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Date: August 05, 2003 at 19:25:06
From: Adrian Mack, [dialup-196-172.wasp.net.au]
Subject: Mechanical resonance of speakers

URL: My website, Peerless XLS 12 Dual PR Sub Project, 18LW1400 vented sub, Horns, etc.....



Hi Brian!

> For example, consider a free-air (or infinite-
> baffle) system. If power input is kept the same
> while the input frequency drops, the driver's cone
> will undergo increased excursion. With that
> increased excursion comes increased distortion.

Exactly. And in systems where excursion is reduced, which is best handled in vented systems, distortion is reduced also. And the point about Acoustic Suspension and IB systems and overring, also applies too, and a reduction in overring is a reduction in harmonic distortion too. But since the biggest culprit in this region is magnetic and not mechanical, I'm not sure how much shift is done to distortion figures in that area. I am sure that overall distortion is reduced, however, and that's a good thing. A cabinet built around Davies implementaion of the Thiele/Small works or a PiAligned cabinet will yield even greater results and less overring than the two aforementioned cabinets.

But the magnetic structure does play a major part in distortion, and poor drivers typcially having really loose suspensions (high Qms) will simply "ring all day long" once you hit them with a few short bursts at resonance, no matter what cabinet we put them in in attempt to damp the driver. Data published by JBL and others attributes this to a concern primarily from the magnetic structure, and not the suspension or diaphram. But just look at the distortion graphs of any woofer, and you'll notice that from about a half octave above resonance, distortion rises dramatically. Certain motors have actually been designed to operate the driver below resonance, so it may be that distortion drops here again. But it is a common trend that I've not seen an exception for - That woofers peak distortion is in the resonance region.

Adrian


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