# 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