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tipatina

impedance vs resistance

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Klipsch Forte II, stock except for Crites crossover, sound great, good output from all drivers when playing music. With my volt meter I get 4 Ohms at the binding posts rather than 8. I realize my voltmeter is sending DC for measurement. Checked volt meter for accuracy with some resistors, seems to be working fine. Any insights? 

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DC will read about 80% or so of what AC impedance will read.  Also, that 8 Ohms figure they gave you is an average.  The actual impedance varies dynamically according to what frequency the driver is producing.  If you want to get an accurate measurement of the driver's actual impedance curve, you'd have to measure it using the PC while playing a frequency sweep to see the whole range. Software that can do this (such as DATS) comes with a special USB cable with alligator clips.  You generally don't need to bother with this level of detail unless you start getting into more advanced building, designing custom crossovers and the like.

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Impedance curve measured by member WMcD in 7th post. of this thread.

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OK, figured I was oversimplifying, will read up on nominal impedance. Thanks for your replies

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Sorry I thought you were looking for an impedance curve, but you were wondering more basically what's the difference. A speaker and its associated crossover is an electrical to mechanical energy transducer. It works with alternating current not direct current. So your direct current reading meter is only telling you the opposition to direct current flow. But there is also an opposition to the alternating current called reactance. This is caused by the capacitors and inductors in the crossover and the inductance of the voicecoils of the drivers. The resistance and reactance when added together yield the impedance. Strangely at some frequencies when added together they equal less than the DC resistance that your meter reads.

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14 hours ago, JasenGibson said:

DC will read about 80% or so of what AC impedance will read.  

Not a good thing to go by.  Impedance, particularly when partnering with an amplifier (phase with impedance) can crush that amplifier.  A lot of speakers with ugly crossover implementations and multiple woofers in particular throw simple theories out the window unless of course you are looking at a single driver used in it's typical pass band and averaged. Then you you are pretty close.

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16 hours ago, pzannucci said:

Not a good thing to go by.  Impedance, particularly when partnering with an amplifier (phase with impedance) can crush that amplifier.  A lot of speakers with ugly crossover implementations and multiple woofers in particular throw simple theories out the window unless of course you are looking at a single driver used in it's typical pass band and averaged. Then you you are pretty close.

True, the 80% usually applies to a driver as a opposed to a whole speaker.

 

Can you expound on your statement: "Impedance, particularly when partnering with an amplifier (phase with impedance) can crush that amplifier." I'm interested in practical theories on how speaker impedance affects amplifiers.

 

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On 4/9/2020 at 1:42 PM, tipatina said:

Klipsch Forte II, stock except for Crites crossover, sound great, good output from all drivers when playing music. With my volt meter I get 4 Ohms at the binding posts rather than 8. I realize my voltmeter is sending DC for measurement. Checked volt meter for accuracy with some resistors, seems to be working fine. Any insights? 

My instinct tells me that a Forte II's DC resistance should be a bit higher than that, maybe in the 5+ range. I don't doubt the Crite's crossover could be the reason for the drop in resistance. You should still consider them an 8ohm impedance speaker. If your amp has multiple ohm taps, you can use the 8 ohm or the 4 ohm with no troubles, the 4 ohm tap usually gives better low frequency response, 8 ohm tap gives better clarity. Usually.

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On 4/10/2020 at 11:41 PM, MechEngVic said:

True, the 80% usually applies to a driver as a opposed to a whole speaker.

 

Can you expound on your statement: "Impedance, particularly when partnering with an amplifier (phase with impedance) can crush that amplifier." I'm interested in practical theories on how speaker impedance affects amplifiers.

 

In the very basic view, the lower the impedance, the higher the current that is needed to push the speaker.  That's why a lot of receivers just don't cut it when pairing with speakers that drop down in the impedance, particularly in the bass area such as dual woofer systems and many speakers using 4ohm woofers in an attempt to provide higher output.  They cause amplifiers to go into protection or inadequately drive the speakers resulting in poor bass response.  My experience anyway.

This site has some interesting explanations https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law/all#current

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11 hours ago, pzannucci said:

In the very basic view, the lower the impedance, the higher the current that is needed to push the speaker.  That's why a lot of receivers just don't cut it when pairing with speakers that drop down in the impedance, particularly in the bass area such as dual woofer systems and many speakers using 4ohm woofers in an attempt to provide higher output.  They cause amplifiers to go into protection or inadequately drive the speakers resulting in poor bass response.  My experience anyway.

This site has some interesting explanations https://learn.sparkfun.com/tutorials/voltage-current-resistance-and-ohms-law/all#current

My KLF-10's have dual 10" woofers wired in parallel. They are 8ohm woofers (6.6ohm DCR) which would make them a 4 ohm load to the amp, and I have found that using the 8ohm taps on my amp underpowers them. Thanks for the info.

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You replaced the crossovers and I'll assume the new crossovers have some different value components; all bets are off regarding the impedance.

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On 4/9/2020 at 5:34 PM, babadono said:

The resistance and reactance when added together yield the impedance. Strangely at some frequencies when added together they equal less than the DC resistance that your meter reads.

 

The reactance value is not simply added to resistance. It is a vector summation of the two values, with the reactance values on the Y axis and the resistance on the X axis. Therefore, the impedance cannot fall below the resistance value.

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11 hours ago, MechEngVic said:

My KLF-10's have dual 10" woofers wired in parallel. They are 8ohm woofers (6.6ohm DCR) which would make them a 4 ohm load to the amp, and I have found that using the 8ohm taps on my amp underpowers them. Thanks for the info.

 

A question inspired by the above....  not a question about it.

 

If you take two 8 ohm woofers, put them in parallel....you would get (in general?) a 4 ohm electrical apparent load?

If you take two 8 ohm woofers, put them in parallel....and install them in a horn....  you get a 4 ohm electrical apparent load on the drivers, however when you horn load them, that load might increase up to ?? 6-8 ohms?

 

So horn loading adds some resistance to the drivers, but the direct radiator does not??  (if accurate, that is because the DR is more in free air without the front load of the horn??)

 

 

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9 hours ago, Coytee said:

 

A question inspired by the above....  not a question about it.

 

If you take two 8 ohm woofers, put them in parallel....you would get (in general?) a 4 ohm electrical apparent load?

If you take two 8 ohm woofers, put them in parallel....and install them in a horn....  you get a 4 ohm electrical apparent load on the drivers, however when you horn load them, that load might increase up to ?? 6-8 ohms?

 

So horn loading adds some resistance to the drivers, but the direct radiator does not??  (if accurate, that is because the DR is more in free air without the front load of the horn??)

 

 

Any cabinet offers a woofer a physical resistance at specific frequencies, whether sealed, ported, or horn loaded, due to resonant frequencies between the woofer's design and the cabinet's dimensions/features. This physical resistance in small regions of the woofer's frequency response cause spikes in the impedance of the woofer. These "spikes" raise the average impedance of the woofer/cabinet system. With a folded horn design, you get both impedance matching of the cabinet and of the horn.

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There is a lot being discussed here.

 

The curves I ran says LMS because it is Loudspeaker Measurement System.  We are measureing the electrical impedance (but call it resistance) at a wide range of frequencies as you can see on the x-axis of the graph.

 

When we use a multimeter we are measuring the resistance at the frequency created by the battery in the multimeter.  That is d.c. (so called direct current).  The frequency is zero Hertz.  It should match what appears on the left-most range of the graph if it could go down that far.  It usually doesn't.

 

It is an oversimplification to say resistance over a wide range of frequencies has nothing to do with resistance versus imepedance.  However the graph has mostly to do with showing how resistance (err) varies with frequency while our multimeter is showing only zero Hertz.

 

We can wade into the effect of horn loading verus a direct radiator speaker.  Essentially we like looking at the input impedance of the driver in a box because it is easy to measure electrically. 

 

Very complicated but the system can be broken up into the electrial part (the magnet and voice coil, the mechanical part (mass and spring of the driver, and the acoustical part which is the air load on the driver daphragm (horn loading changes this per below).  

 

There is a model for all this.  In a way, the electrical resistance as measured at the input terminals is affected by the mechanical and acoustic elements.   So with some deep thinking we can look at the box's electrical input and figure a lot about the mechanical and acoustic properties.  For example we can see the peak in elecrical resitance caused by the mechanical resonance of the drive -- but that is not what we're talking about here.

  

A very simple model is as follows:

 

terminal plus - -  voice coil resistance - - - acoustic resistance - - and a ground return to terminal minus.

 

In a direct radiator system we see:

 

Input terminal plus - voice coil resistance (8 ohms) - - - acounstic resistance (0.8 homs) -- return to ground terminal

 

That acoustic resistance is almost a dead short and like a piece of wire, it consumes no energy because there is little voltage drop across it.  Most of the power just heats up the voice coil.  The system resistance is 8.8 ohms.  Note PWK's comments to the effect of "just heating up the voice coil."  And the speaker is a space heater.

 

In a horn loaded system we have

 

termial plus - -  voice coil resistanc (8 ohm) --- acoustic restance (8 ohms) -  return to ground terminal.

 

Here we see that system resistance is 16 ohms.  You have to take a good look at this.  We see that we no longer have the acousic resistance of near zero (wire-like) to a resistance which does absorb electrial power quite well (light bulb - like.  Also this is why people say that horn loaded system electrical resistance doubles.  Also, it is50% efficient because half the elecrical energy is disipated by the voice coil.

 

It is true that the increase in acousic resistance is due to acousic impedance matching by the horn.  I'm afraid to go into it here.  This matching does not occur at d.c. measures by the mulitimeter, only at freqs where the horn is working to increase the acoustic resistance "seen" by the diaphragm of the driver.

 

WMcD

 

 

 

 

 

 

 

 

 

 

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3 hours ago, WMcD said:

There is a lot being discussed here....

And your comment has a lot going on. I'm going to read read it carefully several times to hopefully digest what your saying, plus I'll have to carefully read the thread babadono posted where you also comment on the complexities of impedance. Based on what I've read so far from your comment and the other thread, I'm seeing that acoustic/mechanical properties of a speaker can increase OR decrease impedance...?

 

I also noticed that the DCR that the OP measured is the same as you did.

 

 

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On 4/13/2020 at 8:43 AM, Don Richard said:

The reactance value is not simply added to resistance. It is a vector summation of the two values

 

I like what I'm reading in this thread.  In regards to that quote, the impedance value is the length of that vector, the hypotenuse in a right triangle.  So it's actually the square root of the sum of the squares, which isn't terribly difficult math, but is indeed more involved than simple addition.  ;)

 

I'm pretty confident that most amplifiers will be happier with a laying-down vector than with one upright.

 

Over a frequency sweep on a speaker system I'd bet that vector looks kinda like a VU meter needle.

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