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5 Hearing

itor, in that case the electrical signal would be caused by the difference in voltage due to

the different distance [8]. Alternatively, a material with a permanent charge (an “elec-

tret” of “ferroelectric” material) could be used with the same effect [9]. These methods

are commonly used in microphones.

On a smaller scale, vibration can also be captured by a piezoelectric crystal [10].

Piezoelectricity by definition is a transducer of vibration/pressure to electricity—these

materials emit voltage when under pressure. In a piezoelectric microphone, the trans-

ducer is such a crystal.

Microphones can also be so small that they can be part of a microelectromechanical

system (MEMS), a special type of computer chip [11]. In this case, the pressure-sensitive

diaphragm is directly etched out of the silicon in the chip. Microphones made in this

way are digital microphones.

Here, I would like to focus on biomimetic acoustic sensors that are either im-

plantable, mimic the attributes hearing possesses additionally to microphones, or use

the acoustic sensor for a different application.

As an example of implantable microphones, Lee’s group is in the process of devel-

oping an artificial hair-cell microphone based on a flexible, piezoelectric film [12] (Fig-

ure 5.6). This system exhibits some of the frequency separation and selectivity of the

basilar membrane. It was possible to align the distribution of vibration displacement

Figure 5.6: Artificial hair-cell microphone based on a flexible, piezoelectric film developed for implemen-

tation [12]. (a) Experiment setup for measuring the vibration amplitude of the flexible piezoelectric film

in response to a sound. (b) The film is scanned to detect vibrations by using scanning points. (c) Peak of

vibration over all scanning points in (i) first harmonic mode and (ii) second harmonic mode.