Qts: total Q

Q is a measure of how sharply a resonance rings. Low Q is heavily damped and settles quickly; high Q rings on. Qts is the total damping at fs, counting every source of loss:

Q_{ts} = \frac{Q_{es} Q_{ms}}{Q_{es} + Q_{ms}}

It is a parallel combination, so the smaller of Qes and Qms dominates, and in nearly every driver that is Qes: the motor does most of the damping.

What it tells you

Qts is the single best predictor of what enclosure a driver suits.

• Below about 0.3 the driver is heavily damped and wants a vented box. In a sealed box it will roll off early and sound thin.
• 0.3 to 0.5 is flexible: it works well vented, and works sealed if the box is not too small.
• Above about 0.5 the driver wants a sealed box. Vented, the alignment tends to demand an impractically large enclosure or produce a peaky response.
• Above about 0.7 it is aimed at a small sealed box, or at infinite baffle.

These are tendencies, not rules. The number that actually decides the sealed response is Qtc, which is Qts raised by the box.

What it trades against

Nothing is free. A low Qts comes from a strong motor (more magnet, more copper in the gap) which also raises efficiency. A high Qts driver in a small box gets you deep bass from a small enclosure, and pays for it in sensitivity. This is the whole content of Hofmann's Iron Law.

Reading published figures

Qts is derived from Qes and Qms, so a datasheet quoting all three that do not satisfy the equation above has a rounding error or a copying error somewhere. Baffle checks this for you and says which figure looks wrong.