The impedance ratio is just the square of the winding ratio, correct?

So a 3000/4-ohm xfmr is the same as a 6000/8-ohm xfmr, etc.

When an audio output xfmr's specs are quoted at, for example, 6K primary/8-ohm secondary, people are led to believe that those are the only two impedances supported.

Why?

The AC impedance of an audio transformer is the impedance measured at a certain frequency (since this is an inductor we are dealing with). I do believe that with output transformers at least, the frequency they have chosen in the past was 400 cycles. Perhaps some other companies choose other frequencies.

Although a ratio is a ratio is a ratio, when it comes down to actual impedance matching, you must use a certain impedance. If not, bass and treble will not load properly, optimum matching will not occur, and it is possible for voltages to be incorrect (mostly plate voltages). As far as bass and treble go, a high impedance circuit needs a high impedance device, and a low impedance circuit needs a low impedance device. If you try to feed a low impedance into a high impedance, treble notes will get lost in the high impedance because high impedances don't like treble notes. On the other hand, if you connect a high impedance to a low impedance, you'll short out the bass notes.

Thomas

There's another thing to keep in mind: Audio transformers also have a rating that is equivalent to "a-c volts per turn." This is related to the amount and permeability of the iron in the core. When a transformer is rated at 3000/4 ohms, as you noted, an assumed plate voltage swing is implied, and when you arbitrarily change the tube and speaker so that it becomes a 6000/8 ohm setup (same transformer) the voltage swing is presumed to go up with the impedance change, for the same power rating. This may, in a marginal transformer, exceed the allowable volts per turn, and tend to saturate the core. The low frequency end of the spectrum would be most affected in this case.

:The AC impedance of an audio transformer is the impedance measured at a certain frequency (since this is an inductor we are dealing with). I do believe that with output transformers at least, the frequency they have chosen in the past was 400 cycles. Perhaps some other companies choose other frequencies.
:
:Although a ratio is a ratio is a ratio, when it comes down to actual impedance matching, you must use a certain impedance. If not, bass and treble will not load properly, optimum matching will not occur, and it is possible for voltages to be incorrect (mostly plate voltages). As far as bass and treble go, a high impedance circuit needs a high impedance device, and a low impedance circuit needs a low impedance device. If you try to feed a low impedance into a high impedance, treble notes will get lost in the high impedance because high impedances don't like treble notes. On the other hand, if you connect a high impedance to a low impedance, you'll short out the bass notes.
:
:Thomas