What's magical about the 4-16 ohms window?

[Jon Silberman]

I'm guessing 99% of guitar amp outputs and speakers are clustered in the 4-16 ohm range but why? Why not a 32 or 50 ohm speaker or a 2 or .75 ohm one? Is it cultural, historical, or technical?

[John Phillips]

Probably just historical. When the speaker impedances were settled on (1930s or 40s I think) the power, voltage and current were not really large enough to make much difference either way.

But, with higher powered amps:

If you go down much below 4 ohms, you start needing really quite large currents to achieve useful power outputs, which increases the necessary size - and cost - of the speaker wiring. At 4 ohms, a 100W RMS amp delivers 5A, which is still within the range 'normal' wiring will take.

If you go much above 16 ohms, you start producing enough voltage to be dangerous - a 100W RMS amp produces 40V with a 16-ohm load, and more when distorted. Over 50V is typically considered dangerous, or needing special connectors (not 1/4" jack). In fact, I'm not sure that there weren't some markets (Canada possibly) where some higher-powered amps did not have the higher impedance settings, probably because of that.

[Curt]

It's simply an industry standard Jon. A loose one, but an industry standard.
No different than 110VAC or 60Hz in this U.S. It could have very well been 130VAC and 70Hz.