Impedance and Sensitivity Question (again)

[xboxweasel]

It seems that no one really knows the answer to this. I've scanned the forum and have not found what I am looking for.

Here is my story: A very high end stereo shop in town (owner just sold a $100K system to a doctor) sells speakers rated at 2 ohms. I don't have the make and model numbers yet. I will get these tomorrow when I go and visit this shop. Anyways, he says that the reason for all these 8 ohm speakers out there is so that the cheap receivers don't melt. Fine. I can understand that.

I read a little about the history of receivers and can understand why manufacturers went to 8 ohms back in the 60's.

What I want to know is what is the difference in a low impedance speaker when it comes to the sound? Because that is what it is all about. I do not care about the rest of the equipment. I care about the sound characteristics. In the end, is it really worth it? Can you hear it? Can you feel it? Can you taste it?

Now how about sensitivity. Same thing. Higher sensitivity = easier for cheaper receivers to handle. Again why go with low sensitivity speakers? Let's say low 80-85dB/2.83v/meter. If you can afford it should you go with separates? Run 30A circuits to all your amplifiers? Is it worth it?

PS: I am not trashing anyones equipment. Cheap and expensive are relative terms. Not everyone makes $500k/year. I am one of those. I have a Yamaha receiver and 8 ohm speakers. I just want to know.

 

[Seth=L]

Lower impedance could offer the ability to produce lower frequency bass in a smaller enclosure space. One speaker can't have all the benefits of a speaker system. It can't be small, be efficient, and be even loaded around 8 ohms, and sound larger or cover a larger part of the frequency band all at once.

That is my understanding, and it is plausible that I could be wrong (it's not beyond me to be wrong some of the time).

If WmAx (Chris) sees this thread he would likely have a better explanation if not completely different that mine.

 

[PENG]

A loudspeaker's impedance does not stay constant throughout the 20-20,000 Hz range. The impedance of an 8 ohm nominal speaker could dip well below 8 ohms. It is reasonable to assume that if a manufacturer can set its goal solely on sound quality; without having to worry about the impedance, it may have more options and therefore better chance of achieving its goal to produce the best sounding product. That being said, I don't think it is necessarily true that two ohm speakers will do better than 4 to 8 ohm speakers, they just may have better chance.

 

[Seth=L]

I agree with PENG in this respect. Just because a speaker is 2 ohms nominal does not make it better, it just has a different characteristic. Some "white van special speakers" are 2 ohm and they are absolute garbage and would damage the equipment of the owner (it isn't likely that someone who owns Halcro monoblocks is going to fall for a speaker scam, only a few people on a budget with budget equipment). But at the same time being 2 ohm rated doesn't mean it is a bad speaker either, and just the same with 8 ohm speakers. There are crappy 8 ohm speakers, and they are excellent 8 ohm speakers. The B&W 802 Nautilus comes to mind, they are 8 ohms nominal speakers and they are great speakers.

 

[mtrycrafts]

A loudspeaker's impedance does not stay constant throughout the 20-20,000 Hz range. The impedance of an 8 ohm nominal speaker could dip well below 8 ohms. It is reasonable to assume that if a manufacturer can set its goal solely on sound quality; without having to worry about the impedance, it may have more options and therefore better chance of achieving its goal to produce the best sounding product. That being said, I don't think it is necessarily true that two ohm speakers will do better than 4 to 8 ohm speakers, they just may have better chance.

Some time back, in one of his emails to me, Dr Toole didn't have kind words for those low impedance speakers and their designers.

 

[mtrycrafts]

It seems that no one really knows the answer to this. I've scanned the forum and have not found what I am looking for.

Here is my story: A very high end stereo shop in town (owner just sold a $100K system to a doctor) sells speakers rated at 2 ohms. I don't have the make and model numbers yet. I will get these tomorrow when I go and visit this shop. Anyways, he says that the reason for all these 8 ohm speakers out there is so that the cheap receivers don't melt. Fine. I can understand that.

I read a little about the history of receivers and can understand why manufacturers went to 8 ohms back in the 60's.

What I want to know is what is the difference in a low impedance speaker when it comes to the sound? Because that is what it is all about. I do not care about the rest of the equipment. I care about the sound characteristics. In the end, is it really worth it? Can you hear it? Can you feel it? Can you taste it?

Now how about sensitivity. Same thing. Higher sensitivity = easier for cheaper receivers to handle. Again why go with low sensitivity speakers? Let's say low 80-85dB/2.83v/meter. If you can afford it should you go with separates? Run 30A circuits to all your amplifiers? Is it worth it?

PS: I am not trashing anyones equipment. Cheap and expensive are relative terms. Not everyone makes $500k/year. I am one of those. I have a Yamaha receiver and 8 ohm speakers. I just want to know.

You may want to ask the dealer if he knows of anyone having a completer measurement of those low impedance speakers: THD, power response on axis and off, proper FR plots, etc. That would tell a whole lot about those speakers.
Yep, you got it right though, when you have such speakers, low impedance, low sensitivity, you need a power station to drive them

I wonder how much that Dr. knew or knows about audio

By the way, here is a speaker impedance curve that is rated at 8 Ohms. Looks like minimum is about 7 Ohms and most of it is well above 8. Interesting.

http://www.av123.com/images/miniz.gif

 

[TLS Guy]

Impedance nothing to do with quality

There is nothing that makes a two ohm speaker better than a four or eight ohm one.

The problem is the high powered amps. Transistors like to deliver current to a point. The lower the impedance the more current the power amp will deliver, up to a point. Quite a few amps now deliver 400 watts or more into low impedance loads. Now if an amplifier manufacturer were to design an amp delivering 400 watts or more into 8 ohms, if a customer connected it to speakers with low impedance it would spell destruction for the amp, unless the designer has a circuit to lower the supply voltage at a certain current set point. This will drive an amp into hard clipping, and high end amp manufacturers do not consider that good for their reputations.

There is a big problem with low impedance speakers, as the resistance of the cables easily gets over 5% of the nominal impedance of the speakers. That is not the best. As stated before loudspeaker impedance is all over the map in a given speaker.

There is another issue also, and that is safety. Speaker terminals are not shrouded. An amp delivering 400 watts into 8 ohms would generate 56 Volts across the speaker terminals. And 400 watts is just the RMS power, and on peaks could produce 80 volts. That's enough for a nasty nip. At 2 ohms the voltage is 28 and the highest reasonable peak would be 40 volts, and that's enough for a good tingle.

As far as sensitivity goes the higher the better, as every 3db increase in sensitivity is the same as doubling the amp power. The only problem is that there is a relationship between a bass driver's sensitivity and bass response.
A lower sensitivity tending to favor a better bass response. That is why very sensitive drivers, such as the Lowthers are loaded by bass horns, to bring the bass output up the level of the mid and HF.

 

[xboxweasel]

Great. Thanks a lot people.

I did not go there today. Maybe next week. I'll see what the store owner has to say and report back.

Later.

 

[no. 5]

Some time back, in one of his emails to me, Dr Toole didn't have kind words for those low impedance speakers and their designers.

Reeeealy... care to paraphrase some of it for the rest of the class?

 

[jostenmeat]

A loudspeaker's impedance does not stay constant throughout the 20-20,000 Hz range. The impedance of an 8 ohm nominal speaker could dip well below 8 ohms. It is reasonable to assume that if a manufacturer can set its goal solely on sound quality; without having to worry about the impedance, it may have more options and therefore better chance of achieving its goal to produce the best sounding product. That being said, I don't think it is necessarily true that two ohm speakers will do better than 4 to 8 ohm speakers, they just may have better chance.

I've really wondered about the bold-faced portion. Thanks Peng.

I agree with PENG in this respect. Just because a speaker is 2 ohms nominal does not make it better, it just has a different characteristic. Some "white van special speakers" are 2 ohm and they are absolute garbage and would damage the equipment of the owner (it isn't likely that someone who owns Halcro monoblocks is going to fall for a speaker scam, only a few people on a budget with budget equipment). But at the same time being 2 ohm rated doesn't mean it is a bad speaker either, and just the same with 8 ohm speakers. There are crappy 8 ohm speakers, and they are excellent 8 ohm speakers. The B&W 802 Nautilus comes to mind, they are 8 ohms nominal speakers and they are great speakers.

Im sure you know already (because you know too much already), but many BW speakers (and Focal for that matter) have minimal impedances at around 3 ohms. I think I've seen minimal impedance specs before for some extremely efficient Klipsch speakers that also dipped to around that point.

Lower impedance could offer the ability to produce lower frequency bass in a smaller enclosure space

I believe thats what BW says about their CM bookshelves. Something like "punchy bass courtesy of a lowered impedance"...

 

[Seth=L]

Im sure you know already (because you know too much already), but many BW speakers (and Focal for that matter) have minimal impedances at around 3 ohms. I think I've seen minimal impedance specs before for some extremely efficient Klipsch speakers that also dipped to around that point.

I believe thats what BW says about their CM bookshelves. Something like "punchy bass courtesy of a lowered impedance"...

I spend too much time here obviously.

The Nautilus 802s are 8 ohms nominal and pretty efficient as well, but there is no way that my Teac CR-H220 could power them. I would bet they dip pretty darn low.

 

[TLS Guy]

Crossovers are tough all the way around

I think everyone is being very hard on the speaker manufacturers. Crossovers are a necessary evil. It is really hard to design a good crossover. There are so many things to juggle. The most important parameter is smooth frequency response. That in itself is tough. Then you have to keep the drivers out of their problem areas. At the same time you try to cause minimal phase shifts, time and group delay. And you end up causing quit a bit of all of that anyway. Fairly far down the list is the impedance curve. After hours of simulation, testing and listening tests, you are ready to let the amplifier designer deal with the impedance bumps. One of my crossovers took me 10 years!

And yes, you do need listening tests. Passive crossovers are not and never will be an exact science. If you read B & W account of the design of the 200 series you will see what I mean.

There is relief in site. Digital crossover are here, and offer roll off as high and even higher than 40 db/octave , with no phase, time or group delay headaches.

It time to put the amps in the speakers, one for each driver and use digital crossovers. Then we can dump those bulky receivers, and those huge amps that cause a hernia with every lift.

 

[mtrycrafts]

It time to put the amps in the speakers, one for each driver and use digital crossovers. Then we can dump those bulky receivers, and those huge amps that cause a hernia with every lift.

Now that would be something of a speaker indeed.

 

[Seth=L]

Now that would be something of a speaker indeed.

Didn't Meridian do something like that?

 

[jelanier]

Let me add something to this correct response..

There is nothing that makes a two ohm speaker better than a four or eight ohm one.

The problem is the high powered amps. Transistors like to deliver current to a point. The lower the impedance the more current the power amp will deliver, up to a point. Quite a few amps now deliver 400 watts or more into low impedance loads. Now if an amplifier manufacturer were to design an amp delivering 400 watts or more into 8 ohms, if a customer connected it to speakers with low impedance it would spell destruction for the amp, unless the designer has a circuit to lower the supply voltage at a certain current set point. This will drive an amp into hard clipping, and high end amp manufacturers do not consider that good for their reputations.

There is a big problem with low impedance speakers, as the resistance of the cables easily gets over 5% of the nominal impedance of the speakers. That is not the best. As stated before loudspeaker impedance is all over the map in a given speaker.

There is another issue also, and that is safety. Speaker terminals are not shrouded. An amp delivering 400 watts into 8 ohms would generate 56 Volts across the speaker terminals. And 400 watts is just the RMS power, and on peaks could produce 80 volts. That's enough for a nasty nip. At 2 ohms the voltage is 28 and the highest reasonable peak would be 40 volts, and that's enough for a good tingle.

As far as sensitivity goes the higher the better, as every 3db increase in sensitivity is the same as doubling the amp power. The only problem is that there is a relationship between a bass driver's sensitivity and bass response.
A lower sensitivity tending to favor a better bass response. That is why very sensitive drivers, such as the Lowthers are loaded by bass horns, to bring the bass output up the level of the mid and HF.

Let me add a thing or two...
The impedance of a loudspeaker is nominal. The impedance peaks at resonance for a woofer (classic sealed box). The peak frequency and it's shape are a function of woofer parameters and box size/design. Remember that power =E^2/R. Loudspeakers are designed to see a source impedance of theoretical ZERO. If the source impedance is high (low damping factor), then the drive voltage at resonance would be higher than the drive at other frequencies. A low output impedance of an amplifier, gives it the ability to provide the same voltage at all frequencies regardless of the impedance. (Damping Factor) Now, any resistance between the amplifier and loudspeaker(Rs) adds to the output impedance of the source (Zo) as seen by the loudspeaker(Zl). (The lower the speaker impedance, the worse the damping factor DF=Zl/(Zo+Rs))
Not only do you lose power in this resistance, but the loudspeaker will sound completely different as it now sees a source impedance different from what it was designed for. Power: since the amplifier tries to deliver the same voltage at all impedances, lowering the impedance of the loudspeaker increases power. If you go from 8 to 4, as an example, the amplifier has to deliver twice the power (twice the current but same voltage).
The amplifier will TRY to do this. If the load has a very low impedance and the amplifier does not have either the current capacity, or power dissipation to handle it, it will try anyway all the way up to it's self destruction! (neglecting protection circuitry)

 

[3db]

A loudspeaker's impedance does not stay constant throughout the 20-20,000 Hz range. The impedance of an 8 ohm nominal speaker could dip well below 8 ohms. It is reasonable to assume that if a manufacturer can set its goal solely on sound quality; without having to worry about the impedance, it may have more options and therefore better chance of achieving its goal to produce the best sounding product. That being said, I don't think it is necessarily true that two ohm speakers will do better than 4 to 8 ohm speakers, they just may have better chance.

PSB's new Synchrony line advertises its nominal and low impedance to be 4 ohms. I checked this out at the Sound & Vision Magazine forum. Apparently according to one of the more knowledgeable speaker people in the forum, this is not the 1st time PSB has designed a speaker where the impedance stayed relatively constant across the audio spectrum. This is an exception I realize as speakers when modelled electrically are nothing but compoments exhibiting resistance, capacitance and inductance with the latter 2 having frequenct dependencies.

 

[mtrycrafts]

PSB's new Synchrony line advertises its nominal and low impedance to be 4 ohms. I checked this out at the Sound & Vision Magazine forum. Apparently according to one of the more knowledgeable speaker people in the forum, this is not the 1st time PSB has designed a speaker where the impedance stayed relatively constant across the audio spectrum. This is an exception I realize as speakers when modelled electrically are nothing but compoments exhibiting resistance, capacitance and inductance with the latter 2 having frequenct dependencies.

Check this one out
http://www.av123.com/images/miniz.gif

Those impedance peaks will certainly affect the performance, at those areas. Try driving a 20 Ohm+ speaker load

 

[TLS Guy]

I'm sorry but I have to correct this

As I have stated in a previous post, there are ways in the crossover to correct the impedance and make it look as resistive as possible. Where feasible this is desirable. Sometimes however the correction causes other problems. Impedance correction greatly increases part count and complexity. The lumps and bumps in the impedance curve do not of themselves produce a poor frequency response. It is just a reflection of the reactive nature of voice coils and crossover chokes and the reactance of capacitors in the crossover. Impedance rises with frequency. Yes, the driver takes less current and power on the impedance peaks, but it does not follow that the output of the driver will decrease at those frequencies.

In the curve shown there is one LF peak indicating a closed box design. A well designed reflex or TL should have a characteristic double hump in the LF response. If you don't get that, then there is something amiss in the design.

These speakers have a high impedance of 65 ohms and 60 ohms at 15 and 45 Hz respectively.

http://mdcarter.smugmug.com/gallery/2424105#127080883

After impedance correction the impedance curve is 8 ohms except for drops to 5 ohms between 150 Hz and 275 Hz and 900 Hz and 3.5 KHz. It rises to 10 ohms at 450 Hz and 14 ohms at 5 KHz. However the part count is 29 components per crossover. There are studies to suggest a slight trend to listener preference for impedance compensation.