Piezoelectric microphones operate based on the electroacoustic transduction principle, where mechanical vibrations are converted into electrical signals. The crystal inside the microphone deforms under acoustic pressure, generating a voltage proportional to the deformation. To ensure sensitivity and frequency independence, the crystal must be positioned in such a way that it responds uniformly across different frequencies. For optimal performance, materials with the strongest piezoelectric effect should be selected. Rochelle salt and ammonium dihydrogen phosphate (ADP) are commonly used in audio microphones, while they are also suitable for underwater and ultrasonic applications due to their durability, even though their sensitivity is relatively low.
In the early 1970s, piezoelectric polymers were introduced for use in both standard acoustic microphones and specialized underwater and ultrasonic devices. These materials offer flexibility and can be shaped into various structures. One common design involves a ring-shaped supporting diaphragm, with a diameter of 30 mm and a support plate curvature radius of 35 mm. To allow bidirectional vibration, small peaks of 0.5 mm are added to the support plate. These peaks help reduce the restoring force on the diaphragm by allowing air to pass through holes in the support plate, thus increasing the microphone's sensitivity. A preamplifier is mounted within the cavity of the membrane to boost the weak signal generated by the piezoelectric material.
The diaphragm in a piezoelectric polymer microphone is curved between the support points. When deformed, the outer edge experiences the highest stress and strain, so the surface remains tent-shaped rather than tapered. This shape helps maintain uniform stress distribution. Additionally, uneven stresses can cause radial curvatures in the diaphragm, as shown in Figure 9-9. These curvatures contribute to a more linear response, as flat diaphragms tend to produce more harmonic distortion.
The preamplifier is directly integrated into the membrane of the support plate. It typically uses a field-effect transistor (FET) with an input impedance of 200 MΩ, ensuring minimal signal loss and high stability. This configuration allows for efficient signal amplification, making the microphone suitable for a wide range of applications from consumer electronics to scientific research.
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