What's with 4 ohm speakers?

Some misinformation here:

They're not hard to drive if you have a good amp. Also, 4 ohms is an average.

4 ohms is not an average. Plus, you can see in the specs of all amps that its not a good thing. No matter what amp you have, the distortion will be higher driving 4 ohms. And usually its the kind of distortion you can easily hear- an amp driving 4 ohms will be harsher and less detailed than if it were driving 8 or 16 ohms.

Sensitivity.

Take a typical "8 ohm" midwoofer and double them up in parallel and you get 6 dB more. Realistically, more like 3.5 to 4 dB but also drops impedance by half. Problem is that impedance varies with frequency and the same midwoofer will have a minimum impedance, usually around 200 Hz. Some more than others.

Putting two 8 ohm speakers in parallel gets you 4 ohms, with 3, not 6 db increase in sensitivity. However the actual efficiency will be found to not have changed. It is important not to confuse efficiency, which is the actual watts that make a speaker move, as opposed to sensitivity, which has to do with the voltage on the speaker at the time. If you have 2.83 volts into the speaker and its 8 ohms, the power is 1 watt, IOW they are the same. But into 4 ohms that 2.83 volts is 2 watts- 3 db 'more'. Its not really more at all, what is happening is your amp is being asked to double its power.

Its important to understand this distinction! Not all amps double power (tube amps for example). Yet such amps can sound quite musical. Regardless in all cases, as stated earlier, amps do sound better (smoother, more detailed) driving higher impedances.

Put simply, if sound quality is your goal, your amplifier investment dollar will be best served by a speaker of 8 ohms or more, all other things being equal (and regardless of the amplifier technology). If OTOH sound *pressure* is your goal than you have a small (3db) argument for going with a 4 ohm speaker.

Ngjockey, Pipedreams has done something like that. I've seen Pipedreams that were 4 ohms, and the exact same model in 16 ohms.

Seems to me the ZU Definition, normally a 6 ohm speaker, is also available as a 30 ohm speaker.

It's easier to design a speakers with a steady low impedance than a steady high impedance, something that some amps seem to have an easier time with. Historically there have been more speakers that can produce wave form fidelity with a lower impedance than a higher impedance.

This is entirely false, plain and simple.

To answer the OP question, I suspect the reason is that many designers don't know how amplifiers work, so we often see crazy loads that are 'hard' to drive.

Folks, there is a reason such speakers are considered hard to drive- the amp has to work harder to do the job. You can always see it in the specs of any amplifier- the harder you make it work, the more distortion it makes. Unfortunately the distortion we are talking about is the kind that makes a system harsher and brighter- the odd ordered harmonics, to which the human ear/brain system is very sensitive.

Part of the problem is that speaker designers often confuse Sensitivity with Efficiency. I can point to examples if anyone is interested. You don't get something for nothing in this world. In electronics, this idea is known as the Law of Energy Conservation, or Kirchoff's Law.

But many speaker designers don't understand this. They think that if they put two drivers in parallel, that the speaker gets easier to drive (sensitivity increases). It does not! It gets *harder* to drive, and the amount of power to make it play a certain sound pressure does not change at all!

If lowering the impedance was actually helpful, why not 1 ohm instead of four? Then the sensitivity would be increased by 9 db! Nearly a 10:1 improvement... but of course that would violate Kirchoff's Law. If you are able to violate Kirchoff's Law, FWIW, you will have created a Free Energy Device which, as far as we know, does not exist.

Yet many speaker designers persist in trying to do exactly that, and many audiophiles that don't understand how this works (its really just math when you boil it down, FWIW) follow along in the fantasy.

So here is the bottom line: higher impedance speakers cause amps to make less distortion (smoother, more detailed). Increasing the Sensitivity of a speaker by decreasing its impedance does not affect Efficiency, but it does make the speaker harder to drive (amp will sound harsher, less detailed).

Put another way: If Sound *quality* is your goal, your amplifier investment dollar will be best served by a speaker of higher impedance, all other things being equal. If sound **pressure** is your goal and you have a transistor amp up to the task, then there is an argument for lower impedances.

Those of us with a little technical knowledge accept and expect it because at lower frequencies dynamic loudspeaker driver output is proportional to voltage.

The statement is ambiguous. We know that doubling power is 3db, and that there is or should be a direct correlation with driver output. Since this is so then driver output is also proportional to power.

Quantity becomes quality when it avoids clipping

This is a bit of a strawman. If the amp is clipping get a speaker with more efficiency or a more powerful amp. Sound quality is not served by quantity if that quantity is also containing annoying distortion products, which are common with a lot of amps without clipping coming into the equation!

Or is there some type of industry standard and everyone measures these type of things the same way?

They are supposed to, anyway. Of course the room is important, about 1/2 of the total system sound, IME.

>The statement is ambiguous. We know that doubling power is 3db, and that there is or should be a direct correlation with driver output. Since this is so then driver output is also proportional to power.

Nope. You're confusing voltage and power where power is voltage squared divided by impedance.

Sorry - not confused. I am quite literal though. If you are saying there is not a proportion then you are contradicting yourself. But I suspect we are arguing semantics. To clarify I was simply stating that +3 db more watts is twice as many watts.

Amplifiers which don't accommodate these physical realities with terminal voltage that's a fixed multiple of input voltage regardless of load impedance aren't universally useful in high-fidelity applications for speakers having impedances that are otherwise compatible causing neither instability nor power dissipation issues.

Certainly this is true. However not all speakers have this requirement of an amplifier- such speakers can be incompatible with amps that are capable of Constant Voltage behavior. Rather than repeat myself ad naseum I invite you to go back to an earlier point in this thread an look for a link I dropped to an article on the Voltage and Power paradigms, so you can catch up on the conversation.

Jjrenman, you are absolutely correct. People commonly assume that Efficiency and Sensitivity are the same but that is only true into exactly 8 ohms.

Won't draw any conclusions but I encourage anybody else to try a similar experiment. Results do go against standard convention. With closer spacing, or other factors, results may vary.

Actually this sounds pretty predictable to me. The Plinius can double power when impedance is halved, so it would make 3 db more output. Depending on where the mic is placed, you will find out about beaming and line source effect as well.

If you ran the same tests with a tube amp you would get different readings- likely more output with the drivers in series rather than parallel.