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 Post subject: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Sat 08, 2017 10:57 pm 
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It's ben so long I can't remember. Haven't been doing much in this category in the past couple decades. I need to determine the impedance of a couple of speakers so I can replace them with the proper impedance. IIRC, there is a way to measure with an ohm meter and then multiply by a fixed number that roughly tells you what the impedance is.
I cannot remember that multiplier.
Can anyone tell me what that multiplier would be?
Mark D.


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 Post subject: Re: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Sat 08, 2017 11:05 pm 
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It is not exact, but what sticks in my head is 1.5. I think that it was Leigh that pointed out that a higher power speaker could have an even lower DC resistance for the same impedance.....i.e. a higher ratio of impedance to resistance.

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 Post subject: Re: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Sun 09, 2017 12:06 am 
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In general, measure the resistance and use the next higher value standard impedance. For example, if the resistance is 2.9 ohms, you have a 3.2 ohm speaker. If you measure 7.2 ohms, it is an 8 ohm speaker. Of course, there are speakers with odd impedances that cause this method to fail but it will usually get you close enough.

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 Post subject: Re: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Sun 09, 2017 12:39 am 
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Thanks! As soon as I saw 1.5 I recalled that was what I used way back when.
The other way, going to the next highest standard impedance brings it to about the same thing for standard speakers. So I guess either way works.

Mark D.


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 Post subject: Re: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Sun 09, 2017 9:16 pm 
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Hi Mark D,
My experience is similar to that of Mark and Jim, posted above. I have found that multiplying the cold DC resistance by about 1.25 or 1.3 will usually put you in the general range of the speaker's impedance.

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Poston


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 Post subject: Re: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Mon 10, 2017 5:13 am 
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Here are a few curves:
https://www.google.com/search?q=speaker ... 20&bih=925
You can probably find a curve to support just about any rule of thumb you've ever heard.....
One of the biggest uncertainties is resonance.

How to not lose sleep over this: recognize that the average person will probably never hear the difference with an impedance mismatch of maybe 2X.

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 Post subject: Re: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Tue 11, 2017 4:36 am 
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pixellany wrote:
How to not lose sleep over this: recognize that the average person will probably never hear the difference with an impedance mismatch of maybe 2X.

Very true! And the older our ears get, the less we notice the difference. :(

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Poston


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 Post subject: Re: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Thu 13, 2017 6:34 am 
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Location: Powell River BC Canada
You could connect your multi meter to the speaker on the 200 mV AC range.

Then get someone to play a wind instrument*, directly at the cone. Whole playing,
have some else connect a resistance box in parallel, until the meter voltage drops
to 50 % of what it was. If you are using a Radio Shack Smart2 meter, switch to dB and
look for a 6dB drop.


* Or another speaker /amp running at 400 Hz.

Absorb what was found.

Then, from then on, know any loudspeaker can hear you.




:shock:

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 Post subject: Re: Determining the Impedance of a Speaker with Ohm Meter
PostPosted: Apr Thu 13, 2017 4:32 pm 
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pixellany wrote:
One of the biggest uncertainties is resonance.

In addition to the impedance plots that Pixellany pointed to I will add this one from the GR 1603-A Z-Y Bridge manual:

Image

This Cartesian graph looks quite different than most of the ones that Pix pointed to, but I like using these coordinates, as they give a better picture of how resistance and reactance vary together with frequency. The point of resonance is the point of maximum resistance and minimum reactance, at about 352 Hz.

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