Why are low impedance speakers harder to drive than high impedance speakers

Perfectly stated Charles. Many roads...

Dave

you took my statement out of context. Here is what I actually said:

Yes- it was not to contradict you, but to use the phrase as a talking point, as I see that approach recommended a lot. Then later people sell off the gear in search of that holy grail... I think if you start with the grail first you're less likely to sell and waste the cash.

Atmashere, the OP said nothing of ESL's. I did make mention of : "Most typical speakers...."

Correct- in fact by 'most typical' is about 95% of the market. In high end, its still about 85% and so is 'mostly accurate' :)

For those outside of that percentage (Charlesdad's speakers are box speakers but they are an example) that voltage thing just does not work. This is entirely due to the designer wanting the speaker to work with a particular kind of amp. In Charlesdad's case, the speakers were originally envisioned on a set of our M-60s and later that manufacturer started making SETs, which work much the same way. SETs in general are much happier on higher impedance loads despite often having 4 ohm taps and they tend to make constant power rather than constant voltage owing to zero loop feedback.

In a tube amp, the 4 ohm tap is not an efficient way to use the output transformer, which often means not only less power (lost due to heat) but also less bandwidth, sometimes up to an octave lost on the bottom end. So its often really worthwhile to avoid low impedance speakers with tube amps even if you have the taps on the output transformer!

atmosphere: "One other point not previously mentioned is the effect of speaker cables! At lower impedances they tend to be far more critical, where shorter distances and much larger gauges are required to prevent loss of definition and impact. This can be a pretty big deal as the series resistance of the speaker cable can have a pretty dramatic effect on the effective output impedance of the amplifier! By contrast the speaker cables are far less critical at 16 ohms- which is why a lot of us kids got by with hardware store zip cord in the old days."

At the risk of taking this thread into the contemptuous nether regions of many others (please no!), this may explain a lot regarding contrary experiences of many on this forum regarding speaker cables' impact on sound.

Good point Ralph.

Dave 

Not sure anyone cares about this, but I thought I would chime in with more information now.

In general, many good drivers are available in 4 and 8 Ohm versions. When this is true, the 4 Ohm is usually 3 dB more sensitive. The low impedance causes extra current flow which provides for increased force against the same magnet. The choice here usually has to do with whether designers will put them in parallel and matching other drivers in the system with less wasted heat.

Some speakers really can't help it. Like ESL's. Essentially they are moving capacitors. The Apogees of old were/are essentially just wires suspended n a magnetic field. Getting up to 1 Ohm impedance was as high as they could get!  Any decrease in the current (i.e. increased impedance) would cut the force down proportionately.

In some cases I have seen crossover design deliberately lower impedance to below 4 Ohms to make speakers seem more "discerning" or "demanding." This added nothing to the sound quality of them. They just added to the snobby appeal.

Best,

E (yes, I'm a snob)

This is a very informative thread. I've benefitted from it. Thanks.