Speaker Impedance

[TheWalkman]

Twenty five years ago, it seems virtually all speakers were 8R. Today, a larger percentage of speakers, it appears, are 4 or 6 ohm. Why is that?

Is the speaker resistance a function of the length of wire in the voice coil? If that were the case, I would them conclude that manufacturers are simply cutting manufacturing costs by using less copper in the windings resulting in lower impedance speakers. On the other hand, wouldn’t stronger magnets in today‘s speakers cause resistance to rise as they interact with the voice coil?

What’s really behind this phenomenon?

(All the EE’s are probably chuckling at this but I’ve never really found a great explanation.)

 

[egellings]

The resistance listed for the speaker is just a nominal value, The so-called resistance, which is actually a complex impedance (has reactive and resistive components) varies quite a bit over frequency, with often a very large rise at resonance. Crossover components, speaker voice coil wire length, driver moving mass and box size and type all interact to give the speaker its signature impedance vs. frequency curve. It's a can of nice little worms.

 

[TheWalkman]

The resistance listed for the speaker is just a nominal value, The so-called resistance, which is actually a complex impedance (has reactive and resistive components) varies quite a bit over frequency, with often a very large rise at resonance. Crossover components, speaker voice coil wire length, driver moving mass and box size and type all interact to give the speaker its signature impedance vs. frequency curve. It's a can of nice little worms.

Thanks, egellings.

I understand that the indicated impedance, 4R, 6R, 8R is an integer representation (dare I say, loosely, average impdedance) over the frequency curve of the speaker from, nominally 20Hz to 20kHz, but back to the original question: why does it seem that I'm seeing an overall trend of fewer 8 ohm speakers today and more rated at four ohms?

I never recall seeing a four ohm speaker in the 1970s whereas today, it seems I rarely see any 8 ohm speakers.

- is there something in the newer speaker design that is lowering impedance?
- were amps in the 70's incapable of higher power demands of low impedance speakers so speakers were designed to protect the old amps?
- is marketing driving this so that they can say they handle more power (to snooker folks who don't appreciate that this is meaningless?

Something has clearly changed with speakers and their design and I'm wondering if someone can tell me what has caused impedance to drop.

I was afraid I was asking a really dumb question when I posted this but I'm beginning to think I have, in fact, opened a can of worms!

 

[voodooless]

I never recall seeing a four ohm speaker in the 1970s whereas today, it seems I rarely see any 8 ohm speakers.

Back in the day Tube amps were still plenty, and they are more comfortable driving voltage rather than current. So higher impedance was actually easier on the amps.

Even the early transistor amps were not very comfortable driving low impedance speakers. Better transistors and mosfets changed this.

- is there something in the newer speaker design that is lowering impedance?

Mostly no, amplifier design changed. Driving low impedance is no more an issue with modern amps. All in all it makes for a higher efficiency system. And also a bit cheaper, since you don’t need as high voltage components like large capacitors.

Regarding speakers I don’t know that much about the design history regarding impedance. Someone else might chime in.

- were amps in the 70's incapable of higher power demands of low impedance speakers so speakers were designed to protect the old amps?

Already answered that

- is marketing driving this so that they can say they handle more power (to snooker folks who don't appreciate that this is meaningless?

Well, surely this plays a role as well.

I was afraid I was asking a really dumb question when I posted this but I'm beginning to think I have, in fact, opened a can of worms!

It is not a stupid question at all. Keep asking

 

[escape2]

Speaker manufacturers have figured out that they can provide smaller speakers with a wider and flatter frequency response (especially on the low end), but this will come at cost of lower overall efficiency. So this means more power from the amp will be needed to drive them to the same high dB level as before. How can you deliver more current/power to the speaker? By lowering the impedance.

 

[voodooless]

Speaker manufacturers have figured out that they can provide speakers with a wider and flatter frequency response (especially on the low end), but this will come at cost of lower overall efficiency. So this means more power from the amp will be needed to drive them to the same high dB level as before. How can you deliver more current/power to the speaker? By lowering the impedance.

Well, sort of. Actually the loss of efficiency is mostly due to the diminishing size of modern speakers coupled with the need to also reach down low. Much lower than in the 70’s. Back in the day you had a 12” woofer in your system that would do 50 Hz, now we want our 6.5” woofer to go down to 35 Hz. So people want small, nice looking speakers that also have wide bandwidth. This will automatically make an inefficient system, and therefor need more power.

So the question is then: which was first, the chicken or the egg? Probably there is no clear answer here. Technology advanced, fashion changed, and it al organically evolved into what we have today.

 

[DonH56]

Speculative: Higher driving force requires greater current since the magnetic field in a (voice) coil is a function of the current through it. Transistor amplifiers more easily generate large currents at lower voltage and the output impedance is typically very low. High-voltage transistors are typically harder to make with gain and output capability comparable to low-voltage transistors, and they have more limited bandwidth compared to lower-voltage transistors. It is not too difficult to put multiple lower-voltage transistors in parallel to provide more current drive, but stacking them to achieve greater voltage is more difficult and usually less reliable.

Tube amplifiers generate smaller currents at higher (usually much higher) voltage and with high impedance necessitating a transformer (usually) to provide a modicum of impedance matching to the speaker, providing lower output voltage and impedance with higher current. Tubes are usually less efficient since they tend to have lower gain (not always) and require a filament (heater) supply, plus they do not last as long (lower lifetime) compared to transistors.

So from an electronics side, the advent of high-current, lower-voltage transistor (solid-state, SS) amplifiers enabled lower-impedance speakers with greater dynamic range since more current can be provided into lower-impedance loads.

I am not a speaker designer so can only guess about the speakers themselves. I would guess lower-impedance drivers can handle more power with larger wires and the higher current handling allows longer-throw voice coils for greater output. Lowering crossover impedances (which plays into overall speaker impedance) provides greater flexibility in design and potentially less loss in the crossovers themselves (larger wire for lower resistance in the lower-value coils so less loss). It should also extend the bandwidth on the upper (higher) side but I suspect that is negligible.

I am not sure about efficiency; IME the lower efficiency is more a function of cabinet size and design trades than impedance but again I am not a speaker designer. Specifically, the trade to extend the bass response using a smaller (e.g. bookshelf-size 5"~6") woofer, which in turn requires speakers to "throw away" midrange and treble power by attenuating the mid/high frequencies to match levels with the woofer, which has a lower output. Especially when a small woofer is pushed lower in frequency in a small cabinet.

Edit: Others above already covered all this more concisely whilst I was composing my post (price of multiplexing and trying to stay in touch with a meeting at work ).

 

[escape2]

Well, sort of. Actually the loss of efficiency is mostly due to the diminishing size of modern speakers

Yes, great point which I've forgotten to note. I updated my post.

 

[egellings]

The trend to lower impedance speaker ratings could be due to the amplifier technology of switchmode amps being used these days, especially in automotive sound systems. For a given voltage swing, a lower impedance speaker will draw more power from that swing than a higher impedance rated one will. The switchmode amps can easily hunk the 'curnt' needed to do that. So low Z rules.

 

[voodooless]

Edit: Others above already covered all this more concisely whilst I was composing my post (price of multiplexing and trying to stay in touch with a meeting at work ).

We still need to figure out the actual why if it all. Surely we know what happened and that it was an organic evolution, but what were the actual drivers here?

Yes, great point which I've forgotten to note. I updated my post.

So why? It’s probably not just because it was possible to do so.. there must be a reason for the trend to have smaller speakers that need to go lower. Last part is probably mostly due to the advent of home theater. For music you really don’t need to go that low in most cases, but in movies there actually some content down there to admire.

why we want smaller: probably because on average we have less room: https://www.theguardian.com/business/2018/apr/08/uk-living-rooms-have-shrunk-by-a-third-survey-finds

 

[EdW]

The problem is they are still called 8 ohm speakers even when they are around 4 ohms! It’s worse than that though the impedance often drops below 3 ohms and it’s a complex impedance. If you read Bob Cordell’s excellent book ‘designing audio power amplifiers‘ he shows that certain drive conditions can prove very significantly more challenging than even the 3 ohm impedance suggests. So a good amplifier needs to be able drive a 2 ohm resistive load in short bursts (at twice the current) as well as 4 ohm continuous. A few thoughts:
1. Semiconductors generally favour lower voltage swings. Higher voltage devices usually have a lower cut off frequency and a disproportionately higher output resistance with a weaker drive into saturation so maybe lower impedance speakers aren’t a bad thing
2. Speaker manufacturers like to quote sensitivity at 2.83V rms and to then pretend that this is 1 watt which it is in 8 ohm but is of course 2 watts in 4 ohms.
3. Even large speakers are now relatively inefficient - just look at the specs for B&W 800, KEF Blade etc. I imagine that the lower efficiency can be traded for improved transient response and lower distortion. But I’m not a speaker designer, just an analog IC designer!
4. All good business for the manufacturers of speaker cables of course

 

[escape2]

So why?

WAF.

 

[voodooless]

2. Speaker manufacturers like to quote sensitivity at 2.83V rms and to then pretend that this is 1 watt which it is in 8 ohm but is of course 2 watts in 4 ohms.

That is just an interpretation issue. Actually this way is allright nowadays, since voltage is the limiting factor, current much less so. Power ratings of driver are often far lower than the amps can deliver.

3. Even large speakers are now relatively inefficient - just look at the specs for B&W 800, KEF Blade etc.

Yes, so they can dig down deep.

I imagine that the lower efficiency can be traded for improved transient response and lower distortion. But I’m not a speaker designer, just an analog IC designer!

Can of worms alert: https://audiosciencereview.com/forum/index.php?threads/transient-response.18745/

 

[egellings]

With woofers, it's always about size, low frequency reach, and efficiency. You can have any two of the three, but not all three at once.

 

[egellings]

Not being a speaker designer (just a grateful listener) maybe efficiency is a trade-off parameter that can be adjusted to work with other design parameters to optimize a speaker design. With the availability of inexpensive, high power, compact, low heat switchmode technology, I can see where efficiency might go south, especially in small speakers. But, as I have always believed, you can't send a 6" woofer out to do a man's job, either.

 

[andreasmaaan]

I imagine that the lower efficiency can be traded for improved transient response and lower distortion. But I’m not a speaker designer, just an analog IC designer!

This is not quite right. Designing a speaker with a sealed enclosure will result in improved transient response vs a bass-reflex or enclosure, but efficiency will not be part of the tradeoff. And (all else equal) the bass reflex enclosure will produce less distortion above the port tuning frequency, and more distortion below it (whether this results in more or less overall distortion will depend on the frequency to which the port is tuned and the spectrum of the signal being played through it).

 

[andreasmaaan]

We still need to figure out the actual why if it all. Surely we know what happened and that it was an organic evolution, but what were the actual drivers here?

Only speculation here, but I find it hard to imagine anything other than the market was the primary driver.

 

[voodooless]

Only speculation here, but I find it hard to imagine anything other than the market was the primary driver.

That does not answer anything, it’s like saying snow is white because it’s white In sales, the market is always the driver.. the question is why it went into this particular direction? Well.. tried to partially answer that in the same post..

 

[andreasmaaan]

That does not answer anything, it’s like saying snow is white because it’s white In sales, the market is always the driver.. the question is why it went into this particular direction?

Fair enough Then the question becomes: Why did the market demand smaller speakers? I presume the answer is mostly that non-audiophiles (and family members of audiophiles) do not appreciate monkey coffins...

 

[trl]

I think you guys are way overthinking this.

If John Smith has an amp that outputs 50W/8Ohms or 100W/4Ohms he will probably choose 4Ohms speakers to benefit of the additional SPL provided by the added power. He doesn't knows that 8Ohms speakers are more amp-friendly (damping factor, less current) and that THD is better, he cares about power.

I guess it's marketing that dictates this.