AES London 2011
Tutorial T9

Sunday, May 15, 17:00 — 18:00 (Room 5)

T9 - Power Limitations in Micro Transducers

Bo Rohde Pedersen, Rohde Consulting
Andreas Eberhardt Sørensen, Pulse HVT

Voice coil temperature is typically a HOT topic when discussing power handling of transducers and many manufactures use IEC tests to specify and rank the input power a specific transducer can endure. For micro speakers this situation is also the case but as we are dealing with small coils and thin wires, the need to know and map the thermal factors are even more important.

The factors are the input power, losses, and cooling effects that happen around the voice coil. In this session these factors will be mapped and investigated as well as demonstrated. The first part of the session holds a mini tutorial in using an infrared thermal camera to measure the voice coil temperature as well as a discussion on what must be in focus when setting up such a system to measure micro transducer thermal aspects.

As the temperature increases our tests shows temperatures of up to 200°C in the voice coil and as a result the Rdc (Dc resistance) also increases and the resulting power dissipated decreases. The theoretical thermal model of the transducer will be examined and revised to fit the findings of our research. The used thermal model is of second order and includes nonlinearities to describe the change of the voice coil resistance and the velocity depending cooling of the voice coil.

Under the test are two micro loudspeakers one with a round and one with a rectangular voice coil because the request for higher output and lower resonance frequencies have moved the development from small round coils to larger (relative to the transducer) rectangular coils. The effect of this transition will be documented and the differences as well as the change in the thermal factors will be explained. We want to prove with this session that the trend has an overall positive effect on the overall thermal behavior of the voice coil and the micro transducer’s capability to endure higher input power.

Finally this tutorial should end with a discussion of the factual situation in the micro transducer market that the customers are asking for more power from the driving amplifier and first question is; will the micro transducer be able to handle that input power? Today the amplifiers deliver 1-1.5W in 8Ohm with a build in voltage step up but customers are asking for 2-3W and micro transducers will have to be designed to handle input abuse of this scale. Once this power handling is in place the next question will be; what is gained on the output side of the micro transducer?

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