Acoustic transducers

Chapter Acoustic transducers

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Acoustic transducersAn acoustic transducer is an electronic component that converts sound waves intosome other form of energy, or vice versa. The other form of energy is usually an al-ternating-current (ac) electrical signal. The waveforms of the acoustical and electri-cal signals are identical, or nearly so.Acoustic transducers are designed for various frequency ranges. The human hear-ing spectrum extends from about 20 Hz to 20 kHz. But acoustic energy can have fre-quencies lower than 20 Hz or higher than 20 kHz. Energy at frequencies below 20 Hz iscalled infrasound; if the frequency is above 20 kHz, it is known as ultrasound. Inacoustic wireless devices, ultrasound is generally used, because the wavelength is shortand the necessary transducers can be small. Also, ultrasound cannot be heard, andtherefore it will not distract or annoy people.Figure 32-4 is a simplified diagram of a piezoelectric transducer. This device con-sists of a crystal, such as quartz or ceramic material, sandwiched between two metalplates. When an acoustic wave strikes one or both of the plates, the metal vibrates. Thisvibration is transferred to the crystal. The piezoelectric crystal generates weak electric612 Wireless and personal communications systemsMetalplatesCrystalSignal input/output32-4Simplified cross-sectionaldiagram of a piezoelectricacoustic transducer.currents when it is subjected to mechanical stress. Therefore, an ac voltage developsbetween the two metal plates, with a waveform similar to that of the acoustic waves.If an ac signal is applied to the plates, it causes the crystal to vibrate in sync withthe current. The result is that the metal plates vibrate also, producing an acoustic dis-turbance in the air. The piezoelectric transducer can thus act either as an acousticpickup or an acoustic emitter. Piezoelectric transducers are common in ultrasonic ap-plications, such as intrusion detection systems.Figure 32-5 shows another type of acoustic transducer that can function either asa pickup or emitter. This is known as an electrostatic transducer. When a sound wavestrikes the flexible plate, the plate vibrates. This causes rapid changes in the spacing be-tween the flexible plate and the rigid plate, producing fluctuation in the capacitance be-tween the plates. A direct-current (dc) voltage source is connected to the plates. As the