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95Honda

Impedance...

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Maybe this should be stickied...

Impedance cannot be measured with a DMM.

Only DCR can be measured with a single DMM, and this isn't impedance.

There is no such thing as "Impedance Rise". There is an impedance curve, but this both rises and falls.....

The system impedance will always be higher than the DCR of the driver under any condition.

Many factors affect impedance, the biggest being the enclosure. Until you measure your complete alignment at all frequencies in it's operating range, you have no idea what the impedance is.

Impedance changes do not change driver power handling, they change how much current you amplifier puts out at a given voltage..

Nobody owns a .7 ohm/coil impedance woofer.... You may have a .7 ohm DCR, but you have a 1 ohm/coil impedance woofer. The impedance is an average measurement over a broad (relative to usable bandwidth) range of frequencies.

Edited by 95Honda

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What is happening physically to the driver in correlation to the peak in the imp. curve?

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Which peak? (there are normally more than 1) Or do you just mean what happens when impedance goes up?

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The real deal is measuring. From what I've been told, over the counter clamp meters for measuring amperage are only good at 60Hz. Any tips for how to measure amperage at various frequencies?

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What is happening physically to the driver in correlation to the peak in the imp. curve?

Which peak? (there are normally more than 1) Or do you just mean what happens when impedance goes up?

e.g. Cone has least resistence, driver is demanding less power???

What physically occurs with the driver that correlates to the peak(s) in imp.?

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There are 2 major factors that I think will kind of help explain what you are asking(?)...

Normally, in a vented 4th order alignment (which is most of what we discuss on here) you are going to have 3 impedance peaks. The first 2, which will be domiant for most of our applications, are the peaks below and above the dip (trough) at tuning. The 3rd peak, and not quite as signifigant (or completely ignorable if you are totally out of the bandwidth) is the gradual peak that is caused by the inherent inductance of the voice coil.

So, an easy way to describe the first 2 peaks.... At resonance, the cone of the subwoofer is highly damped by the entire system attributes (mainly the Hemolitz resonation taking the load from the cone in an efficent manner) and due to the high amount of dampening (control) the reactive electrical properties are at a low point (null) and load is moved into a more resistive (reactance properties start cancelling each other out) realm... Above and below this point dampening suffers quickly and is immediately apparent due to the high amount of impedance shift... I am not sure if this is broke down well for understanding... I haven't really had to explain this many times and there are alot of people out there who could convey this better than I...

The 3rd peak is easy. The voice coil is an inductor, inductive reactance is proprtional to frequency. And a little food for thought, some overhung designs have 4-5mH of inductance, this starts raising impedance signifigantly as low as 100 Hz or so....

This is why I always say there is no such thing as "Impedance Rise".... You can see, the impedance goes up and down alot...

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To be honest, as far as clamp meters, they should be accurate between 50-400Hz.... As this is the most common range of AC power measurement.

The Flukes we have at work get calibrated yearly, and they are within a 1% or so in that range, and these are the ones that aren't very expensive.

I sometimes conduct EMP hardness testing and use very high current rating inductive pickups to measure high power content pulses. These are devices that cost several thousand dollars. When I conducted my clipping effect tests a while ago I used both the high current inductive probes and a standard Fluke clamp and didn't see much of a difference with sine wave measurements in the bottom few octaves of the audio realm. When I was dealing with square waves, I couldn't use the Fluke clamps because I needed a visual indication of real time current on an oscilliscope, and this obviously wasn't possible.

Now if you buy a $20 Velleman kit clamp meter off of e-bay, I wouldn't trust it's accuracy at any frequency...

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To be honest, as far as clamp meters, they should be accurate between 50-400Hz.... As this is the most common range of AC power measurement.

The Flukes we have at work get calibrated yearly, and they are within a 1% or so in that range, and these are the ones that aren't very expensive.

I sometimes conduct EMP hardness testing and use very high current rating inductive pickups to measure high power content pulses. These are devices that cost several thousand dollars. When I conducted my clipping effect tests a while ago I used both the high current inductive probes and a standard Fluke clamp and didn't see much of a difference with sine wave measurements in the bottom few octaves of the audio realm. When I was dealing with square waves, I couldn't use the Fluke clamps because I needed a visual indication of real time current on an oscilliscope, and this obviously wasn't possible.

Now if you buy a $20 Velleman kit clamp meter off of e-bay, I wouldn't trust it's accuracy at any frequency...

50-400Hz is cool, but my subs are low-passed at 60Hz. I'm interested in a meter that will measure from let's say 10Hz and up. Any thoughts on such a clamp meter or is this o-scope territory? I'm not sure if I'm getting side tracked asking about clamp meters so I'll ask this: How do you measure impedance?

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What I was eluding to about the clamp meter is decent ones will read OK in your range.

For power of a sine wave (and only a sine wave) You only have to multiple RMS Voltage by RMS current to get RMS wattage. Simple as that.

Even if you use an oscilliscope you still need a current pickup to take a reading... And remember, most oscilliscopes only read Pk-Pk or Pk voltage, and unless they have a built in calculator, you have to do the math to get RMS values.... And if you are using a current probe, you will have to pay close attention to current vs. voltage representation and be sure you check if it has differing levels at different frequencies...

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Interesting. Is this impedance curve you speak of, one that is derived from using a device like the woofer tester. Or is it derived from measuring under full power?

I once attempted to "clamp my amps" and verify output with an off the shelf AC clamp meter and a multi-meter. I measure for amperage at the positive wire going to the sub and AC voltage at the sub terminals. I measured frequencies at 5Hz increments from 25Hz to 65Hz. Using the magic of Ohms law I calculated watts and ohms with amps and volts. :) By doing so I inherently ended up with a set of impedance, or is it resistance, values for each frequency tested. Here is a link to what I did

My link

Can the off the shelf AC clamp meter be an accepted tool to measure amperage at various frequencies? If so, how can we prove it?

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The impedance curve is just that, an impedance curve. Modeling programs normally predict at small signal levels (a watt or so) and they do change as power increases. They can vary in form, but overall they normally increase overall when thermal effects set in. The main peaks above and below the tuning trough will also shift if the thermal changes and other factors start affecting driver Q. So I guess the answer to your question is yes and slightly no.

I didn't get a chance to read your link, but from what you typed then yes, you were measuring impedance (not resistance). Resistance is not relevent to loudspeakers as they are a reactive load.

Every off the shelf clamp will be different. Most will be OK in the bottom few octaves of the audio spectrum. The only way to prove if your clamp meter is accurate is to compare it to a calibrated current sensor, I have one, but it cost almost $1000. -OR- just don't worry about the few % you will get in error. You could also email the manufacturer of your particular clamp and see if they have a generic calibration graph for your particular model, if they do, you could factor that into your equations.

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Pinning for Mike. ;)

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I think I just learned something new!! :morepower1:

That is what this site is about.

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I think I just learned something new!! :morepower1:

That is what this site is about.

Ive learned a lot about sound systems in all aspects!! Continuing to learn!

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Thanks for putting this up Mike, I too was getting frustrated with all the misconception posts that have been occurring here lately.

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There are 2 major factors that I think will kind of help explain what you are asking(?)...

Normally, in a vented 4th order alignment (which is most of what we discuss on here) you are going to have 3 impedance peaks. The first 2, which will be domiant for most of our applications, are the peaks below and above the dip (trough) at tuning. The 3rd peak, and not quite as signifigant (or completely ignorable if you are totally out of the bandwidth) is the gradual peak that is caused by the inherent inductance of the voice coil.

So, an easy way to describe the first 2 peaks.... At resonance, the cone of the subwoofer is highly damped by the entire system attributes (mainly the Hemolitz resonation taking the load from the cone in an efficent manner) and due to the high amount of dampening (control) the reactive electrical properties are at a low point (null) and load is moved into a more resistive (reactance properties start cancelling each other out) realm... Above and below this point dampening suffers quickly and is immediately apparent due to the high amount of impedance shift... I am not sure if this is broke down well for understanding... I haven't really had to explain this many times and there are alot of people out there who could convey this better than I...

The 3rd peak is easy. The voice coil is an inductor, inductive reactance is proprtional to frequency. And a little food for thought, some overhung designs have 4-5mH of inductance, this starts raising impedance signifigantly as low as 100 Hz or so....

This is why I always say there is no such thing as "Impedance Rise".... You can see, the impedance goes up and down alot...

Thanks for posting this!!

Let's dumb this down a little to see if I'm understanding you correctly.

A simple vented enclosure for example.

If I understand what you're stating above.

As the air in the port resonates it places a load, resistance to movement or a "brake", on the cone and this causes the impedance to increase in value.

I'd venture to guess that if the if the above, my assumption, is correct.

The cone/coil can't cycle "freely" and, for lack of a better term, is causing "feedback" or a higher resistance to movement due to another force fighting the AC voltage being sent to the coil to induce movement.

Is this basically correct or completely wrong?

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There are 2 major factors that I think will kind of help explain what you are asking(?)...

Normally, in a vented 4th order alignment (which is most of what we discuss on here) you are going to have 3 impedance peaks. The first 2, which will be domiant for most of our applications, are the peaks below and above the dip (trough) at tuning. The 3rd peak, and not quite as signifigant (or completely ignorable if you are totally out of the bandwidth) is the gradual peak that is caused by the inherent inductance of the voice coil.

So, an easy way to describe the first 2 peaks.... At resonance, the cone of the subwoofer is highly damped by the entire system attributes (mainly the Hemolitz resonation taking the load from the cone in an efficent manner) and due to the high amount of dampening (control) the reactive electrical properties are at a low point (null) and load is moved into a more resistive (reactance properties start cancelling each other out) realm... Above and below this point dampening suffers quickly and is immediately apparent due to the high amount of impedance shift... I am not sure if this is broke down well for understanding... I haven't really had to explain this many times and there are alot of people out there who could convey this better than I...

The 3rd peak is easy. The voice coil is an inductor, inductive reactance is proprtional to frequency. And a little food for thought, some overhung designs have 4-5mH of inductance, this starts raising impedance signifigantly as low as 100 Hz or so....

This is why I always say there is no such thing as "Impedance Rise".... You can see, the impedance goes up and down alot...

Thanks for posting this!!

Let's dumb this down a little to see if I'm understanding you correctly.

A simple vented enclosure for example.

If I understand what you're stating above.

As the air in the port resonates it places a load, resistance to movement or a "brake", on the cone and this causes the impedance to increase in value.

I'd venture to guess that if the if the above, my assumption, is correct.

The cone/coil can't cycle "freely" and, for lack of a better term, is causing "feedback" or a higher resistance to movement due to another force fighting the AC voltage being sent to the coil to induce movement.

Is this basically correct or completely wrong?

Not sure I follow your analogy, so let me try to make this easier to understand.

Ported enclosures have three impedance peaks.

-The first two are enclosure dependent

-1st peak (lowest F) is directly related to the chosen port

-2nd peak from resonance

-3rd peak from the inductance of the driver – this one you can ignore

The confusion you have is around the 1st peak. Your analogy of a load/resistance is not terribly far off. To simplify this, basically we can change this peak in a couple different ways. First we can change the port size. If the port cannot support enough air at the velocity that the driver is trying to push through it, the impedance will change somewhat proportionally. The reason I use somewhat is that it is not a linear behavior. The other way to change the peak besides port size is to change the power. Obviously if the “too small” port above didn’t work at X power if we lowered the power to some level below X it could come into a range where it was okay and the air could move through the port. A third non-controllable impedance changing device is the temperature of the voice coil. This will obviously affect more than just the impedance, but nonetheless the drivers impedance will change with coil temperature.

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Tweeted the link. :)

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There are 2 major factors that I think will kind of help explain what you are asking(?)...

Normally, in a vented 4th order alignment (which is most of what we discuss on here) you are going to have 3 impedance peaks. The first 2, which will be domiant for most of our applications, are the peaks below and above the dip (trough) at tuning. The 3rd peak, and not quite as signifigant (or completely ignorable if you are totally out of the bandwidth) is the gradual peak that is caused by the inherent inductance of the voice coil.

So, an easy way to describe the first 2 peaks.... At resonance, the cone of the subwoofer is highly damped by the entire system attributes (mainly the Hemolitz resonation taking the load from the cone in an efficent manner) and due to the high amount of dampening (control) the reactive electrical properties are at a low point (null) and load is moved into a more resistive (reactance properties start cancelling each other out) realm... Above and below this point dampening suffers quickly and is immediately apparent due to the high amount of impedance shift... I am not sure if this is broke down well for understanding... I haven't really had to explain this many times and there are alot of people out there who could convey this better than I...

The 3rd peak is easy. The voice coil is an inductor, inductive reactance is proprtional to frequency. And a little food for thought, some overhung designs have 4-5mH of inductance, this starts raising impedance signifigantly as low as 100 Hz or so....

This is why I always say there is no such thing as "Impedance Rise".... You can see, the impedance goes up and down alot...

Thanks for posting this!!

Let's dumb this down a little to see if I'm understanding you correctly.

A simple vented enclosure for example.

If I understand what you're stating above.

As the air in the port resonates it places a load, resistance to movement or a "brake", on the cone and this causes the impedance to increase in value.

I'd venture to guess that if the if the above, my assumption, is correct.

The cone/coil can't cycle "freely" and, for lack of a better term, is causing "feedback" or a higher resistance to movement due to another force fighting the AC voltage being sent to the coil to induce movement.

Is this basically correct or completely wrong?

Not sure I follow your analogy, so let me try to make this easier to understand.

Ported enclosures have three impedance peaks.

-The first two are enclosure dependent

-1st peak (lowest F) is directly related to the chosen port

-2nd peak from resonance

-3rd peak from the inductance of the driver – this one you can ignore

The confusion you have is around the 1st peak. Your analogy of a load/resistance is not terribly far off. To simplify this, basically we can change this peak in a couple different ways. First we can change the port size. If the port cannot support enough air at the velocity that the driver is trying to push through it, the impedance will change somewhat proportionally. The reason I use somewhat is that it is not a linear behavior. The other way to change the peak besides port size is to change the power. Obviously if the “too small” port above didn’t work at X power if we lowered the power to some level below X it could come into a range where it was okay and the air could move through the port. A third non-controllable impedance changing device is the temperature of the voice coil. This will obviously affect more than just the impedance, but nonetheless the drivers impedance will change with coil temperature.

Thanks for answering M5. I guess I'll have to do some reading on this subject to better wrap my brain around the "why".

Perhaps I was trying to make this to simple. Leaving temperature and inductance off the table.

My thought was a medium, air in this case, was causing resistance to cone motion and in turn the impedance increases.

Time to do some homework.

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What happens to the peak at driver Fs once the woofer is in the enclosure? I assume this peak will shift, but would it be in addition to the peaks on each side of the port tuning, or does it become part of the upper peak?

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What happens to the peak at driver Fs once the woofer is in the enclosure? I assume this peak will shift, but would it be in addition to the peaks on each side of the port tuning, or does it become part of the upper peak?

Perhaps some graphs will help:

Driver has an Fs of 41hz.

IB

ib.jpg

sealed 1ft^3

sealed.jpg

sealed 2ft^3

sealed2.jpg

ported 1ft^3 50hz

ported50hz.jpg

ported 2ft^3 50hz

ported2_50hz.jpg

ported 1ft^3 70hz

ported70hz.jpg

ported 2ft^3 70hz

ported2_70hz.jpg

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What happens to the peak at driver Fs once the woofer is in the enclosure? I assume this peak will shift, but would it be in addition to the peaks on each side of the port tuning, or does it become part of the upper peak?

The first peak below tuning is due to the interaction of the driver and the port.

The second peak above tuning is due to the interaction of the driver and the compliance of the enclosure.

So they are both related to the driver and the design of the enclosure as a whole, but I think the answer to your question would be that the second peak above tuning is due to the driver/enclosure compliance interaction just like you would have with a driver operating in a sealed enclosure. The lower peak arises because now there is a second point of interaction for the driver, it's interaction with the port.

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I understand the peaks on each side of the tuning, but I was curious how these peaks interact with peak at driver Fs. The 4 ported graphs show what I was looking for.

Thanks!

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