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In Reply to: RE: Compression - you hardly ever see measured posted by Chip647 on July 29, 2014 at 05:05:19
As far as I can tell the problem is not so much a drivers power compression which is rare to happen in a domestic environment but the fact that the voice coils heat up thus increasing resistance which in turn changes the response of passive crossovers.
This is kind of a double whammy since the crossovers inductors will also heat up and change the response even more.
This is largely avoided by using active speakers which can be driven to much higher volumes without changing sound than passives.
Follow Ups:
"the fact that the voice coils heat up thus increasing resistance which in turn changes the response of passive crossovers"
Well... That would change the response of the system due to the DC impedance shift of the load but not that of the crossover itself. Of course it will heat up also but probably not as much or as fast.
What I found fascinating about your post is you have me wondering if this is the reason or at least part thereof that many listeners seem to prefer a more matched source such as open-loop tubes/transformers. That should largely dominate the the thermal response of the speakers, especially the woofers. Of course it also introduces another artifact: worse damping. But I wonder if that might not be a good thing in many case?
I've orders of magnitude more power than I need so I think I'll try some build-out resistors just for fun. My room is pretty dry acoustically which I think is good in general but maybe wetter woofers would work wonders...
Rick
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: 07/29/14
"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.
That is probably why high efficiency drivers tend to sound much more dynamic than low efficiency drivers. They don't heat up as much playing at any given volume level so they don't suffer from compression caused by rising resistance. Also, because crossover characteristics are based on a fixed resistance of the drivers, changing resistance due to heating messes with how the crossover operates.
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