Resistance will always be a few ohms or less, WAY below "impedance" which is literally a complex function of frequency , amplitude, loading, and speaker environment.
One "preferred" measurement method was to use a 1KC signal; one point of thousands; totally misleading and almost useless.
Just connect each one in its own preferred environment (box, baffle, boffle, horn, etc) and listen to it where you would like to place it (living room, bedroom, car, concert all, etc.).
It isn't always way below the complex impedance. The resistive component at DC is usually as low as it ever gets and the frequency dependent reactive components add to it. It shouldn't depend on amplitude unless you are saturating the magnetic components or about to blow the fuse.
If you measure it on DC and round up to the next power of two in ohms you won't be that far off the nominal impedance. It does vary a lot with frequency depending on the crossover network and drivers used. The OP might find the Wiki entry on loudspeakers nominal impedance helpful:
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When you look and the |Z| and phase response of some hifi speakers it is surprising that they work at all. B&W 802D a case in point - nominally a
8 ohm speaker with excursions 20ohm around 2-3kHz crossover.
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Using a continuous tone sine wave at real power into a loudspeaker is a good way to kill it if you hit a resonance. Real speaker tests are done by sweeping sufficiently quickly not to allow a resonance to build up or more usually with narrow bandpass pink noise.
The ear is surprisingly easily tricked.
An oscilloscope or spectrum analyser is much less easily fooled.
At resonance, impedance rises quite a lot, much reducing power draw. Speakers don't have enough in-band resonance to destroy themselves, or they'd not be reliable or acceptable sounding in use. Excluding outliers like 1920s moving iron speakers etc.
Why do you care? Stated another way, "What are you gonna do with the information when you get it?"
Isn't the impedance going to be critically dependent on the enclosure? And to a lesser degree, the acoustics of the room? 40 years ago, I experimented with trying to get a decent sounding speaker in a pickup truck and ended up using an amplifier with negative output impedance.
With a "12 V" car battery voltage you can get about 4 Vrns output voltage from a half bridge, i.e. 0.5 W into 32 ohms or 8 W into 2 ohms. Some full bridge ICs claim 22 W into 4 ohms, requiring 14.4 V battery voltage (alternator maximum output voltage).
With a DC/DC inverter power supply, the voltage swing and speaker impedance can be more freely selected.
At least the DIN standard specifies that the actual impedance at any frequency should be at least 80 % of the nominal impedance, agreeing well with Phil's rule of thumb.
one drivers that have impedance minima in the band from 250 to 400Hz. "
Sounds like a relatively crude approximation.
Also, correct me if I am wrong, that impedance minima is still higher than the DC resistance I assume. I would find it very hard to believe there is e nough EMF to make it go lower, but stranger things have happened. Like the tunnel diode that should not work.
The impedance of any device must be (equal to or) higher than it's DC resistance or it's a battery.
Why should a tunnel diode not work? Both the tunnel diode and speaker will have "negative (incremental) resistance region(s)" but there is nothing untoward about that.
OK, you got the arithmetic right(ish). The typical car amplifier is more like 50W into a 2-ohm speaker.
Obviously. What do you think a 400W boost regulator costs? What does it add to the cost of the car. What's the market? ...just to "freely select" speaker impedance.
If more power is needed, 1-ohm speakers or dual voice-coil speakers (using two amplifier channels driven from the same source) are a lot cheaper.
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