Dayton Audio ND90-8 3-1 - Manual (Page 5)

with voice coil displacement x affects the input current of the driver. Here the nonlinear source of
distortion is the multiplication of displacement and current. The second effect is the generation of a
reluctance force which may be interpreted as an electromagnetic motor force proportional to the
squared input current.
The flux modulation Bl(i) has two effects too. On the electrical side the back EMF Bl(i)*v produces
nonlinear distortion due to the multiplication of current i and velocity v. On the mechanical side the
driving force F = Bl(i)*i comprises a nonlinear term due to the squared current i. This force produces
similar effects as the variable term Le(x).
Nonlinear Parameters
The displacement limits XBL, XC, XL and Xd describe the limiting effect for the force factor Bl(x),
compliance Cms(x), inductance Le(x) and Doppler effect, respectively, according to the threshold
values Blmin, Cmin, Zmax and d2 used. The thresholds Blmin= 82 %, Cmin=75 %, Zmax=10 % and
d2=10% generate for a two-tone-signal (f1=fs, f2=8.5fs) 10 % total harmonic distortion and 10 %
intermodulation distortion. The thresholds Blmin= 70 %, Cmin=50 %, Zmax=17 % create 20 % total
harmonic distortion which is becoming the standard for acceptable subwoofer distortion thresholds.
Traditionally, Xmax has been defined as the physical overhang of the voice coil, 15% times the
physical overhang, or the point where BL has dropped 70% from its X=0 value (same as X Bl). The
additional nonlinear limits allow us to quantify the other factors that limit a loudspeaker’s performance.
These parameters are defined in more detail in the papers: “ AN04 – Measurement of Peak
Displacement Xmax” , “ AN05 - Displacement Limits due to Driver Nonlinearities.” , “AN17 - Credibility of
Nonlinear Parameters” , “ Prediction of Speaker Performance at High Amplitudes” , “Assessment of
Voice Coil Peak Displacement Xmax” , and “Assessing Large Signal Performance of Loudspeakers”
Symbol Number Unit Comment
Displacement
Limits
thresholds can be changed in Processing property page
X Bl @ Bl
min=82%
4.1 mm Displacement limit due to force factor variation
X C @ C min=75% 5.3 mm Displacement limit due to compliance variation
X L @ Z max=10 % 5.7 mm Displacement limit due to inductance variation
X d @ d2=10% 18.9 mm Displacement limit due to IM distortion (Doppler)
alpha Heating of voice coil by eddy currents
alphaOrg Heating of voice coil by eddy currents (without limits)
Rtv K/W thermal resistance coil ==> pole tips
rv Ws/Km air convection cooling depending on velocity
Rtm K/W thermal resistance magnet ==> environment
tau m min thermal time constamt of magnet
Ctm Ws/K thermal capacity of the magnet
tau v s thermal time constant of voice coil