speaker impedence question

[daz]

If you run 2 speakers of the same model but one is 4 and the other 8 ohms, which one will sound louder and approximately how noticable would it be? Would it make running both a waste of time due to the one taking over and causing it to sound the same as with one?

[The Dude]

The only thing you can say for certain is that more current will flow through the 4 ohm speaker. Therefore, presumably, it would be louder. How noticeable? You'd have to judge that. Are both in the same cabinet? If not, one cabinet might be more efficient.

[pdf64]

In series, parallel or something else, eg different amps?

[Jazz P Bass]

Series or parallel?

[Gnobuddy]

If you run 2 speakers of the same model but one is 4 and the other 8 ohms, which one will sound louder and approximately how noticable would it be? Would it make running both a waste of time due to the one taking over and causing it to sound the same as with one?

All else being the same, and assuming the speakers are in parallel, the 4 ohm speaker draws double the power from the amplifier.

Double the power might sound like a lot, but our ears have progressively less and less response to more and more sound power (the response is logarithmic). This is why decibels were invented, and doubling the power is the same as a 3 dB increase.

So (all else being the same) the 4 ohm speaker will be 3 dB louder. This is certainly detectable, but not very substantial, as far as our ears and brains are concerned.

(One decibel is considered the smallest change our ears can reliably detect. It takes ten decibels to make a sound appear to be twice as loud.)

Only you can decide if it's a waste of time or not...keep in mind you have twice the expense, and twice the weight in speaker to haul around, and your amplifier has to be comfortable driving a 2.66 ohm load (that's 4 ohms and 8 ohms in parallel).

-Gnobuddy

[Enzo]

That all else equal part is the kicker. If one speaker gets twice the power, it is 3db louder for that. But if the other speaker is 3db more efficient, then they wind up being equally loud. In other words there is more to it than simple impedance.

[Gnobuddy]

That all else equal part is the kicker.

Agree with you totally. But Daz did say "of the same model". Speaker designers often try to make their 4 and 8 ohm versions of the same speaker have about the same efficiency (they will tweak both magnet and voice coil to get the same BL product.)

So, if the speaker designer did his/her job properly...

-Gnobuddy

[daz]

Series.

[Gnobuddy]

Series.

Well, that changes things!

Now you have one-third of the total signal voltage across the 4 ohm speaker, and two-thirds of it across the 8 ohm speaker.

That means the 8 ohm speaker has twice as much voltage across it as the 4 ohm speaker does.

The combined effect of twice the drive voltage, and twice the speaker impedance, is to deliver twice the power to the 8 ohm speaker.

So, if all else is the same (same speaker efficiency for both drivers, etc), then it is the 8 ohm speaker that will now be 3 dB louder. Exactly the opposite of the parallel-speaker case.

I should add that it's not unusual for the fundamental resonance frequency of nominally identical speakers to vary randomly by a few percent, and because the speaker impedance peaks and varies sharply around that frequency, it's quite likely that the power sharing between the two speakers at frequencies right around resonance will not follow the simple reasoning outlined above.

But "right around resonance" is only a small part of the overall frequency range, so even if somewhat unpredictable things happen in that region, the big picture shouldn't change too much. All else being the same, the eight ohm speaker will be 3 dB louder.

-Gnobuddy

[Enzo]

The bottom line is that one of the speakers would be a little bit louder than the other, and three feet away you'd never notice that. Nothing cosmic will happen.

So, if the speaker designer did his/her job properly...

You are right of course, but that also assumes the manufacturer has the goal of making them identical, instead of marketably similar. I suspect when Eminence turns out tens of thousands of speakers, that when it comes to 8 versus 16 ohm or whatever, they are going to use the same basket/magnet assembly for both. I could be wrong of course, and they might adjust gaps and magnet charges, but I doubt it. I think they grab a basket assembly from one stack, and a voice coil and cone from another, and put them together. ( I mean for a given model, not for ALL 12" speakers.)

I further suspect that combining the 4 and 8 ohm versions of speakers is not on the list of intended uses by the maker. And they likely are not tweaking anything to get them to sound the same in that application.

[daz]

Thanks guys, that helps. I wouldn't even be asking or thinking of doing this but it's a unique situation rather longwinded so i would go into it. But suffice it to say it's the only solution for what i want given a certain speaker that i love in 4 ohms is so rare it would be impossible to find another in 4.

[mooreamps]

Series.

both speakers should be a little less loud ; since total speaker goes up ; just a little bit.....
Both is right ; the sky will not fall regardless.....

-g

[R.G.]

Welcome back Gary.

[J M Fahey]

Thanks guys, that helps. I wouldn't even be asking or thinking of doing this but it's a unique situation rather longwinded so i would go into it. But suffice it to say it's the only solution for what i want given a certain speaker that i love in 4 ohms is so rare it would be impossible to find another in 4.

Can you be a little more specific?
What spekers are we atalking about?
And what amp?
Are other speakers involved?
Must it fit in an already made cabinet or is it a new buiuld?
because in the latter case I might suggest something.