Figuring Resistance (Impedence) ??

[spudler]

I have found some speakers that are not exactly the Impedence advertised. I know if you have 2 8 ohm speakers hooked up one way you get 4 ohm resistence and another way you get 16 ohm what is the proper term for the 4 ohm wiring in parrallel?? In series means one after the other right or 16 ohm?
What happens if per say I have a 7 ohm and a 8.5 ohm do I come up with a 7.75 ohm? Or does it mess up the whole equation if they are not equal?

[newey]

Spuddy-

When the 2 are equal, the math is easier. But the same equations apply in either case:

For series: Z_T= Z₁+Z₂

For parallel: Z_T= Z₁*Z₂ / (Z₁+Z₂)

EDIT: And, yes, "series means one after the other". Parallel means the leads of each one meet at common points.

Does this help?

[spudler]

Thanks. I always confude those terms. I have since posting found a huge amount of articles on amps and speakers and so on so I am slowly gaining knowledge and learning a ton of stuff I did not know.
Interesting formulas I thought for parallel wiring you just added the the Impedence and then divided that # by the number of speakers to get the new Impedence

[newey]

I thought for parallel wiring you just added the the Impedence and then divided that # by the number of speakers to get the new Impedence

Now wait just a doggone minute there, before anyone starts blowing stuff up!

If you have 2 8Ω speakers in parallel, that's 4 Ohms impedance (See: the formula above, giving 8*8=64, divided by 8+8= 16, result is 64/16, or 4.)

You have it as 8+8, which is 16, divided by the number of speakers, which is 2, giving a result of 8.

Which is not 4 in my math!

[sumgai]

spudler,

Jun 14, 2010 1:48:38 GMT -5 spudler said:

Interesting formulas I thought for parallel wiring you just added the the Impedence and then divided that # by the number of speakers to get the new Impedence

Err, not quite. newey's correct in his statement just above - you don't add anything, you simply divide the Ohm's value of one speaker by the number of speakers, and you're done right there. Of course, we're talking about where all the speakers are of the same impedance rating. Where that's not true, then newey's formula (way up above) gets the job done, as it works for unequal impedances too.

But where you have any number of speakers of unequal impedance ratings, you can get to the answer faster with this little gem:

1 / ((1/speaker a) + (1/speaker b) + (1/next speaker....))

On any handheld calculator, that's easy to do. Just invert the first speaker's rating, add it into memory, and repeat until you've run out of speakers. Recall that total from memory, invert it, and there's your answer. Again, while this works with any impedance values (equal or otherwise), if they're all the same, then it's still quicker to just divide the value of one by the number of speakers, and you're good to go.

HTH



sumgaiu

[ashcatlt]

Yeah, if they're all the same, you just divide the impedance by the number of impeders. There's no adding involved.

I don't really care for the parallel equation newey posted above because it only accomodates 2 objects. It's the same thing as sumgai posted, really, just "simplified". Take a minute to prove it, if you want.

Some rules of thumb to keep in mind, might help in verifying your "answers":

No parallel combination of resistances will ever end up with a total greater than the smallest individual resistance.

The closer the resistances are to equal, the closer it will be to the simplified (resistance/resistors) total.

If one value is significantly greater than another, it can usually be ignored since it will have minimal impact on the total. Many folks use what I call the "at least 10x rule of thumb".

[spudler]

opps sorry I should of tried that methoud first.
Sorry about that. I guess I am just so used to using same resistance speakers what I meant was I take the rating of the speakers and devide that by # of speakers. Ie 8/3= 2.67.
I was way off sorry about that.
Thanks for the formula as I had not had it before so I will put that in with my other info I have so I will have it in the future.
Sorry about the confusion here.
And I have read that the DC resistance of a speaker is not really a good way to figure them anyway as the AC Impedence (which varies with frequency) is actually what the amp sees as it is sending out AC voltage anyway.
I assume the reason they even list DC value is since it is easy to test with a meter so you can see get an idea of what the speaker is if not listed and you can see if it is blown or not.
I have noticed a few that are not what they are listed to be usually only off by 2 ohms. I had some 4 ohm that when I tested them showed between 2.5 and 3 ohms. I had a couple 8 ohm ones that were at 6 ohm also.

[sumgai]

spudler,

In general, most speakers exhibit a DC resistance somewhere between 2/3 and 3/4 of their rated AC impedance. That's not hard and fast, just a good "rule of thumb". But you're correct, all you can really do with a DC reading is check for an open or shorted voice coil.

HTH



sumgai

[wolf]

Or if you want a handy calculator for such calculations, there's this one:
www.1728.com/resistrs.htm

It has the formula there too.

(and yes, that is my website).

[spudler]

cool thanks that is pretty neat I will have to check out what else you have on the site.