Qes and Qms: where the damping comes from

Qts is the total damping at resonance. Qes and Qms say where it comes from.

Qms is mechanical: friction in the surround and spider, and losses in the cone itself. It is usually between 1 and 10. A high Qms means a low-loss suspension, which is generally desirable but says little on its own.

Qes is electrical, and it is the interesting one. When the cone moves, the voice coil moves through the magnet gap and generates a voltage. With the amplifier connected, that voltage drives a current which opposes the motion. The motor brakes the cone electrically:

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

Read backwards, a strong motor, large Bl, means a small Qes, which means heavy damping. Qes is typically 0.2 to 0.8.

Why this matters in practice

Because Qes depends on the electrical circuit, it depends on what the driver is connected to. Series resistance (long thin speaker cable, a series crossover inductor's DC resistance, a valve amplifier's output impedance) adds to R_e and raises Qes, and with it Qts. Add half an ohm in front of a 6-ohm driver and you have changed its effective Qts by around 8%, which is enough to move a carefully chosen alignment.

Baffle models this: the amplifier output resistance field feeds straight into this mechanism, and you can watch the response change as you raise it.

The consistency check

Since Qts is a parallel combination and Qms is nearly always much larger than Qes, Qts sits just below Qes. If a datasheet shows Qts larger than Qes, or the three figures do not combine correctly, one of them is wrong.