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In Reply to: RE: Compression - you hardly ever see measured posted by rick_m on July 29, 2014 at 08:04:27
Firstly there is no such thing as 'DC impedance'. If it's DC it is called resistance.Secondly if the driver impedance changes (resistance is part of it) the crossovers crossing point changes too in not necessarily predictable ways and that is really not good.
This also the reason why a xover designed for 4 Ohm drivers can not be used with drivers of a different impedance.For example a 2nd order low pass at 2000Hz for a 4 Ohm driver requires a 0.593mH inductor in series and a 18.51 µF capacitor in parallel.
A 2000Hz low pass for an 8 Ohm driver wants 1.185 mH and 9.254µF.
As you can see the inductor needs to be bigger while the cap needs to be smaller so a driver which has changed impedance due to heating of the voice coil will play havoc with the correct functioning of the crossover.PS: I do not like under damped woofers at all. They sound 'mushy' and lose a lot of detail.
I do not use tube amps for that very reason.
All my favourite speakers are active and thus the woofer is maximally damped. Gets me closer to the original sound than anything with a passive xover. The worst sound (as in furthest from reality) to my ears are passive speakers driven by tiny SETs. There is just nothing right about the sound from them.
Edits: 07/29/14Follow Ups:
"I do not like under damped woofers at all."
I don't usually either and especially dislike ported speakers. "Hey, your speakers have a great bass note!"
I think having a Zero at DC is just a bad idea and offsetting it with a nearby pole does not really fix things so I buy "acoustic suspension" speakers. Yet I have heard a few ported one's that were at least OK. Implementation rules.
I've never ran low-level crossovers but logically they sure seem like a good idea.
Rick
On the whole I am fine with ported speakers but as you said: Implementation is everything!
The ones I do have problems with tend to be small ported speakers.
Lots of times manufacturers try to squeeze more bass extension out of them than is healthy because it makes good ad copy.
The other problem is that even if they are properly designed without excessive extension porting unloads the driver below the tuning frequency resulting in exponentially rising excursion but nearly no sound being produced. This leads to increased IM distortion and port 'chuffing'.
Because of this IMO small ported speakers should be fed an appropriately high-passed signal.
If done correctly there is no real difference between sealed and ported regarding transient and phase responses.
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