efficieny of a 4ohm speaker vs 8 ohm speaker

[pula58]

And let's say I have an amplifier with a selectable trasnformer tap so I can switch between 8ohm and 4ohm output impedance.

Now, let's say I have a 4 ohm weber 12F150 and an 8 ohm 12F150. If I run the 8 ohm speaker with the amp set at 8ohms and compare it to the volume with the 4 ohm speaker (with the amp set to 4 ohms) is there any difference in loudness?

What I am womndering is whether the 4ohm speaker has less windings around the coil and thus has less sensitivity?

Or, does the 4ohm and 8ohm speakers have the smae number of windings, but, use a different type of wire to account for the difference in impedances?

[SatelliteAmps]

There should be no difference. The sensitivity of the speaker does not change with a difference in impedance.

[donnyjaguar]

Generally it'll be louder at 8Ω than at 4Ω. Not because the speaker is any more efficient, but because the secondary of the output transformer has more turns and will couple a little better. Remember that the unused additional turns for the higher impedance secondary will hamper somewhat the coupling efficiency at the lower impedance. The reality is you're not going to notice much, if any, difference here. But you did ask.

[SatelliteAmps]

Generally it'll be louder at 8Ω than at 4Ω. Not because the speaker is any more efficient, but because the secondary of the output transformer has more turns and will couple a little better. Remember that the unused additional turns for the higher impedance secondary will hamper somewhat the coupling efficiency at the lower impedance. The reality is you're not going to notice much, if any, difference here. But you did ask.

A lot of people seem to think this, but every time I have checked there is no difference in wattage. A lot of people make this mistake by checking with either a mismatched load, or using two different speakers, not the same speaker with just a different impedance.

Running a properly matched impedance with the same speaker of the correct impedance will generate the same wattage at either 4, 8 or 16 ohms. There is no difference in volume or wattage output.

[donnyjaguar]

Its not a big difference, I do concede. It is however possible that the turns ratios aren't exactly ideal for the nominal impedance(s). Hence, the 4Ω tap could be exactly 4Ω whereas the 8Ω tap is 9Ω etc. Also, the gauge of wire for the additional turns required to net 8Ω over 4Ω could be smaller and add resistance. You can determine this with testing of course, but it does tend to fall into the category of who cares/could be bothered? :)

[VacuumVoodoo]

Its not a big difference, I do concede. It is however possible that the turns ratios aren't exactly ideal for the nominal impedance(s). Hence, the 4Ω tap could be exactly 4Ω whereas the 8Ω tap is 9Ω etc. Also, the gauge of wire for the additional turns required to net 8Ω over 4Ω could be smaller and add resistance. You can determine this with testing of course, but it does tend to fall into the category of who cares/could be bothered? :)

No, both will have same number of turns, wire gauge adjusted to give 4 or 8 Ohm, or rather closer to 3.2 vs 6 Ohm dc resistance. You'd want to keep same coil inductance, Q factor of the coil assembly should also be the same so for a 4 vs 8 Ohm it would need some adjustment of its mass.
But these are very small differences indeed.

[pula58]

thanks folks!

[donnyjaguar]

VV, I think you're talking voice coil and I'm talking transformer secondary.

[VacuumVoodoo]

VV, I think you're talking voice coil and I'm talking transformer secondary.

Yes, I (mis?)understood question to be about loudspeaker. My post was about speaker voice coil.
With regard to transformer you might experience a slight difference in high end response due to differences in capacitance between primary and secondary as well as in mutual inductance between the windings when using 4 vs 8 Ohm output tap. Still, in 99.99% of cases one would have to be seriously determined to really detect an audible difference.