Re: the voice coil's DC resistance

Re is the resistance of the voice-coil wire, measured at DC. It is the one Thiele-Small parameter you can check yourself with nothing but a multimeter across the terminals, which makes it a useful sanity test on a record: if the meter and the datasheet disagree, something is wrong before you go any further.

Re is not the nominal impedance

"8 ohms" is a class, not a measurement. A driver's impedance varies with frequency: it peaks at resonance and rises again with the coil's inductance. A single number can only ever be a label, and the label is chosen to tell an amplifier roughly what to expect.

Re is nearly always below it. Of the 1068 drivers in this library whose sources state both figures for a nominal 8 ohms, 1063 have an Re below 8 Ω, with a median of 6.0 Ω. The 725 nominal 4-ohm drivers have a median Re of 3.3 Ω.

That is why the filter on the driver list offers bands of Re rather than a nominal figure: far more sources state Re, and it is the quantity the physics actually uses.

It sets the floor of the impedance curve

Z_e(\omega) = R_e + j\omega L_e + \frac{(Bl)^2}{S_d^2 \, Z_{at,mech}(\omega)}

The last term is what the moving cone reflects back into the electrical circuit, and its real part is always positive, so the motion can only ever add resistance and never cancel it. The impedance curve therefore approaches Re without reaching it, dipping closest between the resonance peak and the inductive rise.

This is the figure that decides whether an amplifier will cope. A dip toward 3.2 Ω is a hard load, and drivers wired in parallel halve it.

Where else it turns up

Re appears in the motor relation, so it is part of what fixes Bl and Qes:

Bl = \sqrt{\frac{2\pi f_s M_{ms} R_e}{Q_{es}}}

and it appears in sensitivity, where a lower Re draws more current at the same voltage and so plays louder, without the driver being any more efficient.

Anything in series with the driver adds to it. Thin cable, a crossover inductor's own DC resistance, an amplifier with a high output impedance: each adds resistance the motor's braking has to work through, raising Qes and with it Qts. Half an ohm in front of a 6-ohm driver moves Qts by around 8%, which is enough to leave a carefully chosen alignment. Baffle has a field for that series resistance, and you can watch the response change as you raise it.

What Baffle does not model

Re rises as the coil heats, because copper's resistance climbs with temperature, so a driver at full power is not the driver on the datasheet. That is thermal power compression, and Baffle deliberately leaves it out. Every curve here is drawn for a cold coil.