Variable impedance - what's the audible impact

[delta76]

A noob question that I have been too afraid too ask

We have been shown that many (or all?) speakers have variable impedance - like a nominal 8 Ohm speaker can dip at around 90-120Hz to 3 Ohm (or even lower, 2.5 Ohm in my case).
what's the audible impact of that? Or would it just "harmful" to the amp if it can't handle it (going to protection etc.) ? If there is an audible impact, how can I test it? (I have never noticed anything, but I usually play music as background instead of listening critically)

 

[fpitas]

Most good amplifiers have a very small output impedance, so the speaker variations don't matter a lot. This article explains the situation:

Audio Myth - "Damping Factor Isn't Much of a Factor"

Myth - "Damping Factor Isn't Much of a Factor" Myth - "A Damping Factor of 10 is High Enough" Myth - "All Amplifiers Have a High-Enough Damping Factor" Where did these Myths Originate? These myths seem to trace back to a well-known paper (PDF) written by Dick Pierce. His analysis shows that a...

benchmarkmedia.com

 

[delta76]

I was told at the beginning of my journey that my speakers are hard to drive as because of impedance dip. So basically hokum?

 

[solderdude]

'hard to drive' for amplifiers that are designed for 8ohm speakers and maybe even for some amps that should not be used below 4ohm.
The 'hard to drive' is not really the case though, that is more related to sensitivity (efficiency) but could be a 'challenging' load for some amps.
2.5ohm will draw almost double the current (at the frequencies it dips to that value) as a 4ohm load would.

Some amplifiers may go into current limiting, some simply will not deliver the desired output voltage it can deliver in 4ohm or higher.

If the speaker only dips that low in the mids or treble there is not much of a problem but in the bass, where most energy is, the amp could overheat or clip at high listening levels.
So the audible impact would only be noticeable at higher SPL and only compared to amps that could drive 2ohm at the same level as 4ohm. Not many amps do.

 

[Doodski]

A noob question that I have been too afraid too ask

We have been shown that many (or all?) speakers have variable impedance - like a nominal 8 Ohm speaker can dip at around 90-120Hz to 3 Ohm (or even lower, 2.5 Ohm in my case).
what's the audible impact of that? Or would it just "harmful" to the amp if it can't handle it (going to protection etc.) ? If there is an audible impact, how can I test it? (I have never noticed anything, but I usually play music as background instead of listening critically)

Do you know how a voltage divider works. If you do I can explain the variable impedance stuff in very simplified terms. What speakers are you running?

 

[Matias]

Most good amplifiers have a very small output impedance, so the speaker variations don't matter a lot. This article explains the situation:

Audio Myth - "Damping Factor Isn't Much of a Factor"

Myth - "Damping Factor Isn't Much of a Factor" Myth - "A Damping Factor of 10 is High Enough" Myth - "All Amplifiers Have a High-Enough Damping Factor" Where did these Myths Originate? These myths seem to trace back to a well-known paper (PDF) written by Dick Pierce. His analysis shows that a...

benchmarkmedia.com

And when the amp does NOT have a small output impedance, frequency response changes happen and are measurable.

SUMMER MUSINGS: No... Not all amplifiers sound the same (but many do! ;-).

A blog for audiophiles about more objective topics. Measurements of audio gear. Reasonable, realistic, no snakeoil assessment of sound, and equipment.

archimago.blogspot.com

Mixing and matching high output impedance amps with speakers of variable impedance gives all kinds of frequency response combinations at random, and audiophiles delight in trial and error and searching what suites them better.

 

[delta76]

these https://www.focal.com/en/home-audio/high-fidelity-speakers/aria-900/floorstanding-speakers/aria-948

Impedance chart

(wish there is a better photo)
I never heard of a voltage divider - just had to look it up. I think I understand the basics (using two resistors to have 2 difference circuits with different voltages ?)

 

[fpitas]

The article I linked shows a two-resistor voltage divider in action.

 

[Doodski]

these https://www.focal.com/en/home-audio/high-fidelity-speakers/aria-900/floorstanding-speakers/aria-948

Impedance chart

(wish there is a better photo)
I never heard of a voltage divider - just had to look it up. I think I understand the basics (using two resistors to have 2 difference circuits with different voltages ?)

One series circuit. The voltage is divided between the 2 series resistors and it's called voltage drops. So if 2, 8 Ohms resisters and a 10 V signal are used then 5 volts would appear across each resister. Notice each of the resisters are the same value of 8 Ohms and so they half the available voltage (50% for each resister) in the circuit. Basic math stuff. No Ohms Law required at this time. So If one of those resisters would decrease in a large way then the voltage drop would occur across the larger resister because of the percentages in the math operations. For example a 9 Ohm and 1 Ohm series circuit would result in 9 Volts across one resister and 1 Volt across the other. So now imagine that the voltage is changing in frequency and is going up and down and the resistance of the speaker (One of the resisters in the series circuit) resistance is changing too and so that means the voltage divider has now become a dynamic thing. Ever changing signal frequency, changing resistance of the speaker resister (voice coil) and the source resistance is changing too.

 

[Ken Tajalli]

Voltage divider:

Lets look at some plumbing (visualization for newbie's)

Here we have a tank of water, some pipes draining the water out to a drain, under gravity.
We have two valves on the way, in dotted blue and red squares.
The blue valve, has a very narrow nozzle, so water does not flow through it very easily. It has a high Resistance to flow of water.
The red valve, well! Not so much. Water goes through it, slow-downs a bit, but not much. This valve has a Low Resistance .
We can deduct:
- the same water flows through both valves equally, after all it is the same amount of water. It means the Current of water is the same through the entire flow.
- There is no pressure at drainage point, water is allowed to drain off easily. We call this point zero pressure ( Zero Volts)
- The pressure of water is a little higher at point B compared to Drainage point.
- The pressure at A is highest, because the blue valve is restricting the water flow, so the pressure does not drop much.
This is a pressure (Voltage) divider. We can divide the pressure through a plumbing system, but using valves of different tightness, one after another.

Here Vin is your tank of water, R1 is your first valve, R2 is your second valve.

 

[DonH56]

Amplifier Output Impedance (Damping Factor) and Speakers

Amplifier Output Impedance (Damping Factor) and Speakers

This is a reprint of an article written ca. 2011-2012. I've been meaning to rewrite and update it, but Life and Work keeps getting in the way, so I am reposting the original as-is. Take it for what it's worth, an introductory take on one of the potential causes for differences in sound among...

www.audiosciencereview.com

 

[PHD]

Most good amplifiers have a very small output impedance, so the speaker variations don't matter a lot. This article explains the situation:

Audio Myth - "Damping Factor Isn't Much of a Factor"

Myth - "Damping Factor Isn't Much of a Factor" Myth - "A Damping Factor of 10 is High Enough" Myth - "All Amplifiers Have a High-Enough Damping Factor" Where did these Myths Originate? These myths seem to trace back to a well-known paper (PDF) written by Dick Pierce. His analysis shows that a...

benchmarkmedia.com

This claim is somewhat irrelevant (only relevant for damping factor calculations). A power amp is a voltage source. So even if it's an ideal (stiff) source with zero output impedance, but the speaker impedance varies with frequency (linear behavior) or varies with current (non-linear behavior), it will impact the output power across different frequencies or cause distortion if the impedance is current dependent. If you seek better control of the speaker, you'll need to design a current-source-based power amplifier and use some acoustic servo to adjust the current to get a flat and constant output power regardless of the frequency or speaker impedance. I think such a solution does exist for subwoofers.

 

[fpitas]

This claim is somewhat irrelevant (only relevant for damping factor calculations). A power amp is a voltage source. So even if it's an ideal (stiff) source with zero output impedance, but the speaker impedance varies with frequency (linear behavior) or varies with current (non-linear behavior), it will impact the output power across different frequencies or cause distortion if the impedance is current dependent. If you seek better control of the speaker, you'll need to design a current-source-based power amplifier and use some acoustic servo to adjust the current to get a flat and constant output power regardless of the frequency or speaker impedance. I think such a solution does exist for subwoofers.

Most speakers are designed and tested driven by low impedance amplifiers.

 

[DualTriode]

Voltage divider:

Lets look at some plumbing (visualization for newbie's)

View attachment 296781
Here we have a tank of water, some pipes draining the water out to a drain, under gravity.
We have two valves on the way, in dotted blue and red squares.
The blue valve, has a very narrow nozzle, so water does not flow through it very easily. It has a high Resistance to flow of water.
The red valve, well! Not so much. Water goes through it, slow-downs a bit, but not much. This valve has a Low Resistance .
We can deduct:
- the same water flows through both valves equally, after all it is the same amount of water. It means the Current of water is the same through the entire flow.
- There is no pressure at drainage point, water is allowed to drain off easily. We call this point zero pressure ( Zero Volts)
- The pressure of water is a little higher at point B compared to Drainage point.
- The pressure at A is highest, because the blue valve is restricting the water flow, so the pressure does not drop much.
This is a pressure (Voltage) divider. We can divide the pressure through a plumbing system, but using valves of different tightness, one after another.

View attachment 296783

Here Vin is your tank of water, R1 is your first valve, R2 is your second valve.

Hello,

I take exception, your analogy of water flow Is incomplete and confusing at best.

Bernoulli is scratching his head trying to make sense of it.

Using your analogy air may even be sucked in between the "valves" that you illustrate.

 

[NTK]

Hello,

I take exception, your analogy of water flow Is incomplete and confusing at best.

Bernoulli is scratching his head trying to make sense of it.

Using your analogy air may even be sucked in between the "valves" that you illustrate.

Bernoulli is based on conservation of energy. The throttling process of flow through restrictive valves (and the electrical analog of current passing through resistors) dissipates energy, and thus energy is not conserved (nor the process is reversible). Bernoulli is perfectly fine because his principle does not apply to the situation described here. From Wikipedia:

 

[tmtomh]

Stupid question time: Is it true or not that load dependence of amplifiers impacts frequency response only at higher frequencies?

I ask because the subject of this thread is "what's the audible impact" of "variable speaker impedance."

And as we know from other threads, a major bone of contention is whether it's necessary or useful to test amplifiers with complex loads. The idea behind testing amps with complex loads is that if you do so, then a load-dependent amp won't just have a slight high-frequency peak or rolloff as it does with a constant resistive 4-ohm or 8-ohm load. The idea is that it will instead have nonlinearities throughout the audible range, as the load's impedance varies.

However, my understanding is that this is not the case - my understanding is that a variable load will only impact a load dependent amp's response in the highest couple of octaves in the audible range.

Would love it if one of our more knowledgeable members would be willing to answer this, as I think it would help clear up a lot of confusion (and help sweep away the FUD around this issue speed by a few of our members).

 

[DualTriode]

Bernoulli is based on conservation of energy. The throttling process of flow through restrictive valves (and the electrical analog of current passing through resistors) dissipates energy, and thus energy is not conserved (nor the process is reversible). Bernoulli is perfectly fine because his principle does not apply to the situation described here. From Wikipedia:

Your highest possible grade is a "C".

There is a throttling valve in a carburetor. Without Bernoulli a carburetor would not function

The energy upstream in the fluid flow will always be equal to the energy downstream in the flow plus the energy loss between the two points. Here is your conservation, energy in equals energy out including friction loss.
Bernoulli is still at work.

 

[Ken Tajalli]

You guys are kidding, right?
My crude drawing was just a visualization , to get a point across . I never meant it was an absolute equal circuit.
Deary me!

 

[amirm]

Stupid question time: Is it true or not that load dependence of amplifiers impacts frequency response only at higher frequencies?

Yes and no. No in that there is no such generic rule. Yes in that class D amps with this issue have a rising output impedance with frequency. It is for that reason that the worst case impact is at the highest frequency.

 

[grogi.giant]

A noob question that I have been too afraid too ask

We have been shown that many (or all?) speakers have variable impedance - like a nominal 8 Ohm speaker can dip at around 90-120Hz to 3 Ohm (or even lower, 2.5 Ohm in my case).
what's the audible impact of that? Or would it just "harmful" to the amp if it can't handle it (going to protection etc.) ? If there is an audible impact, how can I test it? (I have never noticed anything, but I usually play music as background instead of listening critically)

This is not a problem. That's what amplifier is designed to do - give as much current as needed to maintain given voltage. Just don't try to drive the speaker with 50W at that single frequency.

What is audible, with some inexpensive Class D designs - exp. all TPA3255 amplifiers - there is difference in frequency response in higher frequencies with higher impedances. So it is the impedance at ~10kHz and above that matters.