Part III: What fruit flies and jet engines have to do with microspeakers
The first two parts explained the necessity of power efficient components in hearables and how electrostatic microspeakers based on MEMS technology can contribute to that. The electrostatic transducer itself does not dissipate any energy but the drive electronics do. How much energy is “burned” primarily depends on the driving voltage and the capacitance of the transducer. This part focuses on the capacitance of the transducer and its implications.
A very interesting approach is to ask the question:
How much energy is actually necessary to generate a certain sound pressure in the ear canal?
This can be answered by simply balancing the energies involved. In a transducer, the input power is converted into reactive and dissipative parts. The dissipated fraction is simply lost as heat. The reactive power goes into elastic compression of air, it is required to effect the elastic deformations of MEMS microspeaker parts (actuators, pistons, valves and alike) and it also is stored as kinetic energy according to Newton’s laws.
For electrostatic transducers, the dissipative parts can be neglected. That means that the entire energy is stored in the displacement of an elastic structure and transformed into the compression of the air in an ear canal. At one end of this canal, this compression leads to a movement of the ear drum.
Evidently, the energy stored in the elastic deformations and alike, will be returned in some way. But it is also obvious that the transducer is the more efficient the less structural deformation and moving masses are involved. The efficiency is then simply calculated by dividing the energy at the output by the energy provided at the input.
The kinetic energy of a slow-flying fruit fly will do!
Conventional electrostatic transducers are usually used in high end loudspeakers and headphones. The membrane in these transducers needs a certain excursion to enable the required sound pressure level. A rather large gap between the membrane and the electrodes is necessary to allow that excursion. But to provide the required force, high electrical voltages are inevitable – for loudspeakers in the range of several thousand volts and for headphones usually a couple of hundred volts.
As mentioned above, this is certainly not helpful for aiming at a fully integrated drive circuitry, keeping in mind that this voltage needs to be generated out of a 3.6V lithium polymer battery for mobile applications. And who really wants to carry around hundreds of volts in their ear?
The electrical capacitance should ideally be as low as possible
Why is the amount of reactive power required so important? Well, essentially an electrostatic transducer extracts the required reactive power from the electrical energy stored in its electrical capacitance. The higher the need for reactive power, the larger the capacitance needs to be – it is that simple. As we have seen above, the physics of sound requires only a tiny amount of energy per time (i.e. power), to generate the sound pressure of a jet engine by an earbud! So an ideal microspeaker should have a very small electrical capacitance. That is theory – how about reality?
Lessons learned for microspeaker designers:
An electrostatic transducer essentially converts the energy stored in its electrical capacitance into sound. This transformation is done at a certain electrical voltage. Assuming an ideal transducer with 100% conversion efficiency, a transducer capacitance of as little as 31 pF is sufficient to generate a sound pressure of as much as 120 dB SPL. That is an incredibly low number!
At Arioso, we are heading for transducer capacitances in the range of 500 to 800pF. At first glance this may not look very ambitious, compared to the threshold set by nature. But from a technological point of view in fact it is. Actually, the resulting power efficiencies outperform any other existing or emerging microspeaker technology. It’s a breakthrough.
So the ultimate question is how to design a microspeaker with such a performance?
Basically, we know the answer already: Simplify the architecture of your micro electro mechanical machine, i.e. the microspeaker, as much as you can. Minimise the amount of silicon parts that need to move for generating the sound. Avoid parts that need to move at higher speed than absolutely necessary.
That is what nature has taught technologists all along: finding the most efficient solution means avoiding complexity and striving for elegance.