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:
R_{e} - voice coil DC resistance (ohm)
L_{e} - voice coil inductance (H)
R_{2} - resistance due to eddy currents (ohm)
L_{2} - inductance due to inductive coupling of eddy currents (H)
Bl- force factor (T_{m})
Mechanical parameters:
S- membrane effective area (m^{2})
C_{ms} - membrane mechanical compliance (m/N)
M_{ms} - mechanical mass of membrane plus mass of air load on membrane
(kg)
R_{ms} - 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 Q_{es}>Q_{t}.
Typical impedance curve of a loudspeaker that is mounted in a free air
ARTA is freeware to measure Loudspeakers
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.