Fi voice coils

[ChevyBoy95]

I'm looking at buying a single dual 2 fi q 15 to run at 1 ohm but it says online that the true voice coil impedance is actually 1.4 ohms per coil for the dual 2. Does this mean I will really be running at .7 ohms and if so do I need to buy an amp that's stable under 1 ohm??

You will more than likely rise to over 1ohm...maybe even over 2...I would not worry about it at allm Most subs when you use a DMM the coils come out under 1ohm or 2ohm load. my experience anyway.

don't post about what you don't know about... on a musical setup it will be dynamic, therefore the rise is COMPLETELY irrelevant.

[WillPwn4Food]

in what world would you end up going from .7 to over 2 from rise? Maybe to 1 but 2 seems a little excessive.

Wow.

What? Was I wrong?

[Smoove]

So having a bigger box with lots of port won't rise more than a smaller box with a smaller port???

Man this topic KILLS me every time it is brought up.

DCR is the DC resistance of the coil. It is the resistance of the coil when DC current is applied.

Impedance is the AC resistance of the coil when AC current is applied.

Leave rise out of it. For the 1,000,000 time - every sub has an impedance peak at the resonant frequency of the cone - in the box or sitting on your kitchen table. The peak is not just at that one frequency - the impedance starts to rise below the Fs and falls back down above it. Too many people take power readings and assume the impedance is rising due to the box, but most if the time it is just within this peak caused by the driver's resonant frequency.

Look at this impedance plot for a driver in free air:


It is an 8 ohm driver, but the impedance is as high as 35 ohms!! Does that mean the driver will get less power at 40 Hz? Yes, but does it mean the driver will be lose 6 dB at 40 Hz compared to other frequencies? NO!!! It is just how it works, and unless you are burping at a specific frequency or know the impedance at every frequency you will play, don't worry about it!!!!

In the case above, you played a 40 Hz test tone and said "man look how bad the rise is" and then tried to correct it by changing from an 8 ohm coil to a 1 ohm coil. Now what will happen the first time you play 110 Hz????

Enough for now. . .

[CortezDTV]

I was waiting for that Brian!

Dc reaistance is not the same as ac resistance!

You never have dc current eye ur playing music

Only time my subs ever see dc voltage is when I "pop" them to check polarity

[Kyblack76]

Yep.. Brian for the score....

[bkolfo4]

Smoove -

Putting the sub in the box will definitely effect the frequency vs. impedance plot.

One of the big factors is the drivers Resonant Frequency will change, since it is no longer free air. The size of the box will be a major factor in how the resonant frequency changes. The peak is still there, but it may not be as large, and it may have shifted enough that it is no longer in the frequency range you are measuring.

In a ported box, there is actually an impedance peak on each side of the port tuning frequency too (you can use this to find the true tuning frequency of your box with just a resistor and DMM).

There are several other factors, but these are a couple of major ones. This is why I continue to try to stress to people "impedance rise" is VERY misunderstood, and really does not apply to a music system unless you are willing to measure the impedance at each frequency that the system will play. Measuring your power at one frequency and then stating "I have a 4 ohm impedance rise" is not a good statement. But on the other hand it is very possible to have an impedance well above nominal for your subwoofer's entire frequency range. Look at the graph above - from 30 to 70 Hz the impedance is double+ the speaker's 8 ohm rating. Put the sub in a small box and shift that peak, you could totally change the impedance from 30-70 Hz, but it will now be high at a different range of frequencies.

That help any?

[Smoove]

I think it's starting to make sense now. I have built plenty of systems over the years and just never tried to understand rise until now. I know some people on here don't like robot underground, but he has told me some of this stuff before. it just didnt click until now. I guess the last part of this question for me would be. Why is that fosgate bdcp amps are not effected as much by rise as some people have stated? I do realize thats more about how the amp handles the rise, than it is the box or resonate frequency of the cone are seen.

[bkolfo4]

They can create amps that work over an impedance range in several different ways. I would be guessing to tell you how they implemented their design. But they cannot change the woofer impedance, and they DO NOT have to try to maintain constant power through the spike at Fs. As I stated above, the power to the driver at the Fs peak decreases, but the output does not decrease relative to the output at other frequencies. If you input 10 watts into the driver in the example above and plotted the output vs. frequency, there is NOT a big dip in output at 40 Hz. I repeat there is NOT a big dip in output at 40 Hz. This is the point people miss. Yes the impedance is high, but the output is flat in the correctly designed box.

Now if you are burping, you can use the actual impedance at your burp frequency to your advantage (remember, not only is the impedance at maximum at Fs, but the cone excursion is also at its minimum). If it is high, you use lower impedance coils, provide more power, and push the cone to the limit.