Speaker Impedance revisited

[highfigh]

The paragraph below is taken from this article;

http://www.ece.msstate.edu/courses/design/ece4512/2003_spring/impedance_meter/Revised Problem 2.doc

In addition, extreme impedance phase angles can strain an amplifier. This measurement looks at whether the impedance of the loudspeaker is resistive, inductive, or capacitive at any given frequency. With impedance magnitude, the more severe the phase angle, the more difficult the speaker is to drive. The measurement of impedance magnitude and phase angle will give the user an idea of how to match the speaker to the amplifier. If the speaker has low impedance and a high phase angle, a powerful amplifier delivering high current while remaining stable into low impedances will be needed. If a speaker has higher impedance and a low phase angle, the demand on the amplifier is lessened.

Home Theater alludes to this;
http://www.hometheatermag.com/bootcamp/40//index1.html


http://en.wikipedia.org/wiki/Electrical_characteristics_of_dynamic_loudspeakers

http://www.stereophile.com/features/99/index5.html

Just to be clear, I never stated that a large phase angle had anything to do with how a speaker sounds nor have I ever implied it on this thread. I maintained the fact that impedance, phase angle and minimum impedance would help better match a loudspeaker to an amplifier's drive capability. Thats been my postion in this thread all along.

The impedance is usually considered to be comprised of resistance, capacitive reactance and inductive reactance and in a speaker system, these are always present to some degree. Some amps puke at the slightest hint of back EMF and some just shrug it off. Amplifier designers generally make sure to come up with something that isn't particularly touchy- if they have an amp with the electronic equivalent of a glass jaw, they won't be in business for long. Conversely, speaker manufacturers are in the same position- if they make speakers that cause amps to blow up on a regular basis, their longevity will be severely limited. Small receivers and amps have been powering a wide variety of speakers for close to 100 years now. While I agree that it's good to know what we're looking at when we're in the market for equipment, I think that you're looking for something that is less of a problem than you think. If it was a 500HP electric motor, I could see why it would be more important but a speaker driver is a lightweight motor assembly and, especially in a system that has a subwoofer, the phase angle issues are largely avoided because the crossover points are above the Fs, unless it's a woofer with no sub.

These drivers have varying phase angles, yet they're all good drivers.

http://www.parts-express.com/pdf/295-404s.pdf

http://www.parts-express.com/pdf/297-608.pdf

http://www.parts-express.com/pdf/297-449.pdf

http://www.parts-express.com/pdf/297-420.pdf

More detailed specs are great for those who want/need them but if you talk to people, most don't care. If you want them, I would recommend contacting the companies and asking for them. If they won't give them freely and you can find out what the driver part numbers are, you'll probably have better luck looking online. I think the manufacturers don't state many specs because:

-There's no standardization for extensive specs
-Most people don't want to learn about the fine details unless they are the techy type or they want to impress others
-They don't need to. It's not required, so there's no motivation.

Good luck in your quest.

 

[3db]

The impedance is usually considered to be comprised of resistance, capacitive reactance and inductive reactance and in a speaker system, these are always present to some degree. Some amps puke at the slightest hint of back EMF and some just shrug it off. Amplifier designers generally make sure to come up with something that isn't particularly touchy- if they have an amp with the electronic equivalent of a glass jaw, they won't be in business for long. Conversely, speaker manufacturers are in the same position- if they make speakers that cause amps to blow up on a regular basis, their longevity will be severely limited. Small receivers and amps have been powering a wide variety of speakers for close to 100 years now. While I agree that it's good to know what we're looking at when we're in the market for equipment, I think that you're looking for something that is less of a problem than you think. If it was a 500HP electric motor, I could see why it would be more important but a speaker driver is a lightweight motor assembly and, especially in a system that has a subwoofer, the phase angle issues are largely avoided because the crossover points are above the Fs, unless it's a woofer with no sub.

These drivers have varying phase angles, yet they're all good drivers.

http://www.parts-express.com/pdf/295-404s.pdf

http://www.parts-express.com/pdf/297-608.pdf

http://www.parts-express.com/pdf/297-449.pdf

http://www.parts-express.com/pdf/297-420.pdf

More detailed specs are great for those who want/need them but if you talk to people, most don't care. If you want them, I would recommend contacting the companies and asking for them. If they won't give them freely and you can find out what the driver part numbers are, you'll probably have better luck looking online. I think the manufacturers don't state many specs because:

-There's no standardization for extensive specs
-Most people don't want to learn about the fine details unless they are the techy type or they want to impress others
-They don't need to. It's not required, so there's no motivation.

Good luck in your quest.

You keep bring this discussion back to the driver level stating about the quality of the sound/driver and I keep pulling it out to the connector terminals of a speaker system where the crossover also plays into the impedance seen by the amp.

I've seen posts here in Audioholics stating incorrectly that 4 ohm and 6 ohm nominal impedance speakers are difficult to drive. The new PSB Imagine tower speakers rated nominaly at 4 ohms dips down to 3.4 ohms at a frequency of 72 Hz and a negligable phase angle is something that I would consider not be a difficult load at all even when run full range. (I don't care who the speaker is made by...its just a review I've just recently seen) And like you correctly stated, if you add a sub to the mix, the load even lessens.

I agree with you that most people don't care or would not understand this. But there are many people like me who have a degree in Electrical Engineering, or other people who take it upon themselves to read and understand this stuff that would like to know more. If the industry can standardize on versions of HDMI etc, why not start specifiying correctly speaker specifications.

 

[Rickster71]

Phase angle has everything to do with an amp's ability to drive speakers, much more so than impedance. A very high phase angle wastes the amplifier power. Its common practise for heavy industry to try and bring back lagging phase angles to zereo my deploying motors which have a leading phase angle because big phase angles wastes power, just like it would an amplifier.

Exactly.
An 8 ohm load with 60° phase angle presents more of a load on an amp than that of a 4 ohm load with 0° phase angle.

While I do understand your concerns, there is no perfect solution. But; there is a solution.

Buy an amp that doubles it's power when going from 8 ohms to 4 ohms. A cheap Behringer does that. (as was suggested earlier)
Or, add a sub and never even think about the problem again. Regardless of phase angle, it takes almost nothing to move a tweeter and mid-range driver.

 

[highfigh]

You keep bring this discussion back to the driver level stating about the quality of the sound/driver and I keep pulling it out to the connector terminals of a speaker system where the crossover also plays into the impedance seen by the amp.

I've seen posts here in Audioholics stating incorrectly that 4 ohm and 6 ohm nominal impedance speakers are difficult to drive. The new PSB Imagine tower speakers rated nominaly at 4 ohms dips down to 3.4 ohms at a frequency of 72 Hz and a negligable phase angle is something that I would consider not be a difficult load at all even when run full range. (I don't care who the speaker is made by...its just a review I've just recently seen) And like you correctly stated, if you add a sub to the mix, the load even lessens.

I agree with you that most people don't care or would not understand this. But there are many people like me who have a degree in Electrical Engineering, or other people who take it upon themselves to read and understand this stuff that would like to know more. If the industry can standardize on versions of HDMI etc, why not start specifiying correctly speaker specifications.

I have never seen phase angle rated for a speaker, rated at the cabinet's terminals. Ever. I haven't even seen more than a couple of amps get a little jumpy when driven hard because I will assume that either the manufacturers dealt with PA or they coincidentally avoided it by choosing their woofers. I also make sure that any amplifier I sell is matched to the speakers' impedance e.g., if the amp indicates that 8 Ohm speakers are to be used, I don't sell 4 Ohm speakers.

I think you may be fairly unique, because none of the people I know who are EE have ever mentioned this, at least when we discussed audio equipment. I'll have to bring it up next time.

 

[3db]

I have never seen phase angle rated for a speaker, rated at the cabinet's terminals. Ever. I haven't even seen more than a couple of amps get a little jumpy when driven hard because I will assume that either the manufacturers dealt with PA or they coincidentally avoided it by choosing their woofers. I also make sure that any amplifier I sell is matched to the speakers' impedance e.g., if the amp indicates that 8 Ohm speakers are to be used, I don't sell 4 Ohm speakers.

I think you may be fairly unique, because none of the people I know who are EE have ever mentioned this, at least when we discussed audio equipment. I'll have to bring it up next time.

This is the review I read that got me started down this path..

http://ultimateavmag.com/speakersystems/408psbsyn/index6.html

Oops PSB Synchrony towers, not the Imagine like I had thought previously..

The thing is, the amp sees the speaker at the terminals (if not bi-amping)
and not at the woofer. Most people care about the speaker as a whole, not only the drivers. An an 8 ohm speaker with a large PA is much harder to drive than a 4 ohm speaker with a small PA given sensitivty remains the same.

It may have never been mentioned because people took for granted lousy speaker specifications. The fact remains that PA and the frequency at which the impedance dip occurs should be mentioned in speaker specs.

 

[highfigh]

Oops PSB Synchrony towers, not the Imagine like I had thought previously.

Well, that changes everything!

But even though the amp looks at the system from the speaker terminals, the largest PA is in the octave above and below the Fs of the drivers and the HP/bandpass crossovers (per design) should keep this from affecting the total load for anything other than the woofer(s), right? It would seem logical (at least, to me) that if the moving mass of the speaker should be as low as possible with higher compliance to keep the PA lower. Maybe we should revisit MFB designs.

Another fact that can't be avoided- many pieces of great audio equipment have lousy specs but produce great sound.

 

[PENG]

I've seen posts here in Audioholics stating incorrectly that 4 ohm and 6 ohm nominal impedance speakers are difficult to drive.

I see your point and you are absolutely right. Unfortunately we do have people who keep rasing the point about how speaker's impedance specs do not mean anything and how they would dip to 2 ohms or less and at large phase angle blablabla...etc... People could be confused and misled to believe their speakers are so hard to drive that they need to acquire some high power amps in order to get the so called "full potential" of their speakers. I own a few amps but I am not going to tell everyone that they need amps for their speakers. More power never hurts but how much you can benefit from it depends on the individual situations.

If you have the impedance vs frequency and phase angle vs frequency curves it will help you find out if your speakers are really hard to drive. The problem is, such specs are typically not available so you may have to find ways to take the measurements yourselves. It may be easier to just buy a cheap pro amp and find out if it makes a difference for you at your listening level. I found out my amps did not make a difference in my h.t. room despite claims in various forums but I leave it connected just because I have an extra one. If I ever end up in a larger room and/or speakers that are truly hard to drive, then the amp will certainly have a more important role to play.

 

[PENG]

1.) When SM (speaker manufacturers) model their products impedance curve, are they using just a single signal that is swept from 20Hz to 20KHz?
2.) If two or more frequencies are being applied to the speaker, is the impedance calculated to be the vector sum of the impedance of the speaker for each given frequency contained in the signal?
3.) Does the minimum impedance of the speaker always correlate with the max phase angle?

1) I am guessing but I am pretty sure they do.

2) As you know, they typically call it "nominal impedance". There isn't really such a thing as vector sum of impedance, effective/or equivalent impedance may be (but just may be) more appropriate. If you hook up your amp output to an oscilloscope you will just see a time varying signal that does not look like a sine wave at all. In reality the impedance is also time varying and I guess that is a good enough reason for "SM" to only provide "nominal" impedance.

3) I have seen enough curves to know that the answer is no, but it could happen.

 

[TLS Guy]

It's not that hard to add a Zobel network ('network' sounding a bit more impressive than it really is) to smooth the impedance and many manufacturers don't bother with it because at Fs, when the driver would start flapping like sheets in the wind if the impedance graph was ruler flat, the coincidental impedance peak keeps it in check.

A frequency response graph is for indicating only the response. If it has an impedance curve as well, it's usually to impress customers who don't have much knowledge of specs and what goes into designing a speaker. Remember- the spec sheets and all of the info that comes from equipment manufacturers comes through the marketing department and not engineering. In most cases, marketing sells the speakers, not how intelligently the products were designed. Buzz words and BS are far more common than legitimate data.

At some point, it becomes time to listen to the music and not the equipment. If a person can't get past minute flaws in the sound, it's their problem, really. Great sound is possible but perfection is not. It's far easier to make mediocre speakers sound good by treating the room than making excellent speakers sound great in a bad room. Too many people who call themselves "audiophile" do little or nothing to/for the room's acoustics and that's a really good way to eliminate any real satisfaction on their system's sound.

If the acoustics are bad, does a flat impedance graph with little phase angle really matter?

Actually the zobel is used, when required to make the crossover work. Most bass drivers have a rising impedance with frequency, often a steep curve. So the low pass inductor sees a rising frequency, so the HF will not be attenuated to the driver, unless the drivers impedance curve is flattened.

However this does not mean that the impedance curve of the speaker as a whole is flattened.

Phase angles do not correlate with the impedance curve, it is just that adverse phase angles that occur at dips of impedance cause maximal amplifier stress.

The impedance curve does not take into consideration the phase angles.

A loudspeaker stress index has been proposed to give one simple number, that takes into account the impedance curve, and the phase angles so that a number is derived that has a true bearing on how much an amp is stressed.

The problem I see, is that most loudspeaker manufacturers would not want you to see this number.

 

[highfigh]

Actually the zobel is used, when required to make the crossover work. Most bass drivers have a rising impedance with frequency, often a steep curve. So the low pass inductor sees a rising frequency, so the HF will not be attenuated to the driver, unless the drivers impedance curve is flattened.

However this does not mean that the impedance curve of the speaker as a whole is flattened.

Phase angles do not correlate with the impedance curve, it is just that adverse phase angles that occur at dips of impedance cause maximal amplifier stress.

The impedance curve does not take into consideration the phase angles.

A loudspeaker stress index has been proposed to give one simple number, that takes into account the impedance curve, and the phase angles so that a number is derived that has a true bearing on how much an amp is stressed.

The problem I see, is that most loudspeaker manufacturers would not want you to see this number.

I didn't want to infer that the Zobel created a flat line impedance graph and had always understood that it was used only when the peaks were excessive and then, only to temper the range of impedance so, as you pointed out, the crossover would work (because the formula is resistance-dependent) and to keep the amp happy. I have also heard/read that avoiding drastic swings in impedance is just part of good speaker design, which I agree with. As I posted earlier, until fairly recently, I hadn't heard much about PA and never from a speaker company, although I have seen it included on the impedance graph for individual drivers.

Most of the graphs I have seen showed the PA at 0° very close to the driver's Fs, the PA max below Fs and the minimum above. Do you know of speaker manufacturers who have addressed this? In the late '70s/early '80s, I remember Infinity having a reputation as being difficult for many receivers to drive but that just about any decent separates were OK with it. The impedance curve doesn't take PA into account but I have seen many graphs that include both which I assume is because, since both have Frequency on the x-axis.

We agree- speaker companies don't want much info given out for their products.

 

[TLS Guy]

I didn't want to infer that the Zobel created a flat line impedance graph and had always understood that it was used only when the peaks were excessive and then, only to temper the range of impedance so, as you pointed out, the crossover would work (because the formula is resistance-dependent) and to keep the amp happy. I have also heard/read that avoiding drastic swings in impedance is just part of good speaker design, which I agree with. As I posted earlier, until fairly recently, I hadn't heard much about PA and never from a speaker company, although I have seen it included on the impedance graph for individual drivers.

Most of the graphs I have seen showed the PA at 0° very close to the driver's Fs, the PA max below Fs and the minimum above. Do you know of speaker manufacturers who have addressed this? In the late '70s/early '80s, I remember Infinity having a reputation as being difficult for many receivers to drive but that just about any decent separates were OK with it. The impedance curve doesn't take PA into account but I have seen many graphs that include both which I assume is because, since both have Frequency on the x-axis.

We agree- speaker companies don't want much info given out for their products.

The zobel is allow the crossover to work. Peaks are dealt with by series and parallel notch filters.

Basically the impedance curve and phase angles are what you end up with. smooth frequency response always trumps everything else.

You can't start a design with the impedance curves and phase angles, you take what you get.

Now you can flatten the impedance after you are done, however you add enormously to the part count, especially in terms of large expensive inductors and capacitors.

This would end up making the speaker expensive to the point were a robust amp would be the cheaper option.

When I designed and built theses speakers, as a technical exercise, I gave these speakers a ruler flat 8 ohm curve. I worked out everything to be as near a resistive load as I could.

However the part count for each crossover went to 29 components, with three boards in each speaker.

I have to admit the amp stays very cool indeed even when pushing them hard.

The components for the crossovers ran to almost $400.

Now in the budget and mid range speakers, I'm already seeing a lot of miserable iron core inductors with far too small a gauge of copper wire. That hobbles a speaker before you start. If obsessional impedance curve smoothing was added, I just hate to see what the components would look like.

For these reasons, I think moving to powered speakers with active crossovers, is by far the best road to go down.

 

[highfigh]

The zobel is allow the crossover to work. Peaks are dealt with by series and parallel notch filters.

You can't start a design with the impedance curves and phase angles, you take what you get.

Now you can flatten the impedance after you are done, however you add enormously to the part count, especially in terms of large expensive inductors and capacitors.

This would end up making the speaker expensive to the point were a robust amp would be the cheaper option.

When I designed and built theses speakers, as a technical exercise, I gave these speakers a ruler flat 8 ohm curve. I worked out everything to be as near a resistive load as I could.

However the part count for each crossover went to 29 components, with three boards in each speaker.

I have to admit the amp stays very cool indeed even when pushing them hard.

The components for the crossovers ran to almost $400.

Now in the budget and mid range speakers, I'm already seeing a lot of miserable iron core inductors with far too small a gauge of copper wire. That hobbles a speaker before you start. If obsessional impedance curve smoothing was added, I just hate to see what the components would look like.

For these reasons, I think moving to powered speakers with active crossovers, is by far the best road to go down.

With all of the "less is more" and KISS that has been mentioned in audio equipment and systems, I have wondered if the most complex crossovers and amplifier circuits produce the best sound. I still read and hear people saying that they have trouble breathing when they hear about an amp with any NFB, then they turn around and buy speakers that make this a moot point.

"Basically the impedance curve and phase angles are what you end up with. smooth frequency response always trumps everything else."

That's kind of what I have been saying. Impedance peaks can be dealt with and sound quality is king. One of my first posts in this thread mentioned using amps that can drive difficult loads and until it becomes easier/cheaper to deal with difficult speakers, it's what I will do and recommend.

I don't see the speaker companies suddenly spewing details that have previously been "unmentionables". They almost act as if it's a national secret, but I sometimes wonder if it's more a case of "we don't do as much research as we would lead you to believe" or "we modeled the drivers, came up with a crossover, built them and they sounded fine, so we took them to market". Good sounding speakers can be built this way.

I built a pair of 6-1/2" 2-way by entering the parameters in Term Pro, built the box/installed the vent and placed the tweeter. Once these were done, I used an Audio Control 3050A RTA at 1 meter in a reverberant room to find out what, if any, response problems existed before doing anything with crossover design. The only thing I have added is an L-pad for the tweeter level with a mark for using my solid state or tube amp. I think I'll claim some of the credit for the overall sound quality by finding out what the response was before doing the crossover but I also think some luck was involved. Human voice sounds very natural, overall smoothness is better and much more open than speakers I have listened to, with high price tags. The problems in bass response are from the subwoofer/main speaker interface and room modes and I am beginning to look for drivers for a full-range system design. My crossovers are simple first and second order with a Zobel but, like yours, the parts cost me over $100 at wholesale.

Interesting hobby we have, eh?

 

[R-Carpenter]

Not to mention huge insertion loss in a 4th order electrical crossover (which happens to be my favorite).
Parts need to have low resistance and with large size inductors you end up paying $$.

 

[3db]

With all of the "less is more" and KISS that has been mentioned in audio equipment and systems, I have wondered if the most complex crossovers and amplifier circuits produce the best sound. I still read and hear people saying that they have trouble breathing when they hear about an amp with any NFB, then they turn around and buy speakers that make this a moot point.

"Basically the impedance curve and phase angles are what you end up with. smooth frequency response always trumps everything else."

That's kind of what I have been saying. Impedance peaks can be dealt with and sound quality is king. One of my first posts in this thread mentioned using amps that can drive difficult loads and until it becomes easier/cheaper to deal with difficult speakers, it's what I will do and recommend.

I don't see the speaker companies suddenly spewing details that have previously been "unmentionables". They almost act as if it's a national secret, but I sometimes wonder if it's more a case of "we don't do as much research as we would lead you to believe" or "we modeled the drivers, came up with a crossover, built them and they sounded fine, so we took them to market". Good sounding speakers can be built this way.

I built a pair of 6-1/2" 2-way by entering the parameters in Term Pro, built the box/installed the vent and placed the tweeter. Once these were done, I used an Audio Control 3050A RTA at 1 meter in a reverberant room to find out what, if any, response problems existed before doing anything with crossover design. The only thing I have added is an L-pad for the tweeter level with a mark for using my solid state or tube amp. I think I'll claim some of the credit for the overall sound quality by finding out what the response was before doing the crossover but I also think some luck was involved. Human voice sounds very natural, overall smoothness is better and much more open than speakers I have listened to, with high price tags. The problems in bass response are from the subwoofer/main speaker interface and room modes and I am beginning to look for drivers for a full-range system design. My crossovers are simple first and second order with a Zobel but, like yours, the parts cost me over $100 at wholesale.

Interesting hobby we have, eh?

I repeat again.. .. I never equated good sound with large PA.. just concerned about matching amps power capabilities to speaker loads and how people have incorrectly posted 4ohm speakers are more diffiuclt to drive without qualifying that statement as there are two other variables that never get mentioned when issuing these blanket statements.


BTW.. Merry Christmas and Happy New Years everyone. You too highfigh