How can floors and bookshelfs have same impedance? - AVS Forum | Home Theater Discussions And Reviews
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Old 06-26-2016, 09:15 AM - Thread Starter
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How can floors and bookshelfs have same impedance?

So I'm having trouble finding the answer to this on google.

How can a floor speakers (with 2 or 3 of the same woofers) impedance be the same as the bookshelf from the same line with only one identical woofer?

For example: The Elac Uni-fi UB5 bookshelf and the UF5 floor are both rated at 4 ohms, but the floor has more than one woofer. Is it not the speakers themselves that determines impedance? Maybe something in the electronics (crossover)?

Thanks in advance. My confusion stems from the fact that when wiring additional speakers in series, you halve the load on the amp, and in parallel you double it. How does a cabinet with multiple identical woofers maintain the same impedance as ones with only one?
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Old 06-26-2016, 09:27 AM
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Quote:
Originally Posted by Justin Ernst View Post
How does a cabinet with multiple identical woofers maintain the same impedance as ones with only one?
You can't assume that the woofers are identical. All you have to do is use a different voice coil to get any impedance you want, wiring them as required for the net impedance. For instance, two 8 ohm woofers in parallel are 4 ohms, two 4 ohm woofers in series are 8 ohms. OTOH you can wire four woofers as series/parallel pairs to get the same impedance as one, or even nine as series/parallel triplets to get the same impedance as one.
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Old 06-26-2016, 09:42 AM
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Bill is exactly right, most manufacturers offer the same drivers in 4 and 8 ohm (see Parts Express for example).

Also a lot of commercial speakers use passive radiators which can confuse things even more.

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Old 06-26-2016, 10:09 AM
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Many driver suppliers will customize drivers for the designers needs, such as this 14 ohm Scanspeak 18W/8535 which I believed was used in the proac Response 3.8.



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Old 06-26-2016, 11:15 PM
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Wow...you joined over 9 years ago and just did your first post. Congrats!
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Old 06-27-2016, 04:07 PM - Thread Starter
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Thanks for the replies guys. I'm still not super clear on exactly what determines impedance but I'm closer. But to be clear, if a floor speaker has 100 watts running through 3 woofers hypothetically and is rated at 8 ohms, the woofers are splitting the power three ways compared to a single woofer 8 ohms speaker correct?
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Old 06-27-2016, 05:46 PM
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The Impedance is determined by the drivers, their impedance, and the Configuration/Wiring of the speakers. And to some extent by the cabinet.

Also, since manufacturers have immense buying power, they can afford to have speaker custom designed. In various speakers within the same design concept, some could be 4 ohm, some could be 8 ohm, and in the example already given, some could be 12 ohms or 14 ohms.

The ELAC UB5 could be a 2.5-way speakers, meaning that at ultra low frequencies, both the 5" Woofer and the 4" Uni-Fi speaker could be running in parallel. So, at those low frequencies below 270hz, the impedance of two 8 ohm drivers would be 4 ohms. (8/2 = 4 ohms)

Though I can't say with certainty, the ELAC UF5 floorstanding with three 5" bass drivers could use three 12 ohm drivers in parallel (12/3 = 4 ohms). The woofer could in all other respects be essentially the same driver, but simply wound with 12 ohm voice coils.

Now I can't say that is definitely how it works, but it is a reasonable explanation of the difference in the speaker design, but the speaker still have the same 4 ohm impedance.

In other designs, two woofer of 4 ohm each could be in Series for a combined impedance of 8 ohms. Two Series speakers put out the same amount of sound as a single identical driver, but they have twice the power handling and half the excursion, which can be an advantage in the right design.

As someone else mentioned, it is possible to wire 4 speaker to have the same impedance as a single driver -



Though I don't have a diagram, you can also wire 9 speakers with a total impedance of a single driver. Using 8 ohm drivers, if we put three in series, then put three gangs of three in parallel, the result is the same as a single driver.

Three in series is 3 x 8 = 24 ohms.

Three Gangs in parallel is 24/3 = 8 ohms. The entire gang of nine is the same as a single driver.

Steve/bluewizard
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Last edited by bluewizard; 06-28-2016 at 07:44 PM.
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Old 06-27-2016, 07:11 PM
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In short, it's up to the designer. He (or she) has control over the specs they wish to result. That said, there is often a marketing component in stated specs. 8 ohms is the safe choice for manufacturers to advertise, however the real answer is usually much more complex, with dips and peaks in the actual response curve.

The best you can do is to view the actual ohms, across the measured response curve of the speaker, to understand the reality (or not) of that spec.

Last edited by RayGuy; 06-27-2016 at 08:33 PM.
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Old 06-28-2016, 10:51 AM
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Wow...you joined over 9 years ago and just did your first post. Congrats!
My guess is that he was waiting to show off his Proac driver for just the right question. Way to play the long game.
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