A Piezoelectric Transducer for a Hearing Aid Using PZT Thin Film
keywords: Hydrothermal Method, Piezoelectric, PZT, Transducer, Earphone
Conventionally, hearing aid earphones make use of electromagnetism to transmit sound to the impaired ear.
While this works well enough, We are investigating the possibility of improving current hearing aids by making use of piezoelectric properties, which may have the following advantages over electromagnetic-type earphones: They require fewer battery changes, They are smaller and simpler and are more easily fabricated.
As illustrated in Fig. 1, a hearing aid is composed of three parts: a microphone, which catches the surrounding sounds, an electric circuit, which amplifies the sound and an earphone, which transmit the amplified sound. For this process, the inner space between the earphone's diaphragm and the eardrum have to be completely sealed.
If not, the leaking sound is amplified and causes feedback howl.
The earphone and the eardrum are indicated in Fig. 2. The piezoelectric transducer has a bimorph construction and forms a diaphragm for a hearing aid earphone as in Fig. 3.
A bimorph consists of three layers. The center layer is a base material, and the outer layers consist of piezoelectric materials. By using a piezoelectric material, we can change the energy from mechanical to electric or vice versa.
The diaphragm in an earphone, vibrates according to the input electric signal.
The change of volume between the diaphragm and the impaired eardrum makes sound pressure.
Piezoelectric material has a poling direction.
When we apply an electric field to a diaphragm in the poling direction, the diaphragm stretches horizontally in the electric field and shrinks vertically in accordance with Poisson's ratio.
When we apply an electric field in the reverse direction, it shrinks horizontally in the electric field and stretches vertically.
In the hydrothermal method, the poling direction is perpendicular to the radius of the disk from outside to in. As we apply an electric field to our bimorph disk in order to stretch one side of the piezoelectric layer and shorten the other, the bimorph vibrates transversely in accordance with the frequency of the driving voltage.
We have fabricated a piezoelectric transducer using a PZT thin film deposited by the hydrothermal method.
The PZT material can produce sounds efficiently from an electrical signal.
This thin film technology is suitable for fabrication of a small transducer, which is worn in ear.
This method uses chemical reactions between a titanium base material and melted ions: Pb2+, Ti4+ and Zr4+ in solution containing KOH, as indicated in Fig. 4.
By this method, we can easily make a small device of any shape.
The reactions are promoted in a teflon autoclave so that we can apply the solution high temperature and high pressure.
This process was composed of two repetitional reactions. In each reaction, the autoclave was kept at 140 °C (first reaction) or 120 °C (second reaction) for 24 hours. In these processes, 10 mm thick PZT thin film was deposited.
The fabrication process of the transducer has two steps. The first step was the depositing of PZT film by the hydrothermal process. The next step was the vacuum deposition of driving electrodes. Gold was used for the electrodes. Each step is indicated in Fig. 5.
We deposited a 10 mm thick film of PZT on the surface of a base disk made of 5 mm thick titanium foil.
The diameter of the transducer was 6 mm.
The bimorph transducer was fixed at the circular edge by two silicone rubber O rings as illustrated in Fig. 3.
To evaluate the transducer, we measured the vibration amplitude of the diaphragm and the sound pressure.
The vibration amplitude of the transducer at the center was measured using a laser Doppler vibrometer.
For example, the amplitude-frequency characteristic of a 10 mm titanium base diaphragm is shown in Fig. 6.
We used Ear Simulator Type 4157 (B&K) for the measurement of the sound pressure. Using this instrument, we can test an earphone under acoustic load conditions similar to the insertion of the earphone into as an actual human ear.
One example of the frequency response for a transducer of 10 mm titanium is shown in Fig. 7.
We found that the 5 mm transducer disk was able to produce 69 dB (0 dB = 2~10-5 Pa) per 1.3 Vrms driving voltage below the fundamental resonance frequency.
However, at least 110 dB per 1.3 Vrms is necessary for practical use.
To investigate this cause of the low sound pressure, we have measured the displacement distribution of the 5 mm titanium base diaphragm by scanning it.
The results are indicated in Fig. 8.
The center should be the largest displacement point, but we found that there are non-displacement parts in areas near the center of the diaphragm.
We had fixed the diaphragm at the top and the bottom with two silicone rubber O-rings.
These O-rings had deformed the diaphragm because of their bad occlusion. This is one reason why the sound pressure level is low.
To improve the performance, we are aware of two points: the PZT quality and how to fix it so as not to deform the diaphragm.
For example, in order to increase the volume changed between the earphone and the eardrum, we are thinking of fixing the diaphragm at the center and using thinner titanium base foil.
Moreover, before depositing PZT thin film, we will fix the titanium base foil with a titanium jig so that we prevent the diaphragm's unexpected deformation.
