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The thought experiment.

Posted by DrChaos (A) on July 17, 2010 at 01:15:19

In Reply to: Series vs Parallel XO & Maggie Physics. posted by DrChaos on July 17, 2010 at 00:48:14:

Sorry to reply to my own post, but I came up with a physical thought experiment.

Take a regular electrodynamic driver connected to nothing. "thwack" it, which is the technical term for "apply a controlled physical impulse to the driver surface". :) What happens? It oscillates and over time gets damped as the energy is radiated acoustically and dissipated physically in the surround and flexing of the driver.

Take good power amplifier with low output impedance much lower than the driver, turn it on, feed in exactly zero volts, connect outputs the same driver. Thwack it again. What happens? Of course it is instantly damped because of the power amplifier connected across its terminals, since the thwackage creates back EMF but the low-output impedance amp ain't be going along with that.

Now consider two such drivers connected with a parallel XO (left side of Fig 1, http://sound.westhost.com/parallel-series.htm).

Thwack them both simultaneously. In this network, will the back EMF instantly be damped (and simultaneously) as in the single-driver-to-amp case? No, it will be "processed" by the inductor and cap. There will be damping of course and different from the 'amp disconnected' state, but it won't be instantaneous.

Next step, consider the series network, but the drivers are physically separated woof and tweet. Thwack the woof, and the back EMF will cause the tweet to tweet.

But finally consider the maggie, two sections on the same driver. The configuration where one side is totally still and the other is moving is energetically less favorable than configurations where they try to move as one.

Thwack it, generating back-EMF in both woof and tweet and look at the right hand side of the series network. I think it's going to damp instantly.

Look at the series XO network again.

Here is how I envision it. Imagine you had a single panel driver, but the mechanical properties & tension of the magic driver material smoothly changed from left to right. What would happen? Stiffer parts would respond more to high frequencies and looser more dissipative parts would respond more to the low frequencies.

So I consider the inductor and capacitor in the series network to be electrical equivalents of physically modifying the properties of a single driver across the diaphragm.

Now if you had such a single driver, would you not agree that if you connected the leads directly it to an amp fixed to zero potential and you thwacked it, the thwackage would be instantly damped?

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Kimber Kable

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