Since the metal film is formed using the fiber as a die as described above, a specific surface area of the heat generating layer can be increased, and the sound pressure with respect to the unit input power can also be increased. In addition, since a low heat conductive material such as a polymer is used as the fiber layer, a heat insulating effect in the substrate direction can be obtained. Therefore, the temperature change on the surface of the heating element is increased, and the sound pressure with respect to the unit input power can be increased.

(Sample Preparation Method)

A pressure wave generating element was produced by the following method (Sample 21).

An epoxy resin (bisphenol A type) solution prepared using N,N-dimethylacetamide (DMAc) as a solvent was used as a spinning solution. The solution concentration was adjusted to 30 wt %. At this time, additives such as imidazoles can be appropriately used.

Using this solution, epoxy resin fibers were spun on a Si substrate (675 μm thick) by an electrospinning method to form a fiber membrane of a nonwoven fabric. In order to enhance the adhesiveness between the fiber membrane and the substrate, an adhesive layer may be appropriately introduced into the interface between the Si substrate and the fiber membrane.

The electrospinning conditions were an applied voltage of 23 kV, a distance of 15 cm between a nozzle and a substrate, and a film formation time was adjusted so that the thickness of the fiber membrane was about 1 to 80 μm. The average fiber diameter of the fibers was 235 nm.