An electro-dynamic loud speaker tested is described in details. The impedance characteristics of the active coil are found to be just the same as for ordinary telephone receivers. Damped or stationary and motional vector impedance graphs are respectively a straight line and a circle when the frequency is varied But the diameter of motional impedance circle coinsides with the resistance axis, which or ordinary receivers is usually depressed downwards from it. The diameters are 74.0, 194.0 and 290.0 ohms, respectively for the d. c. excitation of 0.5, 1.0 and 1.5 ampere and without the trumpet. The resonance, first and second quadrantal frequencies are respectively about 620 _??_, 616.5 _??_ and 624 _??_ for 1 ampere excitation. Variations of these frequencies for different excitation are very small. And the force factor for 1 ampere excitation is 9.64×106 dynes per absampere. When, however, the trumpet is on, the motional impedance graph is reduce to about 1/16 of the size when it is off, which means little distortion of the diaphram motion for the reproduction of speech although the system without the trumpet has a marked resonance point; and the graph is accompanied by a number of small loops, because the trumpet is a multi-resonance system. Advantages of this instrument for acoustic impedance and similar measurements are explained. The action of such a vibrating system as inductance or capacitance is also pointed out.