impedence?

To figure out the nominal impedance, all you need is a multi-meter. You should get about 5-6 ohms DC resistance for an 8 ohm driver, and 2-3 ohms on a 4ohm driver.

a multimeter will not tell you a driver's impedance, rather you can
use this instrument to measure a voice coil's dc resistance. impedance is
resistance to ac, it takes into account not just the resistance but also
reactance and reluctance, which in any coil are relevant figures.

in short, when you pass a current through a conductor it sets up a
magnetic field. now, either a changing magnetic field affecting a
conductor or a conductor moving through a magnetic field will cause an
induced emf in the conductor, the current flowing from which sets up it's
own magnetic field which is directly in oposition to the field which
created it.

clearly, with dc this field is set up once and then remains static until the
current flow is stopped, hence with dc you read resistance, simply how
hard it is for current to flow through the conductor.

now with ac, you are constantly setting up a magnetic field in one
polarisation, then collapsing it, then setting it up again in the opposite
direction etc, as many times a second as the frequency of your supply.
because the field is not static it is constantly fighting the supply that you
set up (lenz's law), hence why you have a different reading.

potentially confusingly, resistance, reactance and impedance are all
measured in ohms, but that is because they are all effects that resist
the flow of electrical current. you can use something called an impedance
triangle to see how they all affect each other, there is a particularly nice
interactive one available here:

http://webphysics.davidson.edu/physlet_resources/bu_semester 2/
c21_impedance_tri.html

james.

mardy hippy.

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w-man Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 04 May 2006 Location: Australia Status: Offline Points: 12	Post Options Post Reply Quote w-man Report Post Thanks(0) Quote Reply Topic: impedence? Posted: 04 May 2006 at 10:56am
	Hi all, just wanted to know,can you measure a speakers impedence if you dont know what it is?And if so what do you need to do it? Any help would be great. Thanks.
	Keep conventional cabinets alive!

Conanski Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 26 January 2006 Location: Ottawa, Canada Status: Offline Points: 2544	Post Options Post Reply Quote Conanski Report Post Thanks(0) Quote Reply Posted: 04 May 2006 at 3:24pm
	To figure out the nominal impedance, all you need is a multi-meter. You should get about 5-6 ohms DC resistance for an 8 ohm driver, and 2-3 ohms on a 4ohm driver.

pooju Members Profile Send Private Message Find Members Posts Add to Buddy List Registered User Joined: 04 April 2006 Status: Offline Points: 254	Post Options Post Reply Quote pooju Report Post Thanks(0) Quote Reply Posted: 11 May 2006 at 6:45am
	not sure you thinking about this, but, I assume you know that impedance changes with frequency - not that you will notice when testing with a multimeter as above, though its worth considering if you're gettin an new amp. For example fane 12xbs drop down to about 3-4 ohms around the 180 mark if memory serves. but is upto like twenty around the 30s n 40s so to save power cross where its low, Edited by pooju - 11 May 2006 at 6:48am

james folkes Members Profile Send Private Message Find Members Posts Add to Buddy List Old Croc Joined: 08 January 2005 Location: United Kingdom Status: Offline Points: 3064	Post Options Post Reply Quote james folkes Report Post Thanks(0) Quote Reply Posted: 11 May 2006 at 11:51am
	a multimeter will not tell you a driver's impedance, rather you can use this instrument to measure a voice coil's dc resistance. impedance is resistance to ac, it takes into account not just the resistance but also reactance and reluctance, which in any coil are relevant figures. in short, when you pass a current through a conductor it sets up a magnetic field. now, either a changing magnetic field affecting a conductor or a conductor moving through a magnetic field will cause an induced emf in the conductor, the current flowing from which sets up it's own magnetic field which is directly in oposition to the field which created it. clearly, with dc this field is set up once and then remains static until the current flow is stopped, hence with dc you read resistance, simply how hard it is for current to flow through the conductor. now with ac, you are constantly setting up a magnetic field in one polarisation, then collapsing it, then setting it up again in the opposite direction etc, as many times a second as the frequency of your supply. because the field is not static it is constantly fighting the supply that you set up (lenz's law), hence why you have a different reading. potentially confusingly, resistance, reactance and impedance are all measured in ohms, but that is because they are all effects that resist the flow of electrical current. you can use something called an impedance triangle to see how they all affect each other, there is a particularly nice interactive one available here: http://webphysics.davidson.edu/physlet_resources/bu_semester 2/ c21_impedance_tri.html james.
	mardy hippy.