C6.1 - Hearing Aid Microphones: From Electret to Piezoelectret Transducers
- SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
- Proceedings SENSOR 2011
- C6 - Miniature Audio Transducers
- G. Sessler, J. Hillenbrand - Technische Universität Darmstadt (Germany)
- 463 - 467
For several decades, most hearing aids have been equipped with electret microphones. These transducers are of simple design. They consist of an electret, mostly a thin polytetrafluoroethylene film cemented to a back electrode, and a membrane, often a metallized polymer film, separated by a shallow air gap from the electret.
Some years ago, a new type of electret microphone, based on cellular piezoelectrets, was introduced and characterized. Piezoelectrets consist of closed-cell foams, often of polypropylene (PP). Charging the surfaces of the cavities by a corona discharge in a certain way makes the material piezoelectric. Advantages of such polymers are high piezoelectric coefficients, availability in large areas, and low material cost.
Because of their small elastic modulus, piezoelectrets are easily compressed by sound waves. If used as a microphone, an air gap is not required. The design of such microphones is thus very simple. Apart from the piezoelectret film, only provisions for film mounting, shielding, and contacting are needed. With stacked films, the open-circuit sensitivity increases proportional to the number of films. A piezoelectret microphone consisting of a stack of six films has a relatively high sensitivity of about 15 mV/Pa. Piezoelectret films made of layers of the fluoropolymers FEP and PTFE are more stable at elevated temperatures than the PP films, and microphones built with them can be used up to 90°C.
Advantages of piezoelectret microphones are their low harmonic distortion and their relatively low equivalent noise level. An evaluation shows that the A-weighted equivalent noise level (ENL) is only 37 dB(A) and 26 dB(A) for transducers with 1 and 5 layers, respectively. To increase the capacitance of piezoelectret microphones, transducers with coiled film stacks have been designed and tested.
Piezoelectret microphones can also be built as gradient transducers. This is achieved by placing two units close to each other and combining them in a sum and delay network. Dipole and cardioid characteristics were achieved with such directional microphones. The suitability of piezoelectret microphones for use in hearing aids is due to the following features: As compared to electret microphones, they do not require a back volume and are thus thinner and more appropriate for use as SMDs. Compared to silicon condenser microphones, stacked piezoelectret transducers have lower ENL. Also, as pointed out above, piezoelectret microphones are of simpler design than competing microphones.