Relationship of impedance and volume (loudness)

spark1

spark1

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I've seen several posts recently in which the author mentions the impedance of their speakers dropping as more volume is applied.

This does not make sense to me. I would think the impedance curve of a particular speaker is the same regardless of how loud they are being played.

I certainly understand that current demand on the amp goes up with volume (voltage), and that with low impedance speakers, this demand may push the amp beyond its capabilities...but that is not the same as impedance varying with volume.

Am I not understanding this topic correctly?
 
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Impedance can vary at different frequencies, but I am not sure of the relationship to volume/power applied. Impedance will often dip some at low frequencies, which puts an additional strain on the amp.
 
Resistance of copper and aluminum increases with temperature, temperature would increase with more power/higher volume. Not sure how that might play into the actual impedance.

What is the reported observation leading to that assertion?
 
bberkom said:
Impedance can vary at different frequencies, but I am not sure of the relationship to volume/power applied. Impedance will often dip some at low frequencies, which puts an additional strain on the amp.
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Yes, I am aware that impedance varies across the frequency spectrum. And as I mentioned, I fully appreciate that current draw is defined by both impedance and voltage.

What I don't see is how a change in voltage (loudness) changes the impedance of the speaker.
 
whoaru99 said:
Resistance of copper and aluminum increases with temperature, temperature would increase with more power/higher volume. Not sure how that might play into the actual impedance.

What is the reported observation leading to that assertion?
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In the two most recent examples, the poster mentioned it more as an aside...in the nature of stating a well-known fact. Something along the lines of "I have X speakers...rated at 4 ohms, but they drop down to 2 ohms when pushed hard". Seems to me that if they drop down to 2 ohms at some point on their impedance curve, it would be regardless of loudness.

I can see where resistance can go up as the coil heats up, but it would seem to me - without much in-depth thinking about it - that this effect would be relatively small.
 
spark1 said:
Something along the lines of "I have X speakers...rated at 4 ohms, but they drop down to 2 ohms when pushed hard".
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Yes, that sounds like nonsense to me too.
However, let's just consider what might happen when the volume goes up. Just to keep things simple I'll only consider a single bass/mid driver with no crossover.
The voice coil has some resistance and that won't change, other than temperature causing the resistance to go up. The lowest point on an impedance/frequency plot is almost the same as the resistance.
The coil also has inductance, which causes the impedance to rise from roughly 1kHz and up. I can't think how that would change with volume but there may be mechanism that I'm not aware of.
The main impedance variation comes from the speaker's resonant frequency. A sealed speaker (or open baffle) shows a single impedance peak at the resonant frequency due to the extra back emf produced by the voice coil moving more than it does at other frequencies. A reflex speaker has what looks like two peaks but is in fact more like one big peak with a narrow dip in the middle of it. The big peak is due to the back emf caused by the large cone motion, just like in a sealed speaker, but at the system resonance the motion of the air in the port damps the cone's movement to a much lower level - hence less back emf and a lower impedance.
So, is it possible that at high SPLs the air in the cabinet behaves differently and changes the impedance curve? It seems a possibility that turbulence in a port could alter its behaviour - but enough to drop a speaker's impedance from 4 ohms to 2 ohms? Doesn't seem likely to me.

No, I don't think a speaker's impedance drops when played loudly.
 
Unless the speakers are really heating up, the impedance isn't going to change much over the volume range. And, if they are really heating up, there are other serious problems, like too much DC getting to the speakers.

Doug
 
spark1 said:
I've seen several posts recently in which the author mentions the impedance of their speakers dropping as more volume is applied.

This does not make sense to me. I would think the impedance curve of a particular speaker is the same regardless of how loud they are being played.

I certainly understand that current demand on the amp goes up with volume (voltage), and that with low impedance speakers, this demand may push the amp beyond its capabilities...but that is not the same as impedance varying with volume.

Am I not understanding this topic correctly?
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A given loudspeaker impedance varies in relationship with its motion, and not only the frequency being reproduced. This is known as motional impedance : http://machineryequipmentonline.com/electric-equipment/loudspeakersmotional-impedance/ (and doesn't imply it only drops ). This is - among others - an electro-mechanical parameter of a considered driver. However this phenomenon is quite marginal and should not be of any importance in a commercial product since it has already been taken into account by its designer.
 
It's possible the other poster didn't fully grasp the concept or just misspoke, but it's hard to say without all the context. Maybe they just meant with bass heavy music, the impedance drops?
 
Thierry said:
A given loudspeaker impedance varies in relationship with its motion, and not only the frequency being reproduced. This is known as motional impedance : http://machineryequipmentonline.com/electric-equipment/loudspeakersmotional-impedance/ ...
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Quite true. It's the back emf I spoke of in post #6.
Thierry said:
However this phenomenon is quite marginal and should not be of any importance in a commercial product since it has already been taken into account by its designer.
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No, it isn't marginal at all - it's entirely responsible for the speaker's impedance curve below about 400Hz.
 
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spark1 said:
In the two most recent examples, the poster mentioned it more as an aside...in the nature of stating a well-known fact. Something along the lines of "I have X speakers...rated at 4 ohms, but they drop down to 2 ohms when pushed hard". Seems to me that if they drop down to 2 ohms at some point on their impedance curve, it would be regardless of loudness.

I can see where resistance can go up as the coil heats up, but it would seem to me - without much in-depth thinking about it - that this effect would be relatively small.
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Sounds like it's just a misunderstanding/misstatement.

As far as voice coil resistance increasing it is otherwise known as power compression or thermal compression. Low levels no big deal but cranking it up/at high output is a real consideration.
 
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Doug G. said:
Unless the speakers are really heating up, the impedance isn't going to change much over the volume range. And, if they are really heating up, there are other serious problems, like too much DC getting to the speakers.

Doug
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Doesn't need to be DC, just power.

Speakers are very inefficient in conversion of amplifier output to sound. Typically 95% or more of that amp power to a speaker goes up in heat in the voice coil.
 
whoaru99 said:
Doesn't need to be DC, just power.

Speakers are very inefficient in conversion of amplifier output to sound. Typically 95% or more of that amp power to a speaker goes up in heat in the voice coil.
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That's why I said, "...LIKE too much DC..." I wasn't being exclusive.

Doug
 
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