --
it was connected to a set of shutters that opened and closed in front of
the ranks of pipes. Pulling the pedal back caused the shutters to
close, making the sound softer and 'darker,' while opening the shutters
boosted both the volume and 'brightness' of the sound. So, even
neglecting the work of Mr. Fletcher and Mr. Munson, an expression pedal
that tries to mimic a pipe organ should darken the sound on the way
towards minimum volume. (Remember, the Hammond Organ was *supposed* to
be a 'perfect replacement for a pipe organ.') This is the reason why an
expression pedal rarely goes to zero -- the pipe organ's shutters don't
completely block the sound, either.

Most clonewheels do a decent job of emulating these effect in their
software. It's difficult to do the same thing in an external volume
pedal: One could make a proper design, but it would only work well if
the clonewheel and amp had specific output and input impedances,
respectively.

Regards,

-BW

Bruce Wahler
Ashby Solutions.com^(TM)
bw@ashbysolutions.com
http://music.ashbysolutions.com
877.55.ASHBY (877.552.7429)

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