how to stop inpedence rise

change box? i dont think you can stop it but a new box might help drop it a little

magic, or witch craft.

you sure its not a d2?

is it bigger or smaller box to help lower imp rise?

Click to expand...

Smaller

Your wiring is not causing the impedance rise //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

What you need to realize is that contrary to what you might think, resistance and impedance are not the same thing. Resistance is merely a part of a system's overall impedance. When your sub is just sitting there, it has an impedance of one ohm per coil. That number is a resistance. However, as soon as you start feeding your sub a signal that isn't just DC, aka, a music track with different frequencies, you start dealing with not just the resistance of the coil, but impedance of the coil which is taking into account capacitance and inductance.

Here, the main culprit is inductance. Basically all a speaker voice coil is at rest is a resistor with whatever resistance is stated on the side of the box. Under a signal, it is behaving more like an inductor (aka a low resistance coil which stores energy in a magnetic field), meaning that the current flowing through it is creating a magnetic field, which when interacting with the polarity of the motor, creates the excursion. At the system's resonant frequency, which is far more a factor in ported enclosures than sealed, the impedance is naturally going to rise for reasons I won't bore you with. Note that it's impedance, NOT resistance, so changing your wiring out won't make a single difference.

The way to "fix" it is to make your resonant frequency higher, aka, make your box smaller. However, for obvious reasons, you still need an audio setup that works the way you want it to, thus people factor in the rise in impedance as part of the design so that they know how to get the most out of their amplifiers. I hope that helped //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

http://forum.soundpressure.com/showthread.php?t=6796

Click to expand...

If you read through the whole thread, there are 2 opposing viewpoints, both of which are equally valid, to the extent of my knowledge.

1) Over-powered burps, tend to do well in a peaky small box will help the woofer maintain the damping necessary to control the cone at higher than recc. power. Since burps are short, you will have a tougher time burning up the coil (to a point)... so it's the way most SPL folks run.

2) Bigger than recommended box, plenty of port area, and high tuning can give a more efficient setup, with nice low impedance rise (at resonance)... but only if you are running a normal power level through it.

From my understanding, the impedance rise is a tug-of-war between the "resistance" generated by large AC current running through an inductor (the coil) and the decrease in current caused by this added "resistance." I would love for an expert to chime in on the true reason behind it, though.

Many people disagree with Option 2), but I can verify that a big efficient box can have low impedance rise. My subs are request 0.9-1.2 cf, but I have them in 2.5 cf each, along with monster port area (for 2 10s). My impedance rise is 1.0 --> 1.8-2.0 @ 52 Hz...

I am nearly bottoming them out, and they **** royally @ 30 Hz... but most SPL boxes don't extend well.

If you read through the whole thread, there are 2 opposing viewpoints, both of which are equally valid, to the extent of my knowledge.
1) Over-powered burps, tend to do well in a peaky small box will help the woofer maintain the damping necessary to control the cone at higher than recc. power. Since burps are short, you will have a tougher time burning up the coil (to a point)... so it's the way most SPL folks run.

2) Bigger than recommended box, plenty of port area, and high tuning can give a more efficient setup, with nice low impedance rise (at resonance)... but only if you are running a normal power level through it.

From my understanding, the impedance rise is a tug-of-war between the "resistance" generated by large AC current running through an inductor (the coil) and the decrease in current caused by this added "resistance." I would love for an expert to chime in on the true reason behind it, though.

Many people disagree with Option 2), but I can verify that a big efficient box can have low impedance rise. My subs are request 0.9-1.2 cf, but I have them in 2.5 cf each, along with monster port area (for 2 10s). My impedance rise is 1.0 --> 1.8-2.0 @ 52 Hz...

I am nearly bottoming them out, and they **** royally @ 30 Hz... but most SPL boxes don't extend well.

Click to expand...

I just explained it in the post above yours //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

Sorry if it seems like I was stealing your thunder. Not my intent.

I was trying reason out WHY impedance rises to a certain level and stays there. (hence the tug-of-war analogy)

Since you fluent in electricals, could you explain why impedance rise is different between a 0.5 , 1.0, and 2.0 cf box?

Specifically, how does the sub determine the final impedance value?

Believe me, there's no thunder to steal when telling something you know and someone might not. They likely have something they know and you don't as well //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

I'll do my best to make this make sense, so to help make it clearer, I'll use analogies:

Let's assume that you know what resonant frequency is already, but if not, the theoretical definition is where an input is 360 degrees in phase with the output. In electrical circuits, it's where the circuit has zero imaginary parts, aka, where the stuff talked about previously with inductance and capacitance cancels out to just leave no imaginary numbers. Since that might be a bit hairy if you don't study electronics, let me do my best to explain this with an example that is oft used in physics: a child's swing. When learning to swing by yourself by "pushing" with your legs, you initially end up with poor results because you are trying far too hard to get the swing to go higher. This is because the swing works like a simple pendulum, whose length determines the resonant frequency, and at said resonant frequency, energy is stored in the form of the swing's motion. When you push at the resonant frequency of the swing, your pushing (INPUT) is in phase with the movement of the swing (OUTPUT). Again, if you push improperly, you'll end up just wasting your own energy and the swing won't change its arc.

The longer the chain on the swing, the lower the resonant frquency, or the slower you need to push to gain amplitude. The same applies to a speaker enclosure, the larger the box, the lower the resonant frequency of the box, and since the lower frequencies more often coincide with the values where your audio information is for your subwoofer, you end up having to deal with impedance rise there.

Thiele or Small, I can't remember which, once stated that any speaker system can be described as a high pass filter. If you model the system as such, there is going to be a natural frequency to the circuit close to which smaller and smaller amounts of input are needed to create the same amount of output, because again, your input is in phase with the output. In circuits, this is where the imaginary parts of the impedance cancel out. I really wish I could write out what I'm trying to display in terms of variables, but I don't have math type here so that might be difficult.

So, if you model your sub system as a high pass filter, using pretty elementary electrical theory, you can determine the SYSTEM's resonant frequency (governed by an equation which I don't know since I don't have any reference books here as I just moved into a new apartment lol). I don't know the formula for using the high pass model for a speaker, but the general electronic circuit formula is w=1/sqrt(L*C) where w is your frequency in radians, or (2*pi)*(frequency in hz), and L and C are obviously the system's inductance and capacitance respectively.

Now, since impedance is made of complex numbers AND resistance, when the complex numbers cancel out, resonance will therefore be merely resistance, and since resistance is not frequency dependent, the impedance therefore at the resonant frequency will be at where the circuit's impedance is at a minimum. Since there is no impedance to damp the circuit at the resonant frequency, the oscillations can get out of control and become what is called unbounded. So, when your particular subs are causing vibrations in your particular box in phase with the entire system's resonant frequency, the impedance of the entire SYSTEM will be at a mininum.

Your SPEAKERS, however, will not have a minimum impedance, rather the frequency at which the system is resonant is going to cause an increase in the speakers' impedance, which is taking into account the imaginary parts we've been talking about before. It's a relationship which is sort of hairy and is honestly a bit shady myself to understand since I'm still learning as well, but I'm trying my best here. Your inductance is related inversely related to the change in current through the inductor, and at your resonant frequency, this change will be at a minimum, and this will thus raise the inductance of the circuit, and conversely increase the impedance at that frequency. At frequencies on either side, however, it slopes off strongly.

I'm really trying hard to make this make sense, but I'm struggling here, and I already feel that I lost you, PLUS, it's not yet clear enough to me either to make simplifications without worrying whether or not they're entirely valid. With that said, I'll let someone with a bit more experience come in here and try and clarify what I've started. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

Wow, that really sounds more like rambling and not a coherent thought train. I'll clean that up when i get back //content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif

No need to clean up. I was able to follow all of it. I'm working on my PhD in mechanical engineering, specializing in controls... so I may not have enough practical experience, but I can understand "no imaginary parts" just fine. //content.invisioncic.com/y282845/emoticons/wink.gif.608e3ea05f1a9f98611af0861652f8fb.gif

Side Note: It took me 2 classes and a summer of leisure studying, to finally learn bode plots... and how I got it? By realizing that FRF = "speaker frequency response plots." //content.invisioncic.com/y282845/emoticons/veryhappy.gif.fec4fed33b4a1279cf10bdd45a039dae.gif

I did have a question. When you said "the larger the box, the lower the resonant frequency of the box"... I didn't quite follow you there. Are you referring to box volume's resonance, port's resonance, or the resonance of the combined system (sub-->box volume-->port)?

Stating things, so I can rationalize/internalize:

Box resonance - Since there is more air in the box, the compliance goes up... Thus, the natural frequency drops. This is why manufacturers suggest certain box sizes (0.8, 1.2, etc). They attempt to get the total Q of the sub/box near 0.707 which is "optimally controlling" the woofer.

Side note 1: Does the damping change with box size?

Side note 2: Has anyone designed a box that resonates due to internal volume AND standing waves (caused by the physical lengths)? I wonder if you'd get a significant contribution...//content.invisioncic.com/y282845/emoticons/confused.gif.e820e0216602db4765798ac39d28caa9.gif

Port resonance - The port adds a complicating factor, because it allows flow between the air excited by the front and back waves. At resonance, it allows a significantly more airflow.

Guess: When you are above resonance frequency, the port adds damping to the system. This is why at higher frequencies, ported boxes don't have much cone movement. Conversely, lower than resonance frequencies will decrease the damping of the system and cause the woofer to overshoot and "get floppy."

----- I'll start from here when I get more time. But I think (with help) we're getting to the bottom of ported box theory. Thanks again, for good explanations.