Electrical Model for Free Air

Calculate Res - Lces - Cmes for the electrical speaker model

The speaker equivalent circuit is the actual core of the theories crafted by Thiele and Small.
Every component of a speaker can be “transformed” into an electrical component, to form a circuit that acts exactly the same as the speaker (electrically).

Now imagine that you get different resistors, inductors and capacitors, and arrange them in such a way, that when you measure the impedance of this newly formed circuit, it’s exactly the same as the impedance of the speaker. A similar thing happens when you place the speaker in a box. Additional components are added to the circuit. That is how Thiele and Small manage to predict the behavior of enclosures.

  • Re = DC resistance of a loudspeaker transducer, measured in ohms.
  • Le = Voice coil inductance measured in millihenries (mH) (measured at 1 kHz).
  • fs = is the free-air resonant frequency of a speaker
  • Bl = The product of magnet field strength in the voice coil gap and the length of wire in the magnetic field, in tesla-metres (T·m).
  • Mms = Mass of the diaphragm/coil, including acoustic load, in grams. Mass of the diaphragm/coil alone is known as Mmd
  • Cms = Compliance of the driver's suspension, in metres per newton (the reciprocal of its 'stiffness').
  • Rms = The mechanical resistance of a driver's suspension (i.e., 'lossiness') in N·s/m

  • Cmes = Cone Suspension
  • Lces = Cone mass
  • Res = Cone Losses

Voice Coil DC Resistance, Re [Ω] =
Voice Coil Inductance, Le [mH] =
Resonance Frequency, Fs [Hz] =
Motor Force Factor, Bl [Tm] =
Moving Mass, Mms [g] =
Compliance, Cms [mm/N] =
Mechanical Resistance, Rms [kg/s] =

You will need the following tools:

LTSPICE: http://www.linear.com/designtools/software/

Speaker datasheet or own measurements

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