Thanks TBTL, of course, but what is to make difference between higher and lower than Fs frequencies? Assuming that with small signals counteracting by suspension compliance is constant and independent by frequency, necessarily below Fs there must be a decreasing in force exerted by the coil on the membrane. Since effectively the impedance graph below Fs shows that current still flows, if energy is still spent but force decreases, where does it go energy not used?
May it come into play an auto-braking effect by the coil due to electrical damping factor, that below Fs is not counteracted by effects of cone mass and up Fs is instead? If yes, is auto-braking effect due to cone velocity? Should this explain the necessity for low Qts (and so Qes) drivers to work in little space to contain cone velocity and mitigate the auto-braking effects?
If yes, roll-off below Fs is due to a velocity question rather than an acceleration one?
.....what does it mean velocity rises with 6 dB/oct? Isn't velocity measured in m/s? So sound pressure level is proportional to air velocity and not to air displacement?
What dos exactly is acoustic impedance, does it increase with increasing frequency or viceversa?
Coil force does not vary with frequency, it is the same Bl*i for all frequencies. As has been said several times, sound pressure is proportional to acceleration.
For a speaker in a sealed box or on a large baffle, cone displacement is approximately constant below Fs. The box/suspension dominates the motion. In that region, the constant force from the coil moves the cone a constant amount x = F/spring constant. Velocity is then proportional to Frequency * displacement below resonance. The amount it moves back and forth times the frequency it moves back and forth.
If cone displacement x
X(t)=x*sin(wt)
then Velocity u
u(t)=x*w*cos(wt)
and acceleration a
a(t)=x*w^2(-sin(wt))
Above resonance the mass dominates, and you have a mass that it takes more and more effort to move, the faster you move it. Since you only have a fixed force input, the acceleration stays constant, which means displacement drops. F=m*a (ignoring the time varying part) is also F=m*x*w^2. So to have constant acceleration a=x*w^2, the displacement must drop as a/w^2.
Auto braking, back EMF, are the same thing. They play a part at resonance but are not important to the behavior above and below resonance.
The peak at resonance is the velocity maximum, the place where it is easiest to get the cone moving, where the stored energy in the suspension and cone mass are equal. Near resonance the motion can be considered to be damping-dominant. The amount of total damping (so Qts, not Qes or Qms alone) affects the amount of motion (sound output) at resonance. The ratio of total to mechanical affects the impedance rise.
Peak impedance at resonance = Zmax=Re(1+Qms/Qes)
