4 ohm vs 8 ohm speakers

[Johnny2Bad]

There are some other differences that come up with different loudspeaker impedance loads. (I say "loudspeaker impedance loads", which is a repeatable measured value specific to a loudspeaker, versus "4 ohms" or "8 ohms" which may be just a general non-measured designation).

For example, the Damping Factor (DF) of the amplifier-loudspeaker circuit will rise with a corresponding rise in the loudspeaker impedance load (all things being equal, an 8 ohm load will result in twice the DF as a 4 ohm load), and at the same time factors such as the resistance of loudspeaker cabling (both ways) will have a smaller effect on the overall DF as the loudspeaker impedance load rises. This can be a more significant issue when long speaker cables are required.

Exactly what effect a higher DF has on overall Sound Quality (SQ) is subject to debate, is amplifier-dependent (varies with the design of the amplifier stage) and has a practical limit where any further increase has little or no effect. None the less it is a measurable value that has, at least in theory, some effect on overall SQ. In practical terms it's most likely to be an issue when the DF is very low.

With regard to power delivery, it is possible to design an amplifier to generate the greatest power output into a specific load. Again, in practical terms, you will sometimes come across this on modern switch-mode (Class D) amplifiers, where (for example) power output is greatest at, say, 6 ohms, and falls in comparison at both 4 and 8 ohm loads.

 

[3db]

There are some other differences that come up with different loudspeaker impedance loads. (I say "loudspeaker impedance loads", which is a repeatable measured value specific to a loudspeaker, versus "4 ohms" or "8 ohms" which may be just a general non-measured designation).

For example, the Damping Factor (DF) of the amplifier-loudspeaker circuit will rise with a corresponding rise in the loudspeaker impedance load (all things being equal, an 8 ohm load will result in twice the DF as a 4 ohm load), and at the same time factors such as the resistance of loudspeaker cabling (both ways) will have a smaller effect on the overall DF as the loudspeaker impedance load rises. This can be a more significant issue when long speaker cables are required.


Exactly what effect a higher DF has on overall Sound Quality (SQ) is subject to debate, is amplifier-dependent (varies with the design of the amplifier stage) and has a practical limit where any further increase has little or no effect. None the less it is a measurable value that has, at least in theory, some effect on overall SQ. In practical terms it's most likely to be an issue when the DF is very low.

With regard to power delivery, it is possible to design an amplifier to generate the greatest power output into a specific load. Again, in practical terms, you will sometimes come across this on modern switch-mode (Class D) amplifiers, where (for example) power output is greatest at, say, 6 ohms, and falls in comparison at both 4 and 8 ohm loads.

Again, you are not looking at the phase angle and makes your above statement incorrect. An 8 ohm speaker with a high phase angle is harder to drive than a 4 ohm speaker with a low phase angle.

 

[j_garcia]

The indication on a speaker is always "Nominal", as we know it varies with frequency, so it isn't as if every speaker is presenting the same load to the amp, and even if it is, it isn't doing the same thing all the time. 4 Ohms says to me it starts off below 8 Ohms and dips low enough that it will present a more difficult load and I think that is sufficient to warn users.

 

[PENG]

Again, you are not looking at the phase angle and makes your above statement incorrect. An 8 ohm speaker with a high phase angle is harder to drive than a 4 ohm speaker with a low phase angle.

The OP only asked about impedance, but for better clarity I agree one has to consider other factors that have effects on how hard they are to drive. So let me try to include the obvious ones as below:

a) Nominal impedance - This is not well defined, so manufacturers can, and probably do manipulate it to their advantage.

b) Minimum impedance - Some manufacturers, e.g. KEF, B&W, Focal include it in their specs.

c) Phase angle - I am not aware any manufacturers who specify phase angles.

d) Sensitivity - Most do provide this, some specify it in dB/Watt @ 1m, others dB/2.83V @ 1m

e) Maximum output in dB - Some manufacturers, e.g. KEF include this spec.

f) Power handling, or maximum power input - Some manufacturers, e.g. Klipsch, tend to provide this spec.

a), b) & c) have direct impact on whether the speakers is relatively hard/easy to drive, though they don't really mean much, unless you get to see the graphs that show how they vary with frequency.

e) and f) don't have direct impact on how hard it is to drive as such, but if the figures are low enough, then there may not be much point worrying about them. For example, the speaker may be rated 4 ohm nominal and has terrible phase angles in the 20 to 150 Hz range (highly unlikely) such that we may consider it hard to drive, but driving it with a 500W amp is not going to help if it's maximum output is only 96 dB @1m and can take a maximum of 100W or less.

 

[3db]

The OP only asked about impedance, but for better clarity I agree one has to consider other factors that have effects on how hard they are to drive. So let me try to include the obvious ones as below:

a) Nominal impedance - This is not well defined, so manufacturers can, and probably do manipulate it to their advantage.

b) Minimum impedance - Some manufacturers, e.g. KEF, B&W, Focal include it in their specs.

c) Phase angle - I am not aware any manufacturers who specify phase angles.

d) Sensitivity - Most do provide this, some specify it in dB/Watt @ 1m, others dB/2.83V @ 1m

e) Maximum output in dB - Some manufacturers, e.g. KEF include this spec.

f) Power handling, or maximum power input - Some manufacturers, e.g. Klipsch, tend to provide this spec.

a) and b) have direct impact on whether the speakers is relatively hard/easy to drive, though they don't really mean much, unless you get to see the graphs that show how they vary with frequency.

e) and f) don't have direct impact on how hard it is to drive as such, but if the figures are low enough, then there may not be much point worrying about them. For example, the speaker may be rated 4 ohm nominal and has terrible phase angles in the 20 to 150 Hz range (highly unlikely) such that we may consider it hard to drive, but driving it with a 500W amp is not going to help if it's maximum output is only 96 dB @1m and can take a maximum of 100W or less.

I only see item c from publications such as Soundstage but its importance is equal to that of the impedance value.

 

[PENG]

I only see item c from publications such as Soundstage but its importance is equal to that of the impedance value.

Stereophile typically includes them in their full reviews but Soundstage is more organized as you can find them all in one place.

 

[Johnny2Bad]

Again, you are not looking at the phase angle and makes your above statement incorrect. An 8 ohm speaker with a high phase angle is harder to drive than a 4 ohm speaker with a low phase angle.

I said nothing of how easy or difficult a load would be to drive. In fact I specifically stated that I would not be addressing that. Nor does Phase Angle have any effect on Damping Factor, the area I did address.

You need to improve your comprehension skills.

And for what it's worth, Phase Angle is also a vast simplification of how difficult a load a given loudspeaker is to an amplifier, although it certainly is a factor. For a fuller exploration of the issue of amplifier stress with various complex loudspeaker loads (impedance, capacitance, inductance, and phase angle, and the effect of the frequency of the various load factors) see:

"Input Current Requirements of High-Quality Loudspeaker Systems," Audio Engineering Society (AES) 73rd Convention, March 1983
"Peak Current Requirement of Commercial Loudspeaker Systems," AES 79th Convention, October 1985
"Audio Power Amplifiers for Loudspeaker Loads," Journal of the Audio Engineering Society, Vol.42 No.9, September 1994

 

[3db]

I said nothing of how easy or difficult a load would be to drive. In fact I specifically stated that I would not be addressing that. Nor does Phase Angle have any effect on Damping Factor, the area I did address.

You need to improve your comprehension skills.

And for what it's worth, Phase Angle is also a vast simplification of how difficult a load a given loudspeaker is to an amplifier, although it certainly is a factor. For a fuller exploration of the issue of amplifier stress with various complex loudspeaker loads (impedance, capacitance, inductance, and phase angle, and the effect of the frequency of the various load factors) see:

"Input Current Requirements of High-Quality Loudspeaker Systems," Audio Engineering Society (AES) 73rd Convention, March 1983
"Peak Current Requirement of Commercial Loudspeaker Systems," AES 79th Convention, October 1985
"Audio Power Amplifiers for Loudspeaker Loads," Journal of the Audio Engineering Society, Vol.42 No.9, September 1994

You mentioned in your post that a 8 ohm load provided a higher damping factor than a 4 ohm load. Phase angle alters what the amplifier sees as impedance and simply stating 8 ohms without qualifying a phase angle is only half the story. If you would have qualifed 8 ohms at a specified phase angle, then I would have not brought up point. I suggest very strongly that you assess the accuracy of your statements and include relevant parameters.

Im very well versed in impedance. One cant have impedance with out a frequency dependent component such as a capacitor or an inductor from which phase is derived. Anything else u care to add about impedance?

 

[TLS Guy]

Oh dear! This thread seems to be going off the rails. I think mainly because the original question has no practicality.

A four ohm speaker and an eight ohm speaker can not be otherwise identical. That fact alone got this thread off the rails.

Now if there could be, the sensitivity of the speakers would be identical on a one watt one meter spec. On a 2.83 volt one watt one meter spec the four ohm speaker would be 3 db more sensitive, but draw twice the power from the amp.

Now when I as a lad, all amps were tube. A tube amp is a voltage amplifying device. Tubes do not like high anode/cathode currents. So a transformer is required to match the output tubes to the speaker. So the speaker impedance is not critical. In fact there used to be taps on the output transformer to optimally match the amp to four, eight or sixteen ohm loads. However, tube amps always were, and continue to be at a disadvantage driving speakers with widely varying impedance with frequency.

Now a transistor on the other hand is a current amplifying device primarily. The output devices have been directly coupled to the speaker since the early seventies. Now high operating voltage between collector and emitter makes for unreliability, as high voltages tend to punch unwanted holes through the semiconductor and cause device failure. The amp can never produce a higher voltage at the speaker terminals than the rail voltage, in fact it is always a little less.

Now obviously it is voltage that drives current. So in order to get decent power to the speaker load requires a lower impedance. So speaker impedance trended downwards in the transition to solid state amplifiers.

Now this all contributed to higher currents to get the power at satisfactory transistor operating voltages. However current generates heat flowing through a resistance including the internal resistance of output devices. Heat is also a device killer.

The upshot of this is that all amps are voltage and current limited at a certain point. However cheaper devices are blown at lower current flows than more expensive ones. So in general as the price goes up amps are able to provide more current to a lower impedance loads without either clipping, because of inability to provide the required current, activating protection or self destructing.

Now lets look at the speaker end of the equation. First off lets dispose of the damping factor myth. Once you have a passive crossover there is no damping, period. A moving coil speaker can only interact with an amp and have useful damping if it is directly connected to the amp, via a short wire.

The next issue is that the impedance of any speaker is all over the map with frequency. There are bass tuning peaks, and peaks at crossover in addition for impedance of all drivers to rise with frequency, because the VC is an inductor, and that is how they behave.

Not only that, the voice coils and crossover components form reactive loads such that voltage and current are not in phase. This has two consequences. Effective impedance becomes lower than the measured value as the phase angle becomes negative. A gap is generated between true power and apparent power. However an amp must provide enough current to provide for apparent power needs or clipping will result. Passive crossovers are responsible for most of these ill effects and these become much more acute and prevalent as the crossover point is lowered.

This point was obliquely alluded to in a recent thread in which Dr Floyd Toole participated. He referred to costly high end speakers requiring "arc welder amps". He inferred that speakers requiring such amplification were incompetently designed.

He was referring to a problem in high end passive three way speakers, or more, with low crossover points. Now far too often these speakers have dips of impedance to less than three ohms and sometimes to the 2 ohm range or less. Now if the impedance of the speaker/crossover combination is less then the impedance of the drivers, then that combination is resonant. The speaker/crossover combination is ringing and the design is incompetent and not fit for purpose. That speaker will not be an accurate reproducer. Funnily enough in the high end these speakers abound.

Lastly, and perhaps more importantly, we have the issue of baffle step compensation and passive crossovers. This is the frequency at which a monopole speaker transitions from a half space to an omni directional radiator. That frequency increases as the width of the front baffle decreases. Unless the drive to the speaker is increased by 6 db below that frequency, then the speaker will sound thin.

Now only an active circuit can provide boost. A passive circuit can only provide cut, or attenuation.

So in a passive crossover the impedance has to drop below the baffle step frequency. What actually happens is that the impedance is allowed to rise to attenuate the higher frequencies.

So, if you make a speaker truly 8 ohms in the power range, it will be 16 ohms in the higher frequencies, and often more. Since solid state amps are voltage limited, that sets up for clipping in the higher frequencies due to voltage limiting.

If you design to avoid this, then the impedance will drop to four ohms and sometimes a bit less in the power range. This will embarrass amps that are current limited and may blow them up.

The consequences of this is that all decent passive speakers for solid state amps are in fact four ohm speakers, no matter what the manufacturer specifies. If not then amps will be voltage limited rather then current limited. Manufacturers collectively lie through their teeth about the impedance of their speakers.

Any amp that can not provide respectable power into a four ohm impedance is not a very good amp.

Now I think you can see that most of these difficult problems can be solved by transitioning to active speakers.

The bottom line is that amplifiers belong not in receivers, but speakers.

Again Dr Toole referred to this last week. We are only going to make substantial progress and improve sound quality at reasonable cost when this transition takes place.

 

[3db]

Oh dear! This thread seems to be going off the rails. I think mainly because the original question has no practicality.

A four ohm speaker and an eight ohm speaker can not be otherwise identical. That fact alone got this thread off the rails.

Now if there could be, the sensitivity of the speakers would be identical on a one watt one meter spec. On a 2.83 volt one watt one meter spec the four ohm speaker would be 3 db more sensitive, but draw twice the power from the amp.

Now when I as a lad, all amps were tube. A tube amp is a voltage amplifying device. Tubes do not like high anode/cathode currents. So a transformer is required to match the output tubes to the speaker. So the speaker impedance is not critical. In fact there used to be taps on the output transformer to optimally match the amp to four, eight or sixteen ohm loads. However, tube amps always were, and continue to be at a disadvantage driving speakers with widely varying impedance with frequency.

Now a transistor on the other hand is a current amplifying device primarily. The output devices have been directly coupled to the speaker since the early seventies. Now high operating voltage between collector and emitter makes for unreliability, as high voltages tend to punch unwanted holes through the semiconductor and cause device failure. The amp can never produce a higher voltage at the speaker terminals than the rail voltage, in fact it is always a little less.

Now obviously it is voltage that drives current. So in order to get decent power to the speaker load requires a lower impedance. So speaker impedance trended downwards in the transition to solid state amplifiers.

Now this all contributed to higher currents to get the power at satisfactory transistor operating voltages. However current generates heat flowing through a resistance including the internal resistance of output devices. Heat is also a device killer.

The upshot of this is that all amps are voltage and current limited at a certain point. However cheaper devices are blown at lower current flows than more expensive ones. So in general as the price goes up amps are able to provide more current to a lower impedance loads without either clipping, because of inability to provide the required current, activating protection or self destructing.

Now lets look at the speaker end of the equation. First off lets dispose of the damping factor myth. Once you have a passive crossover there is no damping, period. A moving coil speaker can only interact with an amp and have useful damping if it is directly connected to the amp, via a short wire.

The next issue is that the impedance of any speaker is all over the map with frequency. There are bass tuning peaks, and peaks at crossover in addition for impedance of all drivers to rise with frequency, because the VC is an inductor, and that is how they behave.

Not only that, the voice coils and crossover components form reactive loads such that voltage and current are not in phase. This has two consequences. Effective impedance becomes lower than the measured value as the phase angle becomes negative. A gap is generated between true power and apparent power. However an amp must provide enough current to provide for apparent power needs or clipping will result. Passive crossovers are responsible for most of these ill effects and these become much more acute and prevalent as the crossover point is lowered.

This point was obliquely alluded to in a recent thread in which Dr Floyd Toole participated. He referred to costly high end speakers requiring "arc welder amps". He inferred that speakers requiring such amplification were incompetently designed.

He was referring to a problem in high end passive three way speakers, or more, with low crossover points. Now far too often these speakers have dips of impedance to less than three ohms and sometimes to the 2 ohm range or less. Now if the impedance of the speaker/crossover combination is less then the impedance of the drivers, then that combination is resonant. The speaker/crossover combination is ringing and the design is incompetent and not fit for purpose. That speaker will not be an accurate reproducer. Funnily enough in the high end these speakers abound.

Lastly, and perhaps more importantly, we have the issue of baffle step compensation and passive crossovers. This is the frequency at which a monopole speaker transitions from a half space to an omni directional radiator. That frequency increases as the width of the front baffle decreases. Unless the drive to the speaker is increased by 6 db below that frequency, then the speaker will sound thin.

Now only an active circuit can provide boost. A passive circuit can only provide cut, or attenuation.

So in a passive crossover the impedance has to drop below the baffle step frequency. What actually happens is that the impedance is allowed to rise to attenuate the higher frequencies.

So, if you make a speaker truly 8 ohms in the power range, it will be 16 ohms in the higher frequencies, and often more. Since solid state amps are voltage limited, that sets up for clipping in the higher frequencies due to voltage limiting.

If you design to avoid this, then the impedance will drop to four ohms and sometimes a bit less in the power range. This will embarrass amps that are current limited and may blow them up.

The consequences of this is that all decent passive speakers for solid state amps are in fact four ohm speakers, no matter what the manufacturer specifies. If not then amps will be voltage limited rather then current limited. Manufacturers collectively lie through their teeth about the impedance of their speakers.

Any amp that can not provide respectable power into a four ohm impedance is not a very good amp.

Now I think you can see that most of these difficult problems can be solved by transitioning to active speakers.

The bottom line is that amplifiers belong not in receivers, but speakers.

Again Dr Toole referred to this last week. We are only going to make substantial progress and improve sound quality at reasonable cost when this transition takes place.

Stating impedance without phase angle is meaningless. Its only half the equation. You know better than this TLS. Don't propagate the BS.

 

[TLS Guy]

Stating impedance without phase angle is meaningless. Its only half the equation. You know better than this TLS. Don't propagate the BS.

The significance pf phase angle is clearly discussed in my post.

I will say one thing though, that these highly adverse phase angles are a function of passive crossovers.

I'm not propagating any BS.

When you actually look at hard to drive speakers with low impedance dips and negative phase angles, that situation is actually indicative of much more serious problems.

The more I consider this over the years, the more I'm convinced that passive crossovers have had heir day, and its time to move on. Passive crossovers in three way speakers should have been gone a long time ago.

Another upside of this also would mean the end of those dreadful receivers.

 

[3db]

The significance pf phase angle is clearly discussed in my post.

I will say one thing though, that these highly adverse phase angles are a function of passive crossovers.

I'm not propagating any BS.

When you actually look at hard to drive speakers with low impedance dips and negative phase angles, that situation is actually indicative of much more serious problems.

The more I consider this over the years, the more I'm convinced that passive crossovers have had heir day, and its time to move on. Passive crossovers in three way speakers should have been gone a long time ago.

Another upside of this also would mean the end of those dreadful receivers.

The point I keep making is that even a 4 ohm impedance can be benign as long as the phase angle is small. I don' care if an amp or AVR is driving it, I don't care what causes this. I'm simply stating that impedance without phase is meaningless. Yet you refer to low impedance without phase... and then go off on a tangent of AVR and power amps..

 

[TLS Guy]

The point I keep making is that even a 4 ohm impedance can be benign as long as the phase angle is small. I don' care if an amp or AVR is driving it, I don't care what causes this. I'm simply stating that impedance without phase is meaningless. Yet you refer to low impedance without phase... and then go off on a tangent of AVR and power amps..

It is not totally meaningless. True you really do need a graph of the phase angle superimposed on the impedance curve.

The next issue is that few members here know how to calculate the actual impedance that the amp sees when doing the trigonometric calculation from the phase angle at a given frequency.

I have done these calculations often. The bottom line is that for the vast majority of speakers this correction changes the actual measured impedance very little.

In practice the rule of thumb that the overall impedance the amp sees can be calculated from the minimum impedance plus 10%. To that, I would personally add one caveat that the minimum impedance occur in the below 600 HZ range.

Now I know that there are speakers, mainly high priced exotics, that have wide swings of phase angles, with negative phase angles coinciding with drops of impedance. I strongly suspect these speakers are basically rubbish designs with the crossover resonating, and not really worthy of consideration.

I feel it is important to consider speaker and output stage as a unit. This is not really done outside of active designs, as that is the only situation that makes it practical.

I don't feel we are going to make a whole sale advance in this area unless we change our design strategy and design amps and speakers in one box.

The real hold up is that receiver/amp manufacturers are not often speaker manufacturers and amp manufacturers are not often amp manufacturers. This is where we are long overdue for fundamental change.

 

[ski2xblack]

TLS, in your example of passive networks with very high impedance at higher frequencies, it would take a truly awful amp to become voltage limited due to high impedance at high frequencies, wouldn't it? Given the voltage required (highly attenuated in this case, presumably)? The whole point is to attenuate the treble for bsc or other voicing considerations anyway, which the higher impedance achieves, and I'm hard pressed to imagine the sort of amp that couldn't deliver the much reduced voltage in such circumstances.

I'm thinking of an extreme example in Klipsch's use of transformers in the networks of their classic designs, with impedance peaks in the midrange often closer to triple digits than the "nominal 8 ohm" ratings provided, and they didn't have a rep for being difficult loads whatsoever.

The only speakers I've heard of that fried amps due to impedance issues at high frequencies were the electrostatics, and that was due to extremely low impedance.

 

[3db]

It is not totally meaningless. True you really do need a graph of the phase angle superimposed on the impedance curve.

The next issue is that few members here know how to calculate the actual impedance that the amp sees when doing the trigonometric calculation from the phase angle at a given frequency.

I have done these calculations often. The bottom line is that for the vast majority of speakers this correction changes the actual measured impedance very little.


In practice the rule of thumb that the overall impedance the amp sees can be calculated from the minimum impedance plus 10%. To that, I would personally add one caveat that the minimum impedance occur in the below 600 HZ range.

Is this because a majority of speakers don't vary their phase angle dramatically?

 

[TLS Guy]

TLS, in your example of passive networks with very high impedance at higher frequencies, it would take a truly awful amp to become voltage limited due to high impedance at high frequencies, wouldn't it? Given the voltage required (highly attenuated in this case, presumably)? The whole point is to attenuate the treble for bsc or other voicing considerations anyway, which the higher impedance achieves, and I'm hard pressed to imagine the sort of amp that couldn't deliver the much reduced voltage in such circumstances.

I'm thinking of an extreme example in Klipsch's use of transformers in the networks of their classic designs, with impedance peaks in the midrange often closer to triple digits than the "nominal 8 ohm" ratings provided, and they didn't have a rep for being difficult loads whatsoever.

The only speakers I've heard of that fried amps due to impedance issues at high frequencies were the electrostatics, and that was due to extremely low impedance.

Most likely you are correct. However there is still a lot of power in the 1 to 2.5 K range and if the impedance really rises voltage limiting could become an issue, especially in lower sensitivity speakers. Remember the frequency response graphs are pretty much never drawn anywhere close to full power, subs being the exception to that.

I just took a look at some curves of speaker that could be considered 4 ohm and those that could legitimately be rated 8 ohm. Impedance rises to 20 ohms are not uncommon for 4 ohm speakers in the 1 to 2 k range. For 8 ohm speakers this doubles to the 35 to 40 ohm range.

So to produce 100 watts into 35 ohms requires a voltage of 60, while only 30 for 8 ohm. So,yes I do believe voltage as well as current limiting does have to enter into the equation. Obviously voltage limiting will not fry and amp, but it could add roughness at power in the higher pass bands.

The problem is that we do not measure everything, and even less do we look at what we measure under different power conditions.

The fact is that an impedance of 4 to 6 ohms in the 100 Hz to 500 Hz range has been found in practice to be optimal for most solid state output stages.

 

[TLS Guy]

Is this because a majority of speakers don't vary their phase angle dramatically?

That is one reason, but the main reason is that the peak negative phase angle seldom coincides with the nadir of impedance. If it does, then I have come to the conclusion there are major problems on the overall design of the speaker.

You need to be very wary of three way passive designs with crossover points below 350 Hz and even then and above you can run into trouble.

 

[ski2xblack]

Thanks, TLS. My question concerns this part:

So to produce 100 watts into 35 ohms requires a voltage of 60, while only 30 for 8 ohm.

100 watts into a tweeter seems a bit much. Most conventional dome tweeters that I'm familiar with can't withstand that sort of power, would never be subjected to such in the home, and generally cannot be crossed at or below 1.5khz. At the power levels they do actually see, and at frequencies well within their limits on the low end, voltage limiting would probably be extremely rare, no?

Just thinking out loud here.

 

[TLS Guy]

Thanks, TLS. My question concerns this part:



100 watts into a tweeter seems a bit much. Most conventional dome tweeters that I'm familiar with can't withstand that sort of power, would never be subjected to such in the home, and generally cannot be crossed at or below 1.5khz. At the power levels they do actually see, and at frequencies well within their limits on the low end, voltage limiting would probably be extremely rare, no?

Just thinking out loud here.

I really don't know, but I suspect you are right.

The problem is that these impedance rises often occur well before the crossover point.

If this is a problem, the result will be clipping before you expect it. It will be sudden on any peak. It would not behave like current limiting, as power supply reserve would not mitigate it.

 

[PENG]

The next issue is that few members here know how to calculate the actual impedance that the amp sees when doing the trigonometric calculation from the phase angle at a given frequency.

I don't see much point in even trying to calculate it though degree electrical engineers (obviously self taught individuals could do that too if they are good) should know how to do it. If one is to do it properly one would likely have to spend some time reviewing their old text books and/or notes, not just electrical circuit analysis but also advanced math including Laplace transforms as minimum. One would also need the crossover circuit diagram complete with values of every single components used. So except for DIY people such as yourself, it will be much easier to either search for lab measurements (e.g.Stereophile and Soundstage websites) or take their own measurements and plot their own graphs if they have the equipment.

In practice, I would agree for most reasonably well designed speakers, phase angles shouldn't be too bad, while there are certainly no shortages of good sounding speakers that are truly 4 ohm or less nominal. I think 3 dB's point is that it is a factor that should not be ignored, but I guess in doing so his choice of word ("meaningless") appeared a little strong for some.