In this note, we are going to know about the Crystal Microphone Diagram. And also about its function, characteristics, and applications as well. Welcome to Poly Notes Hub, a leading destination for engineering notes.
Author Name: Arun Paul.
What is Crystal Microphone?
A Crystal microphone is based on the principle of piezoelectric effect, which produces a difference of potential between the opposite faces of some crystals when subjected to mechanical pressure. The crystals that show this effect are quartz, tourmaline, Rochelle salt, and ceramic. Rochelle salt has a high piezoelectric effect but is susceptible to moisture. Also, it cannot withstand high temperatures of more than 50°C in outdoor use. Quartz and tourmaline have a low piezoelectric effect. Ceramic is most suitable for crystal microphones as it is not susceptible to moisture and can also withstand high temperatures up to 100°C.
When pressure is applied to the crystal, it deforms, and the momentary charge displacement occurs within the crystal structure. This creates a difference of potential between its two surfaces.
📌 Characteristics of Crystal Microphone –
- Sensitivity: This type of microphone has good sensitivity, about 50 mV (or 26 dB below 1 volt) for 0.1 Pa pressure.
- Signal to Noise Ratio: It is not prone to pickup background noise. Generation of noise inside the microphone is also low. So, it has HIGH signal to noise ration about 40 dB.
- Frequency Response: 100 to 8000 Hz for ±1 dB.
- Distortion: Low, about 1%.
- Directivity: Omnidirectional.
- Output Impedance: High, about 1 M-Ohm.
📌 Other Features of Crystal Microphone
- It is not as rugged as the moving coil microphone. But, it is more rugged than the ribbon one.
- It can be spoken into at close range.
- Its high impedance requires relatively short leads to the amplifier‘s input circuit to prevent loss of higher audio frequencies.
- The mixer circuit will load it and cause severe loss of bass.
- It does not need a bias supply.
- It should not be exposed to direct sunlight for a long time.
- Its cost is low.
Crystal Microphone Construction
Below we have describe the crystal microphone diagram and construction of Crystal Microphone –
The crystal is cut along certain planes to form a slice. Metallic foil electrodes are attached to the two surfaces to carry the potential difference to the output terminals.
Two thin crystal slices, suitably cut, are placed in an insulating holder with an air space between them. A large number of such elements are combined to increase the emf. A diaphragm, made of aluminum, is attached to the crystal surface through a push rod. The whole unit is encased in a protective case. There is a protective mesh cover (not shown in the figure) over the diaphragm.
Crystal Microphone Working Principle
When there is a sound wave of compression, it compresses the crystal. In case of rarefaction, the converse takes place and the crystal is extended and is under tension. Due to this compression and extension, a varying potential difference is generated which is proportional to the mechanical pressure applied to the crystal by the sound waves. The crystal elements are connected in such a way that the potential differences developed in the elements are added up we get a good voltage output (about 50 mV) for feeding to the amplifier.
A crystal microphone’s output equation is based on the piezoelectric phenomenon, which converts mechanical sound waves into an electrical voltage. The generic equation may be represented as –
Where;
- Vout = Output voltage of the crystal microphone
- k = Sensitivity constant (depends on the piezoelectric material)
- F = Applied force (caused by sound pressure waves)
Advantages of Crystal Microphone
Here we have listed some crystal microphone advantages –
- Crystal microphones offer high sensitivity (about 50 mV for 0.1 Pa pressure), making them useful for collecting sound with little effort.
- They have a high signal-to-noise ratio (about 40 dB), which means they pick up little background noise while producing clear sounds.
- With only about 1% distortion, these microphones produce relatively accurate sound reproduction.
- Their ability to collect sound evenly from all angles makes them ideal for general-purpose audio recording.
- Crystal microphones, unlike other microphones, do not require a bias supply, making them more convenient to use.
Disadvantages of Crystal Microphone
Here we have listed some crystal microphone disadvantages –
- Short connecting leads are necessary to prevent loss of higher audio frequencies, as the impedance is roughly 1 MΩ.
- The mixing circuit might overload the microphone, resulting in severe loss of bass frequencies.
- It has a limited frequency range (100 Hz to 8000 Hz), rendering it unsuitable for high-quality audio applications.
- It is more durable than ribbon microphones, but not as tough as dynamic moving coil microphones.
- Physical pressure or hard handling may reduce its performance over time.
Applications of Crystal Microphone
The crystal microphone is used in –
- Home recording system.
- Amateur Communications system.
- Mobile Communication systems as well.