What causes an 8 ohm midbass driver operate like a 4 ohm in a speaker?

[audiojerry]

Is it because of the inductor or trying to match it to the tweeter or ....???

[fpitas]

Huh?

[tom_s]

I think I know what you are asking. This explains things better than I can -

https://www.sites.google.com/site/un...iverimpedances

[billfitzmaurice]

Nothing does. Where have you seen that?

[fpitas]

If I meditate on this will I reach enlightenment?

[audiojerry]

I think I know what you are asking. This explains things better than I can -

https://www.sites.google.com/site/un...iverimpedances

I appreciate the link and Paul's explanation, but I don't think it satisfies the reason for my question. I raised the question specifically because of my build of Paul's Amiga, and generally for my education.
The tweeter in the Amiga is rated at 4 ohm, and the midbass is rated at 8 ohm and stays above 7 ohm within its passband.

But the Amiga measures at 4 ohm according to Paul. I'm just curious what would be the main factor for the lower overall impedence rating.

4 ohm is not ideal for my tube amp. It makes me wonder if I could replace the midbass or alter the xover to achieve a higher impedence.

[billfitzmaurice]

According to his chart it's an 8 ohm speaker. Where tube amps are concerned it would be very rare to find one not happy with a 4 ohm load. Tube amps are the opposite of SS. SS amps have a minimum impedance rating, tube amps have a maximum impedance rating.

[Kornbread]

In Tom's link "When loudspeaker manufacturers give you a "Nominal Impedance" of a speaker, the number is incredibly misleading and a wild guess at best! The actual impedance of the speaker is really all over the map (depending on the frequency), and most likely dips below the quoted "nominal impedance" at certain frequencies. ...". Other than planars and some exotics, flat .imp is rare. Have you ever measured the .imp of a speaker?

[djg]

"4 ohms" is a very general term, used to advise users to how difficult the speaker will be to drive. Here is the Amigas' Impedance curve. Note it varies greatly depending on frequency. This is in Paul C.'s writeup.

Sometimes, designers include an "impedance flattening" optional add on for the crossover, to help those with tube amps.

[djg]

I built these for my Dynakitparts ST35 tube amp. Note the impedance normalization addon. I made mine switchable.

http://speakerdesignworks.com/Singularity_4.html

[Kornbread]

Sensitivity, imp, and phase angle all effect how hard a speaker is to drive.

[djg]

PE sells the Amiga as an 8 ohm speaker.

https://www.parts-express.com/Amiga-...-Only-300-7122

[rpb]

Is it because of the inductor or trying to match it to the tweeter or ....???

The cap in the woofer low-pass filter has low impedance at mid, and high frequencies, so it reduces the speaker impedance. Also, as you approach the xo frequency, both woofer, and tweeter are shorting some of the signal to ground.

[Regore]

Just measure the DC resistance of the driver with an accurate VOM and multiply by 1.25. This will be very close to its impedance.