Electro acoustics deals with recording and reproduction of sound. The quality of the reproduced sound depends highly on the properties of the microphones and loudspeaker in the signal chain, due to the influence of their frequency response, directivity and nonlinear effects. Transducers come in many shapes and sizes, ranging from tiny microphones and loudspeakers used in hearing aids to large loudspeakers used in PA systems for stadium concerts.
The electrodynamic loudspeaker is completely dominant in the market, except for hearing aids and in-ear headphones where the balanced armature receiver is preferred. For the microphones several principles are competing in the market: The dynamic microphone is competing with the condenser principle and its variant the electret, which has the advantage of working without the need of a large polarization voltage. The latter dominates in numbers as it is used in mobile phones.
The current trend for the home market is for loudspeaker systems to become smaller and smaller, which makes it a major challenge to improve or even maintain the ability to produce sufficient output at low frequencies, while maintaining efficiency and linearity.
In order to predict transducer behavior one must be able to understand the electrical, magnetic, mechanical and acoustic subsystems and how they couple and influence each other. A low frequencies and low levels the transducers can be modelled with relatively few parameters, such as moving mass, suspension compliance etc. But several effects complicate the behavior: Eddy currents in the iron of the magnetic circuit influences the voice coil inductance, higher order vibration modes in the diaphragm create large variations in frequency response and directivity pattern, and viscoelastic effect in the mechanical parts leads to parameter changing with frequency. Nonlinear effects which create audible distortion reduces the sound quality of the reproduced sound, this is especially is a concern for loudspeakers.
Simple lumped parameter models based on analogy circuits can be used to predict the basic frequency response of transducers, and it is possible to increase the accuracy and frequency range of these models by using more advanced elements in the models, but to really model the behavior at the higher frequencies one most use Finite Element Model calculations in order to capture the behavior of effect such as diaphragm vibrations and eddy currents. In miniature transducers viscous and thermal losses also play an important role.
Nonlinear domain behavior can be modelled using lumped parameter time domain models extended with parameters that depend on state variables such as position or current. Alternatively FEM calculations may be done, but these are still computationally very expensive, and therefore often not used in iterative design processes.
The research at ACT cover most aspects of transducer as mentioned above, but particular focus is given to:
Nonlinear behavior in miniature transducers
Eddy currents and distortions in the magnetic system
Nonlinear compensation of transducers
Nonlinear system analysis and synthesis
New transducer principles