fi coil reading?

[Nikitaaa]

You are measuring with a meter that can only measure DC resistance. DC resistance is like a car battery or a 9 volt battery, AA battery, etc.

If you applied a 9 volt battery to you sub coil, the cone would to position X and sit there. This is the type of resistance your meter will measure.

Your meter will not measure AC impedance. Impedance is the "resistance" of the coil when an AC voltage is applied. AC is like a test tone or music - the voltage alternates between positive and negative at some frequency X (a wall socket is an AC voltage of 120 volts at 60 Hz). Impedance varies with frequency for multiple reasons. The two main reasons for a sub are 1. The resonant frequency of the moving assembly, and 2. The natural low pass filter effect of the coil acting like an inductor.

When the sub is rated "2 ohms nominal", this is like an average impedance over the usable bandwith a couple of other considerations on how low the impedance is at the lowest point (it will never be lower than the DC resistance).

A big issue people have is the BIG spike at Fs (and it is not just at that frequency - it is liek a bell curve if you plot it). People 90% of the time have no idea the spike is even there and call it "impedance rise" thinking it is occurring due to the sub being placed in a box. They have no idea it would still be there if the sub was out of the box sitting on a table. A lot of the subs we use in the car have an Fs around 30-35 Hz, which creates a much higher impedance in the frequency range we like to use them. That is why you constantly see people clamping the output from their amps and then they say "I got X watts rising to Y ohms". It really is not "rising" like they think, that is just the impedance of the sub at that frequency.

The good thing is that this spike does not matter for playing music. I repeat - it DOES NOT MATTER. The sub still plays just as loud at Fs as it does where the impedance is at a minimum. If you really want to know what is going on with your amp and sub, you would need to clamp at each frequency in the usable range.

And one final thing on impedance - 95% of power to a sub turns to heat. Coils get very hot. Resistance and Impedance increases once you are playing the sub for a couple of minutes. . .

I was completely unaware that the climb in impedance rise had to do with the fs of the subwoofer. I guess it's not as useless of a perimeter ad some suggest.

[bkolfo4]

Just don't forget for music it really does not matter.

Let's say the sub has an impedance of 1 ohm at 60 Hz, but it is 2 ohms at 35 Hz due to Fs (just made that up for an easy example). That means if you plan on playing the sub at 60 Hz, your amp has to be 1 ohm stable, but it does not mean that it will be 3 dB louder at 60 Hz vs. 35 Hz..

[SnowDrifter]

My subs have an Fs of 49hz. If that's the case why have I never measured where that's the bulk of my rise (or spike)? My impedance spike is right around 38-40hz

[CortezDTV]

U might have 2 spikes then

[Nikitaaa]

How long was the build time on your sp4's, guys?

[bkolfo4]

There are additional impedance spikes from port tuning and the Fs of the driver/system can, and most likely will shift slightly once the sub is in the box.

That is why I have said many times that when people say smaller boxes have less rise, there is a good chance those impedance peaks have moved based on a higher system resonant frequency (although every time I have measured impedance of a system for various tuning frequencies, the peak does not move much).

My point of the post was just to make sure the OP understands a dual 2 ohm sub is not just a flat 2 ohms. . .