# Computer based Loudspeaker Impedance Measurement

The measurement of the loudspeaker impedance is based on the system shown in Fig. 1
The referential resistor R is connected between the signal generator and a loudspeaker impedance Z.

Fig. 1 Circuit for the loudspeaker impedance measurement

Impedance is defined in the frequency domain - Z (f). If we measure voltages U1 (f) and U2 (f), at both end of the referential resistor, we can estimate the loudspeaker impedance as:

Fig. 2

The simple measurement configuration is shown in Fig. 3 The soundcard line-out and an external power amplifier is used as a signal generator output. The soundcard left line-input is used for recording the voltage U1 and the soundcard right line-input is used for recording the voltage U2.

Fig. 3

An electrodynamic loudspeaker that is mounted in the infinite baffle is characterized with following physical parameters:

Electromagnetic parameters:
• Re - voice coil DC resistance (ohm)
• Le - voice coil inductance (H)
• R2 - resistance due to eddy currents (ohm)
• L2 - inductance due to inductive coupling of eddy currents (H)
• Bl- force factor (Tm)
Mechanical parameters:
• S- membrane effective area (m2)
• Cms - membrane mechanical compliance (m/N)
• Mms - mechanical mass of membrane plus mass of air load on membrane (kg)
• Rms - mechanical resistance plus membrane radiation resistance (kg/s)
It is easy to estimate Thiele-Small parameters if we have measured data for the loudspeaker impedance.

At low frequencies the influence of a voice coil inductance is small and the expression for the loudspeaker input impedance has the following form:

where s = jw , Ts = 1/ws.

The impedance has the maximum value at the resonance frequency: because Qes>Qt.

Typical impedance curve of a loudspeaker that is mounted in a free air

The ARTA software consists of following programs:
• ARTA - program for the impulse response measurement and for real-time spectrum analysis and frequency response measurements.
• STEPS - program for frequency response measurements with stepped-sine excitation.
• LIMP - program for the loudspeaker impedance measurement and loudspeaker parameters estimation.

The ARTA program has functions of following measurement systems:
• Impulse response measurement system with signal generators: periodic white noise, periodic pink noise, MLS, linear and logarithmic swept-sine.
• Dual channel Fourier analyzer with signal generators: white noise, pink noise, periodic white noise and periodic pink noise.
• Single channel Fourier analyzer with signal generators: periodic white noise and periodic pink noise.
• Spectrum, octave band and THD analyzer with signal generators: sine, two sine, multitone, white noise, pink noise, periodic white noise and periodic pink noise.

The LIMP impedance plot window

Download ARTA here...

Also very good and complete freeware is Room Eq Wizard    Have a look at Room Acoustics Software

Thiele/Small parameters, a guide to the world of loudspeakers specifications.

<<< Back