1. SUPERCONDUCTING VOICE COIL – A ROUTE TO INCREASE EFFICIENCY OF AN ELECTRODYNAMIC LOUDSPEAKER Ivan Djurek *1, Danijel Djurek *2, Branko Somek *1 *1 Faculty of EE and Computing, Zagreb, Croatia *2 AVAC, Zagreb, Croatia

2. INTRODUCTION Complex system Several steps of energy transformation Loss of energy Hard to construct the loudspeaker with low distortion level and high efficiency Possible route: SC voice coil

3. CONVENTIONAL LOUDSPEAKER Example of forced damped oscillator M – mass of voice coil and membrane R S, X S – real and imaginary part of emission impedance k – elastic restoring force R M – friction part

4. CONVENTIONAL LOUDSPEAKER Emission impedance Imaginary part represented with aproximation of Struve function [3]

5. CONVENTIONAL LOUDSPEAKER Characteristics of used commercial loudspeaker Radius of the membrane8 cm R C (working temperature 50°C) 8  B – magnetic field0.4 T l – length of voice coil wire in mag. field12 m M - Mass of the membrane14 g k – factor of elastic restoring force1400 N/m R M (Q es =2.93) – fricton factor1.2 kg/s Manufacturer data

6. CONVENTIONAL LOUDSPEAKER Driving force Current from back electromotive force (BEF) Amplitude of oscillation

7. CONVENTIONAL LOUDSPEAKER Amplitude at resonant frequency

8. CONVENTIONAL LOUDSPEAKER Total power Efficiency

9. CONVENTIONAL LOUDSPEAKER Efficiency of used commercial loudspeaker

10. LOUDSPEAKER WITH SC VOICE COIL Changes: One turn in voice coil: l = 0.075 m R C = 0  No inductance - L Total mass (calculated): 11 g

11. LOUDSPEAKER WITH SC VOICE COIL Efficiency

12. EXPERIMENTAL Test of previous calculations Copper voice coil cooled to 130 K (-143°C) Vibration amplitude vs resistance of voice coil

13. EXPERIMENTAL Amplitude and resistance measurements f=40 Hz

14. DISCUSSION Advantages: Disappearance of electro-mechanical coupling terms > lower distortion Reduction of voice coil inductance L Reduction of mass Higher efficiency Problems: Impedance matching between SC loudspeaker and common amplifiers Cooling