A Study on a Ferro-electret Film- type Speaker

In this paper, we report our work on developing a film-type speaker based on a porous ferroelectret film. This speaker was made of a micro-porous polypropylene film. The advantage of this porous PP film is that it is lightweight and flexible and contains multiple void structures which allow it to store a large number of space charges and to have a piezoelectric effect. Even though micro-porous polypropylene film has been used in many applications for sensors and actuators, the application for speakers has been limited as it has a poor low frequency response. To break through this limitation, we developed two fixtures which can introduce tension for changing the void structure and for changing the density of the porous film for enhancing the acoustic intensity and overcoming the low frequency response. The two PMMA fixtures possessed an array of rectangular or circular holes to create the ferro-electret film array. They contain a sealed cavity to create negative pressure which can be used to create a curvature to each film for changing the void structure and film density. Our experimental finding showed that the acoustic response at low frequency was successfully enhanced and became smoother after introduction of curvature to the film. We found that the acoustic response of a 12.5mm diameter film was better than film with a diameter of either 10mm and 7.5mm. The optimization of the array fixture and experimental studies are discussed in this paper.