Acoustic transducers

The acoustic transducer is an electronic component which converts sound waves into some other form of energy, or vice versa of it. The other form of energy is usually an alternating-current electrical signal. The waveforms of acoustical and electrical signals are identical.Acoustic transducers are designed for the various frequency ranges. The human hearing spectrum extends from around 20 Hz to 20 kHz. But the acoustic energy can have frequencies below 20 Hz or above 20 kHz. Energy at frequencies below 20 Hz is called as infrasound; if frequency is over 20 kHz, it is called as ultrasound. In the acoustic wireless devices, ultrasound is commonly used, because the wavelength is short and necessary transducers can be small. The ultrasound cannot be heard, and thus it will not distract or annoy people.

Figure given below is a simplified diagram of a piezoelectric transducer. This device consists of a crystal, like quartz or ceramic material, sandwiched between 2 metal plates. When the acoustic wave strikes one or both the plates, the metal vibrates. This can be is transferred to the crystal. The piezoelectric crystal generates weak electric

                              Figure--Simplified cross-sectional diagram of a piezoelectric acoustic transducer.

currents when it is subjected to the mechanical stress. Thus, an alternating current voltage develops between the 2 metal plates, with the waveform similar to that of acoustic waves.

If an alternating current signal is applied to plates, it causes crystal to vibrate in sync with current. The result is that metal plates vibrate also, generating an acoustic disturbance in air. The piezoelectric transducer can thus behave either as an acoustic pickup or an acoustic emitter. Piezoelectric transducers are common in the ultrasonic applications, like intrusion detection systems.

Figure given below shows another type of acoustic transducer that can function either as a pickup or emitter. This is called as an electrostatic transducer. When a sound wave strikes flexible plate, the plate vibrates. This causes quick changes in spacing between flexible plate and rigid plate, producing fluctuation in capacitance between plates. A direct current voltage source is connected to plates. As the capacitance fluctuates, the plates alternately charge and discharge. This causes a weak alternating current signal to be produced, whose waveform is similar to that of acoustic disturbance. The blocking capacitor allows this alternating current signal to pass to the transducer output, while keeping the direct current confined to plates.

If an alternating current signal is applied to terminals of transducer, it passes through blocking capacitor and onto metal plates. The signal creates a fluctuating voltage between plates. The direct current source, combined with the alternating current signal, results in an electro static field of constant polarity, but of fast varying intensity. This exerts the fluctuating force between plates, causing flexible plate to vibrate and generate an acoustic wave in air.

The most well known type of acoustic transducer has a coil surrounding a permanent magnet or electromagnet. A diaphragm or cone is attached to the coil or the magnet. When diaphragm vibrates as a result of acoustic waves striking it, alternating current is produced in coil. Conversely, if alternating current is applied to the coil, it produces electromagnetic force which causes the diaphragm to vibrate. This device is called as dynamic transducer. Most of the headphones, microphones, and loudspeakers are of this type.Acoustic transducers are used in the security systems. They are used in robotics to help mobile machines navigate in their surroundings. Acoustic transducers are employed in depth finding apparatus usually found on boats.

                                      Figure--Simplified diagram of an electrostatic acoustic transducer