Difference between 8 ohm & 6 ohm speakers

[TLS Guy]

I see your logic (8 ohms in parallel is 4 ohms) but its not as simple as that. There is the impedance of the crossover network to consider as well.

I have designed enough of these that I know what actually takes place. In the range below where the diffraction compensation cuts in the impedance of all drivers is pretty flat and only a little above the DC resistance. As stated before there will be narrow tuning peaks from the enclosure, one for sealed and TL, two for reflex porting. The combined impedance rise of the series inductor and voice coil inductance does not rise until above cut off. More often than not you have to include a zobel circuit, which eliminates this rise to a large degree, so the woofer cuts off. The impedance at the mid HF crossover is far more often than not a slight dip. Most people, and I usually don't, correct the impedance rise of the tweeter with frequency, so you usually see a significant rise of frequency above the high pass crossover region. There is not much power here. That is why it makes no sense to average the speaker impedance frequency curve to come up with a "nominal Impedance."

And one other thing, the phase angles between current and voltage always makes the situation worse than you would think from the impedance curve. The degree to which it is made worse varies greatly from design to design, complicating matter further.

In an ideal world all amplifiers should be designed to cope with this reality. However at the price point of receivers, and given the heat constraints of having everything in one box, this is not going to happen. However every speaker designer I have ever known, and I include myself, never takes amplifier current limitations into account.

 

[3db]

TLS

Hare the curves measured for the T45s. As you can see from the measured curves, chart 5 impedance drops down to a low of 4 ohms for a band of 100 Hz from 150 to 250. The rest of the impedance curve sits above this.

http://www.soundstagemagazine.com/measurements/speakers/psb_image_t45/

In order to keep the impedance from dipping to four ohms, the speakers would have to be 16 ohms above 600 Hz, which would drop the sensitivity 3 db.

It is not the frequency impedance range you need to average, but the power
spectrum versus frequency The frequency power divide is 400 Hz. Below that area the impedance of a 2.5 way has to drop except for the narrow tuning peaks. As the impedance is halved the amp has to double the power output. So in a 2.5 way design the amp is providing around two thirds of its output into a four ohm load or less. That is the price of diffraction compensation. There is no free lunch.

The bottom line is that most receivers are only delivering half their rated output into properly diffraction compensated speakers. The good news is that this is only a 3db reduction in spl. The reason that the power is halved is that most receivers, although they may tolerate a four ohm load, they can not increase their current beyond what they can push into an eight ohm load at full power. Therefore there power output is halved into a four ohm load. A good external amp will be able to double its power into a four ohm load. This in a nutshell is the essence of the benefit of external amplification.

 

[TLS Guy]

TLS

Hare the curves measured for the T45s. As you can see from the measured curves, chart 5 impedance drops down to a low of 4 ohms for a band of 100 Hz from 150 to 250. The rest of the impedance curve sits above this.

http://www.soundstagemagazine.com/measurements/speakers/psb_image_t45/

That just makes my point. The area of low impedance is were most of the power is, especially if the speakers are set to small with a crossover between 60 and 80 Hz.

 

[3db]

That just makes my point. The area of low impedance is were most of the power is, especially if the speakers are set to small with a crossover between 60 and 80 Hz.

I guess I need a translation to understand what you are saying. I'm looking at it from a frequency vs impedance point of view.

 

[highfigh]

"The impedance at the mid HF crossover is far more often than not a slight dip. Most people, and I usually don't, correct the impedance rise of the tweeter with frequency, so you usually see a significant rise of frequency above the high pass crossover region. There is not much power here. That is why it makes no sense to average the speaker impedance frequency curve to come up with a "nominal Impedance.""

"But, that's the way we have always done it".

This is a response I would expect from many speaker manufacturers and in fact, a lot of speaker spec sheets show "Nominal Impedance- 8 Ohms, 6 Ohms or 4 Ohms."

"In an ideal world all amplifiers should be designed to cope with this reality. However at the price point of receivers, and given the heat constraints of having everything in one box, this is not going to happen. However every speaker designer I have ever known, and I include myself, never takes amplifier current limitations into account."

Right, but you're not crimping a cap in series with the tweeter and calling it "Done". The number of speakers with this "crossover" is incredible. Are these some of the cheapest garbage I have seen? Absolutely but they sold tens of thousands of pairs and at the time, people who wanted a cheap stereo thought they were OK. Oddly enough, some thought they were great until they blew up.

I know you mentioned designing a crossover that is almost purely resistive but how many speaker manufacturers see that as economically feasible?

It's all about acceptable compromise, don't you agree?

 

[lsiberian]

I see your logic (8 ohms in parallel is 4 ohms) but its not as simple as that. There is the impedance of the crossover network to consider as well.

This is true,

But toss in the phase angle which gets to 40 degrees on my speakers. And I get 0.76 * the 5 ohm bottom modeling of my driver. Which is below 4 ohms. four.

Do that with a 4ohm model and it's around 3ohms as a true load. It's definitely fun calculating all this.

This is so much fun. I love when threads become educational.

 

[TLS Guy]

I guess I need a translation to understand what you are saying. I'm looking at it from a frequency vs impedance point of view.

I know you are and that is not useful. You have to look at power delivered to maintain constant spl. to really have something useful. In most music program 50% of power is below 400 Hz and 50% above 400 Hz. So if you had a speaker of constant impedance then half the power would be dissipated above 400 Hz and half below, for typical program. Now if you start to halve the impedance below 400 Hz, like in the speaker you mentioned, then about two thirds of the amp power is delivered below 400 HZ. That is totally dominant to the situation, and for all practical purposes the speaker is a four ohm speaker.

Looking at it like a speaker manufacturers add department is pure sleight of hand.

 

[Ed Seedhouse]

No one seems to have mentioned phase angle. In a straight wire if you increase voltage then you increase current at exactly the same time. But when we deal with impedance in capacitators and inductors that is not so and the current may lag behind the voltage. This is measured by phase angle, in degrees. A full wavelength of lag would be a 360 degree phase angle.

Most speakers exhibit phase angle anomolies across the audible band that are quite large, especially if the crossover filters are complicated. A combination of low impedance with a high phase angle can really mess a poorly designed amplifier up.

All in all a simple single figure specification of impedance tells you virtually nothing of any use to anyone. A frequency plot of the impedance tells you more, but to really understand the load you also have to see a plot of phase angle by frequency as well.

 

[CraigV]

Wow, you guys put way too much thought into this. The answer is 2 ohms

 

[srk16]

One can't really answer that question accurately without knowing the impedance curves of the speakers? It can be that some 8 ohm speakers could have its impedance drop below 4 ohms where as a 6 ohm speaker may have a much tighter impedfance curve and barely drop below 5 ohms. There are just too many variables to answer that question correctly. What you may want to ask is what speakers you consider using with what AVR in a room with X dimesnions listening to low, moderate or loud listening levles. This would help us out alot more in answering your questions.

Thanks for your reply 3db, Iam thinking to buy Wharfedale Vardus Speakers(6ohm impedance) for either an Onkyo 607 or Pioneer 1018 a/v receiver.My other speaker options are Mission MV8(8ohm impedance),Jamo 506 series(6 ohm),KEF(8ohm).My lounge is 320 cm * 250 cm size,it may become bigger later in future.Most likely I will go Wharfedale speakers as they fit my budget.

 

[highfigh]

No one seems to have mentioned phase angle. In a straight wire if you increase voltage then you increase current at exactly the same time. But when we deal with impedance in capacitators and inductors that is not so and the current may lag behind the voltage. This is measured by phase angle, in degrees. A full wavelength of lag would be a 360 degree phase angle.

Most speakers exhibit phase angle anomolies across the audible band that are quite large, especially if the crossover filters are complicated. A combination of low impedance with a high phase angle can really mess a poorly designed amplifier up.

All in all a simple single figure specification of impedance tells you virtually nothing of any use to anyone. A frequency plot of the impedance tells you more, but to really understand the load you also have to see a plot of phase angle by frequency as well.

TLS did a few posts ago.

 

[Tomorrow]

Here is an example of how silly "nominal impedance" numbers can be. This is the impedance graph for my speakers. What do you think the impedance rating should be? The manufacturer says "6 ohms". My receiver begs to differ...especially when it dips below 3 ohms near 400 Hz.

 

[3db]

I know you are and that is not useful. You have to look at power delivered to maintain constant spl. to really have something useful. In most music program 50% of power is below 400 Hz and 50% above 400 Hz. So if you had a speaker of constant impedance then half the power would be dissipated above 400 Hz and half below, for typical program. .

Clear so far...

Now if you start to halve the impedance below 400 Hz, like in the speaker you mentioned, then about two thirds of the amp power is delivered below 400 HZ. That is totally dominant to the situation, and for all practical purposes the speaker is a four ohm speaker.

This is the part I don't get. Are you looking at the impedance curve to see that speaker's impedance has dropped down by half at 400 Hz and below?

 

[3db]

Thanks for your reply 3db, Iam thinking to buy Wharfedale Vardus Speakers(6ohm impedance) for either an Onkyo 607 or Pioneer 1018 a/v receiver.My other speaker options are Mission MV8(8ohm impedance),Jamo 506 series(6 ohm),KEF(8ohm).My lounge is 320 cm * 250 cm size,it may become bigger later in future.Most likely I will go Wharfedale speakers as they fit my budget.

Opps forgot to ask for budget? How much are you willing to spend. The PSBs that I have whose plots I've shown is a really nice speaker, versatile, smooth sounding with good tight bass down to high 30Hz, life like vocals and nice smooth highs.

 

[TLS Guy]

Clear so far...



This is the part I don't get. Are you looking at the impedance curve to see that speaker's impedance has dropped down by half at 400 Hz and below?

Yes, the speakers impedance is four ohms at 100 Hz and it does not reach five ohms until 500 Hz. So the amp driving them is going to be delivering the majority of its power into a four ohm load.

 

[Lordoftherings]

lsiberian, some tweeters drops their impedance in the 2 to 4-ohm regions.

But then, what does it requires from an amp?

Stability in the highs perhaps?

 

[Lordoftherings]

You got to see the graph to really know.

Or put your hand on top of the amp.*

* Referring to speakers having dips in the 2-ohm region.

 

[Lordoftherings]

Wow, you guys put way too much thought into this. The answer is 2 ohms

Hey Craig, how did you come up with the right answer?

 

[Lordoftherings]

Here is an example of how silly "nominal impedance" numbers can be. This is the impedance graph for my speakers. What do you think the impedance rating should be? The manufacturer says "6 ohms". My receiver begs to differ...especially when it dips below 3 ohms near 400 Hz.

Congratulations! That's a very nice speaker (Aerial 7B), built by a good team of people lead by a great speaker designer, Michael Kelly.

I'll say this 7B model is a 5-ohm nominal impedance speaker, with a 3-ohm minimum rating. Needs a true amp. Receivers not recommended, unless THX Ultra2 certified, and even then, it doesn't mean much at all.

* Another very important point that many people seems to forget; it's the electrical phase of a speaker.

 

[Lordoftherings]

If you guys like to see a great speaker frequency response and electrical phase curves looks like,
try to Goggle: "Ampion Athene".
It's a speaker from Finland.

It is one of the best curves I've ever seen.
Talk about a true high resolution music reproducer, with a maximum soundstage.
And cost much less than $2,000 USA per pair.