Thiele-Small Parameters and Dual Voice-Coil Drivers

by Isaac MCN
f4ier@hotmail.com


 

 
 

0. INTRODUCTION

I'm sure somone with some website has already posted something similar to this page, but the fact that the information here has been sought after time and time again indicates that those other websites are either hard to find or rare.

In this page, I illustrate the five possible wiring configurations of a dual voice-coil, DVC, driver unit followed by the corresponding changes in the Thiele-Small Parameters.


1. DUAL VOICE-COIL WIRING CONFIGURATIONS

Parallel Voice-Coils

Revc/2
Levc/2
Qms
Qes
Qts = Qms*Qes/(Qms+Qes)
Bl

Series Voice-Coiils

2*Revc
2*Levc
Qms
Qes
Qts
2*Bl

One Disconnected Voice-Coil

Revc
Levc
Qms
2*Qes
calculate Qts
Bl

One Shorted Voice-Coil

Revc
Levc
Qms = (Revc/((Revc*Rmd)+(Bl*Bl)))*sqrt(Mmd/Cmd)
2*Qes
calculate Qts (unchanged, actually)
Bl

Adjustment Resistor Connected to one Voice-Coil

Revc
Levc
Qms = ((Revc+R)/(((Revc+R)*Rmd)+(Bl*Bl)))*sqrt(Mmd/Cmd)
2*Qes
calculate Qts
Bl

One Amplifier per Voice-Coil

Revc
Levc
Qms
Qes
Qts
Bl

Notes:

Revc = DC voice-coil resistance   ohms
Levc = voice-coil inductance   H
Qms = mechanical damping factor    
Qes = electrical damping factor    
Qts = total damping factor    
Bl = electrical-mechanical conversion factor   Tesla.meters
Sd = effective cone area   square-meters
Fs = resonant frequency of driver   Hz
Mmd = mass of driver's cone   kg
Cmd = compliance of driver's suspension   m/N
Rmd = mechanical resistance in the driver's suspension   N.s/m
Vas = equivalent volume of the driver   cubic meters
         

1a. Parallel Voice-Coils

Using this wiring configuration, the effective resistance is half that of one voice-coil's resistance. Likewise with the voice-coil inductance -- the effective voice-coil inductance is cut in half.


1b. Series Voice-Coils

This wiring configuration yields system Revc, Bl and system Levc that is twice that when using just one voice-coil.


1c. One Disconnected Voice-Coil

Instead of having 2 voice-coils to electrical damp the resonance of the driver, this particular wiring configuration only uses one voice-coil -- the other voice-coil is simply not used or "open-circuited." Because of this, Qes increases by a factor of two.


1d. One Shorted-Voice Coil

Shorting one of the voice-coils lowers Qms because the shorted voice-coil provides electrical damping. On the other hand, Qes doubles because only one voice-coil is connected to the amp.


1e. Adjustment Resistor on one Voice-Coil

The system Q can be adjusted by putting a suitable resistor across one of the voice-coils. Qms is a little lower than 2*Qes when one voice-coil is shorted while discarding one of the voice-coils does not change Qms. Values of Qms midway between these two extremes can be achieved by using a suitable resistor across one of the voice-coils and it can be calculated with the above-mentioned equation. Qes, due to the other voice-coil, is double that of the value as with the parallel wiring configuration.


1f. Separate Amplifier for each Voice-Coil

Treating each amplifier as ideal voltage sources (having no output resistance at all), this particular wiring configuration is essentially the same as the parallel voice-coil configuration. Thus, the three system Q values are equal that of the ones corresponding with the parallel wiring configuration.


The following parameters remain unchanged regardless of wiring configuration,

Fs
Mmd
Cmd
Rmd
Sd
Vas

2. END

I hope the table above will help you in calculating the corresponding Thiele-Small parameters of your dual voice-coil driver. Comments, suggestions and corrections are welcome of course.


3. Corrections Made

- 25 June, 2002: The old highlighted equations had R when I meant to say total R -- including Revc. The updated equations above now clearly states this. I apologize for any inconvenience the old equations might have caused.