From bruce@ashbysolutions.com Mon Jan 13 21:14:32 2003
Subject:The top and the bottom of it ...

Hi All,

There has been a lot of discussion about what makes a Leslie sound like a Leslie, etc. Let me try to add a little info from a design engineer's perspective, as well as my experience.

Tony, the techs are right when they say that "most of the sound comes from the horn," but they aren't talking about sheer volume numbers. They are referring to the fact that most of the Doppler and FM effects are heard above 800Hz. The woofer drum is not large (or long) enough to provide much effect -- other than pulse -- in the lower frequencies. This can be demonstrated by pulling the plug to the lower Chorale motor: the characteristic "Leslie sound" is still pretty much there when only the horn rotates.

This doesn't mean, however, that the lower frequencies are unnecessary. If you pull the plug to the lower SPEAKER, the resulting sound may remind you of a Leslie, but it will not have that huge, wood-like tone. The low end is a major part of the Leslie sound we all love.

What complicates things is that the Leslie sound is composed of a very delicate balance of highs and lows. This balance is attained naturally in a Leslie 122 or 147 by a combination of the speakers, cabinet, amplifier, and crossover, as well as the coupling between the tweeter driver and the rotating horn. The difficult part in simulating the sound -- whether entirely electronically, or through the low-end-only simulation of a Pro-3T or the new Leslie 21xx-series -- is that the original woofer-in-the-box is quite efficient, although not extremely even in frequency response. (If you don't believe me, stop your rotors and use the Leslie as a hi-fi amplifier -- it's pretty awful!) Modern PA-style system low ends have a much more even response, but generally sacrifice some efficiency to get there. So, when you try to use a horn tweeter rotor with a stationary PA-style woofer, the amount of raw watts required to get "the sound" may seem all out of proportion. When we were playing
around with the Pro-3 in the original Clone Shootout, it was obvious that the 40W of tweeter needed a couple hundred watts of woofer to accurately simulate the sound that the original Leslie created with only 40W of amp!

This sounds surprising, but it's really not. I remember reading about an audio experiment years ago where the testers were trying to see how much amplification was required to make a recording of a pair of paper shears (big scissors) sound just like the actual shears. The test was to record the shears with a very accurate mic, then play back the recording and use the real items in a blindfolded test. If the subjects could tell the difference, especially during the first "snip," then more amplification was added and the tests were repeated. Guess what? They got up to 1200 WATTS, and listeners could still tell the difference! Why? Because if you think of the shears as a "musical instrument," it only makes one noise, but it does it very efficiently and naturally. The general-purpose audio amp, however, starts out with a handicap, particularly at the first sound impulse. The moral of the story: Don't get hung up on watts alone when creating or evaluating a particular sound.

As for Keith Emerson and Seth Justmann (J. Geils Band) using double tweeters: The issue is one of boosting the volume. The original 15" Leslie woofers were good for about 65W or so. Even in 1970, I could easily bump that to 150-200W and still remain in a 15" speaker format; using 2003 technology, I can go to 600-1000W. However, I can't triple the horn output in a 3/4" throated horn -- certainly, not with 1970's technology. The only easy way to add volume to the tweeter was to increase the number of tweeters; hence, extra Leslie tops were used. It's a little easier today, but there still is the issue that the throat of the horn is 3/4" wide, and so only a certain amount of air can be moved through that opening at any given frequency.

Regards,

-BW

--
Bruce Wahler
Design Consultant
Ashby Solutions™ http://consult.ashbysolutions.com
978.386.7389 voice/fax
bruce@ashbysolutions.com