With the development of digital hearing aids, recent years have seen the realization of a hearing aid that allows hearing with higher sound quality by optimally correcting audibility according to the degree of hearing impairment of a user.
On the other hand, hearing aids are a necessity for carrying out everyday life, and are used by being worn continuously over a long period of time. As such, there is a demand for further miniaturization of hearing aids so that the users do not get tired even during prolonged wearing.
Specifically, there is a demand for further miniaturization of small speakers which are inserted into an ear hole and used as a hearing aid receiver.
In particular, among open fitting hearing aids, the miniaturization of the receiver is important because there is a need to secure an air passage within the outer ear canal.
In an open fitting hearing aid, an air passage is provided between the receiver and the ear hole, and thus the receiver does not completely cover the ear hole when worn. This results in improvements in wearing comfort, such as minimal reverberation of the user's own voice and minimal sensation of ear hole obstruction during use.
Furthermore, in the case of the open fitting type, at or below the fundamental resonance frequency of the speaker, the sound pressure level decreases with the lowering of frequency, and thus reproduction in the low frequencies becomes difficult. Therefore, there is a demand for a speaker which is small and, at the same time, has a low fundamental resonance frequency.
At present, typical hearing aids utilize a balanced armature speaker which is one form of magnetic speaker. The balanced armature type has a structure in which an armature (moving iron), to which a diaphragm is attached, is disposed between two sets of coils and magnets.
In a balanced armature speaker, current is passed through the respective coils to change the magnetic field within the armature. With this, the armature which is held at the center part of a magnetic air gap and is balanced by the magnetic attractive forces of the two magnets vibrates, and sound waves are generated as a result.
Although miniaturization is structurally possible for balanced armature speakers, the magnetic air gap width between the magnets and the armature is narrow, and thus reproduction of low frequencies which requires large amplitude is difficult. In view of this, the electrodynamic system is available as a system for realizing low frequency reproduction.
FIG. 11 is a cross-sectional view of a structure of a conventional electrodynamic small speaker described in Patent Literature (PTL) 1.
The conventional electrodynamic speaker shown in FIG. 11 includes a yoke 1, a magnet 2, a plate 3, a voice coil 4, a diaphragm 5, a magnetic fluid 6, and a suspension 7.
In this structure, stiffness can be reduced by using, for the suspension 7, a material that is softer or a material that is thinner than the diaphragm 5. As a result, the fundamental resonance frequency is lowered, and, with this, reproduction of low frequencies becomes possible.