What's with 4 ohm speakers?

They're not hard to drive if you have a good amp.

Depends on what you mean by good.

4 ohms is an average.

Not exactly; 4 ohm "nominal" has no commonly accepted definition. Plenty of speakers rated by manufacturer as 4 ohm nominal have very different impedance minima and maxima, which in turn will place very different demands on the amplifier that is driving it. Not to mention phase angle and sensitivity.

Depending on the frequency of the music you are playing, the speakers resistance will vary.

While true in the absolute sense, some speakers have a relatively flat impedance curve; i.e. they do not vary much w frequency. Others vary widely. And that's exactly the reason why amplifier-speaker matching is important and why there are many "good" amplifiers that will not mate well w a 4 ohm nominal speaker. And many good amplifiers that will not mate well w a 16 ohm nominal speaker.

I read a review of my 8 ohm speakers and it noted a drop to 3.something ohms here and there. Changed to the (tube) amp's 4 ohm output and huzzah...all better. Luckily there is plenty of info out there to make sure you don't mis-match yer stuff, although exceptions exist...i.e. high impedence/high efficiency things (Zu, etc.).

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.

I don't know the answer to the OP's question but I do wonder if it has something to do with cost. Take a 4 ohm and an 8 ohm version of the same driver unit: Is it less expensive to manufacture the 4 ohm version?

Four ohm versions are usually more sensitive. A good amplifier properly mated to a 4 ohm speaker will usually deliver more output than it would into an 8 ohm load. With more output, you can play louder with less distortion and more headroom.

Well, I guess I should have checked the thread again before posting because Atmasphere practically wrote just the opposite and I trust his knowledge about such things far more than mine. He didn't answer the OP's question, though, so somewhere along the value chain, I still wonder if it has something to do with cost.