The magnitude of the force on the coil is given by
F = BIl newtons
where B is the flux density (Wb/m2), I is the current (A) and l is the coil length (m).
The velocity of the coil/cone is found from
ν = F/Z m/s
where F is the force (N) and Z is the mechanical impedance (mech. Ω).
To avoid distortion with large amplitude signals, and hence large coil movements, ...