Crossover impedance spike effects

[webba]

Hi all!

I am building my first DIY speakers. I've had some assistance with the crossover (actually I did design my own but eventually asked for some help and had a better one 'donated'!).

My question relates to the impedance spike (which I've seen on many crossovers in the same area around 20-50hz) - This is around the resonance frequency of the driver, yes? Could anyone explain what causes it to be so high? Average impedance is around 4 ohm but spike reaches 24ohm.

Also, what are effects this has on the amps ability to deliver current in this range? And the effects on sound output here?

My 'beginners' knowledge tells me it shouldn't give acceptable results in this range. But if thats the case - why do so many designs have this?

I have a valve amp - any particular considerations special to valves?

Thanks in advance!

Andy.

[planet10]

The rise in impedance at low frequancy is the findemental resonance of the system. This is where your speaker is rolling off. Except for some careful designed aperiodic systems where one works hard to suppress this resonance, it is a feature of all systems.

Tube amplifiers often have a higher output impedance (not a bad thing, just has to be considered in the light of the amp/cable/speaker system). If you have impedance deviations elsewhere, ie 2 ways commonly have a big spike in impedance at the XO, then the interaction of the impedance of the speaker and the output impedance of the amp will cause variations in the FR that follow the impedance

dave

[webba]

Just for my information: The fundamental resonance of the system. Is this the sum of the interaction of the drivers / air / enclosure / port? Ie changing any of these would change the resonance? If its one of those things we just have to live with I can understand that, but I'm trying to understand the physical processes that produce it / act on it.

Is it more to do with the magnetic field in the coil at low frequency ie the inductive effect?

My amp has 4 and 8 ohm taps. How does this relate to the output impedance you mention?

Presumably ohms law is applicable here, so if the voltage is constant, but the impedance (resistance) increases, then less current must flow. Is that right?

If less current flows that means the driver doesn't get what it needs to reach the pistonic movement it should have for the signal its being asked to reproduce? (aka roll-off)

Please correct or congratulate me on today's little puzzle. I have many more still to come. (Related to inductor / capacitor choices in crossover and their effects on electrical phase - and while we're at it the effect this may (or may not?) have on acoustic phase? - Anyway leave that for another post...)

Thanks in anticipation...