View Full Version : How to test for ohm's


jfine
02-14-2007, 07:19 PM
I know, probably been covered before but I couldn't find it.

How can you test a speaker to see what ohm rating it is, 4, 6, 8, etc.,

This is not always a single speaker test, but rather let's say you have a 3-way system, so you want to measure the ohm at the terminal on the back of the enclosure.

This may sound stupid, but can this be done with an ohm meter?

.

onplane
02-14-2007, 10:01 PM
I know, probably been covered before but I couldn't find it.

How can you test a speaker to see what ohm rating it is, 4, 6, 8, etc.,

This is not always a single speaker test, but rather let's say you have a 3-way system, so you want to measure the ohm at the terminal on the back of the enclosure.

This may sound stupid, but can this be done with an ohm meter?

.


You can measure with an ohm meter and read the DC resistance of just the woofer + coil. Is this the "ohm rating" of the speaker system? No! It's not.

Actual speaker impedance is NOT a constant. Impedance of a speaker system will vary with frequency. Manufacturers measure impedance over the speaker's entire audio range and then publish an "average".

You simply can not do the same with just a multi meter.

Regards,
Jerry

MarkAnderson
02-14-2007, 10:16 PM
What speakers are we talking about?

Lefty
02-14-2007, 10:43 PM
You can measure with an ohm meter and read the DC resistance of just the woofer + coil. Is this the "ohm rating" of the speaker system? No! It's not.

Actual speaker impedance is NOT a constant. Impedance of a speaker system will vary with frequency. Manufacturers measure impedance over the speaker's entire audio range and then publish an "average".

You simply can not do the same with just a multi meter.

Regards,
Jerry


Unfortunately the units of measurement for resistance and impedance are both ohms and that tends to be confusing for those new to electronic theory.

They are not the same thing. Impedance of a component or circuit is the sum of it's resistance and it's reactance. Reactance is frequency dependendant as onplane stated and can be either inductive reactive or capacitance reactive. There are complex measurement devices that can read impedance directly but it is certainly not something that a DMM can measure directly. Impedance can be measured with a small array of devices working together and I've seen it published in a few speaker DIY sites where they show how to design and measure speaker crossover circuits.

So when you measure the resistance of say a speakers voice coil the DMM will display the DC resistance of the voice coil but without the value of the induction of the voice coil one can't calculate the impedance of the VC.

To simplify it, in basic electronic courses, they frequently define impedance as AC resistance as they then go on to explain the fundamentals of AC electronics. This is why such courses usually start with DC electronic then move on to AC electronics next. The math also steps up a notch moving to AC theory also.

Make sense?

Lefty

jfine
02-15-2007, 12:16 AM
Thanks so far guys, I don't know the actual make of the speakers I have, they are DIY (by someone else) and they may be some brand name drivers but they may not.

But really what I'm after is, if you have an unknown like this, and let's say you have a 2325, and you want to try to use the Main and Remote together, but according to another thread, you don't want to use 4 ohm on both Main and Remote at the same time, well then there must be way to find out what you have so you don't ruin something???

.

onplane
02-15-2007, 12:31 AM
Thanks so far guys, I don't know the actual make of the speakers I have, they are DIY (by someone else) and they may be some brand name drivers but they may not.

But really what I'm after is, if you have an unknown like this, and let's say you have a 2325, and you want to try to use the Main and Remote together, but according to another thread, you don't want to use 4 ohm on both Main and Remote at the same time, well then there must be way to find out what you have so you don't ruin something???

.

OK, that's a much different question!

Use your multi meter to measure the DC resistance of the woofer + coil. Since vast majority of power is consumed by the woofer, this will help you decide whether you will tax your amp.

If you measure 3 to 4 ohms, assume you have a 4 ohm speaker system.

If you measure 4 to 5 ohms, assume it's a 6 ohm system

If you measure 5+ ohms, assume it's an 8 ohm system

These are "rough" guides, but for purposes of protecting your amp will be good enough.

Regards,
Jerry

jfine
02-15-2007, 12:53 AM
OK, that's a much different question!

Use your multi meter to measure the DC resistance of the woofer + coil. Since vast majority of power is consumed by the woofer, this will help you decide whether you will tax your amp.

If you measure 3 to 4 ohms, assume you have a 4 ohm speaker system.

If you measure 4 to 5 ohms, assume it's a 6 ohm system

If you measure 5+ ohms, assume it's an 8 ohm system

These are "rough" guides, but for purposes of protecting your amp will be good enough.

Regards,
Jerry

Jerry thanks.

So, question. Would it make any sense to measure this at the end of the speaker cable that attaches to the amp--would the ohm measurement be different? Since I dont know, I am just assuming something like resistance thru length of cable....

Also, when you say "woofer + coil", it sounds like I am not measuring at the 2 speaker terminals? Or why not use the terminals at the back of the enclosure? Wouldn't this take into consideration the crossover and whatever all else is in there?

.

onplane
02-15-2007, 02:04 AM
Jerry thanks.

So, question. Would it make any sense to measure this at the end of the speaker cable that attaches to the amp--would the ohm measurement be different? Since I dont know, I am just assuming something like resistance thru length of cable....

Also, when you say "woofer + coil", it sounds like I am not measuring at the 2 speaker terminals? Or why not use the terminals at the back of the enclosure? Wouldn't this take into consideration the crossover and whatever all else is in there?

.

You could measure the resistance at the end of the speaker cables, but then you are NOT measuring the speaker alone. You are also measuring any resistance in the speaker wires.

You SHOULD measure at the speaker terminals at the back of the enclosure. The crossover for the mid and high frequency drivers always consists of caps. These caps block DC and prevent any meanfuling measurements.

Net result is you are measuring the DC resistance of the woofer and it's "in series" coil. This is plenty good enough if you are just trying to make sure you don't over tax your amp.

Regards,
Jerry

osmium
06-29-2011, 02:18 PM
I don't know if this would work and I have not tested it. Maybe some of the other posters here would have an idea.


Disconnect your speakers.
Generate a 60hz sine wave tone and set your amp to a fixed gain level.
Use a multimeter and find the AC voltage, V.
Without adjusting the gain on your amp, connect the negative (common) lead of your speaker wire to your speaker.
Use your multimeter as an AC ammeter and complete the circuit with the red multimeter probe connected to the other speaker wire lead and the black probe to the positive (hot) terminal. You should get a current reading returned, I.

Use Ohm's law: V=IR. You know V (voltage) and I (current). Solve for R (resistance). R=V/I.


Since speaker impedance is an average across many frequencies, perhaps this procedure can be done with pink noise with a true RMS multimeter instead of a 60hz sine. Only a true RMS multimeter would be able to find the output voltage of the amp producing pink noise. Cheaper multimeters base their voltage determination on an assumed 60hz signal. Of course you could always use a oscilloscope if you had access to one. Using a very short length speaker wire would minimize the effects if any of the resistance of the speaker wire.

Thoughts?

Jon_Logan
06-29-2011, 02:32 PM
I, mostly, agree with this method, but the meter acting as an ammeter will have some series resistance. It would be better to have a second meter (rms ACVM) across the speaker terminals so that the series current and the voltage across the load can be known at the same time.

Since the impedance will vary with frequency, having pink noise as the stimulus will make for a rapidly changing measurement result. The result would have to be integrated to give a smoothed, average impedance (voltage) result. Since the impedance varies so much with frequency, it would be better to step/sweep frequencies, then plot the E/I for the different frequencies.

Note: be carefull how much voltage you apply at the upper frequencies. Mids and tweets are delicato.

mhardy6647
06-29-2011, 02:49 PM
How to measure speaker impedance:
Choice 1)
http://sound.westhost.com/tsp.htm

Choice 2)
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=390-804&FTR=woofer%20tester

The nominal impedance of most speakers is based on the impedance plateau above the resonant frequency (Fs) - typically, for a woofer, the value in the neighborhood of 100 to 200 Hz. It is not an average value in any way, shape, or form.

mikeybc
06-29-2011, 03:12 PM
OK, that's a much different question!

Use your multi meter to measure the DC resistance of the woofer + coil. Since vast majority of power is consumed by the woofer, this will help you decide whether you will tax your amp.

If you measure 3 to 4 ohms, assume you have a 4 ohm speaker system.

If you measure 4 to 5 ohms, assume it's a 6 ohm system

If you measure 5+ ohms, assume it's an 8 ohm system

These are "rough" guides, but for purposes of protecting your amp will be good enough.

Regards,
Jerry



I agree with these guidelines for the most part though both pairs (all 4) of my 8 ohm rated Energy 22's measure exactly 4.1 ohms so there's probably some exceptions or different methods used by manufacturers to determine overall system impedance.