Sound is a sequence of waves of pressure that propagates through compressible media such as sound that is perceptible by humans and has frequencies from about 20 Hz to 20,000 Hz. During propagation, waves can be reflected, refracted, diffracted, or absorbed.
Digital recordings are considered higher quality than analog recording not necessarily because they have higher fidelity (wider frequency response or dynamic range), but because the digital format can prevent much loss of quality found in analog recording due to noise and electromagnetic interference in playback, and mechanical deterioration or damage to the storage medium. A digital audio signal must be reconverted to analog form during playback before it is applied to a loudspeaker or earphone.
MICROPHONE
Definition: "A microphone is a transducer which converts the sound signal into electrical signal".
Different types of microphones are available for the proper reproduction of various types of sound like music, speech, drama, and sports commentators. Microphones are usually called Mic. Schematic Symbol of microphone.
CARBON MICROPHONE
Principle: The carbon microphone works on the principle of change in resistance of carbon granules filled in a small enclosure. The total resistance varies linearly with sound pressure variations. Hence this device comes under pressure pressure-operated type microphone.
CONSTRUCTION:
It consists of a diaphragm and a brass cup filled with fine carbon granules. Two metal electrodes are placed in the carbon granules one is fixed and the other is movable. The diaphragm is in contact with granules through the movable electrode. The diaphragm is very light and stiff. It is made with duralumin. A battery is connected between two metal plates When the load is connected current flows through carbon granules and the load. The output is obtained through a transformer to eliminate the DC content of the microphone output and for Impedance matching.
Working: When no sound waves strike the diaphragm the carbon granules offer a constant resistance. During compression conditions, it presses the carbon granules. Hence resistance offered by carbon granules decreases and hence current through the circuit increases. During rarefaction, it loosens the carbon granules. Hence resistance offered by carbon granules increased and hence current through the circuit decreased.
The net resistance of the carbon granules is given by:
R-Ro+r
R= Net resistance in Ω
Characteristics :
• Sensitivity: 100 mV.
Frequency Response: 200-5000 Hz.
• Signal to Noise Ratio: Poor.
• Directivity: Omni-directional.
• Output Impedance: 100Ω (medium).
Advantages:
1. High electrical output or high sensitivity.
2. Rugged construction.
3. Small size.
4. Low cost.
Disadvantages
1. High self noise.
2. Limited Frequency Response.
CONSIDER MICROPHONE
Principle: A condenser microphone (also called a capacitor microphone) works on the principle of change in capacitance between a fixed plate and tightly stretched metal diaphragm with sound pressure variations. Hence device comes under pressure pressure-operated microphone.
Construction: constructional details of the condenser microphone. It consists of a lightweight metal diaphragm generally made of steel or aluminum. This diaphragm is separated by a fixed metal back plate to form a capacitance of a few pico farads. A fixed d.c. voltage of about 200 V is applied between the back plate and movable plate (diaphragm) through a resistance R.
Working: When no sound wave strikes the diaphragm, the capacitor plate charges to the applied voltage V through resistive R. When a sound wave strikes the diaphragm it moves to and fro. When the diaphragm moves towards the fixed plate, the resultant capacitance increases (because in the formula C (epsilon_{o}*epsilon_{r}*A)/d d' decreases and C increases). Similarly, when the diaphragm moves away from the fixed plate, the capacitance is decreased. This change in capacitance produces a change in voltage, according to the equation V= Q/C. The varying voltage across the microphone due to a change in sound pressure is equal to the voltage across the resistor R. The net voltage V_{t} at any instant of time 't' is given by
Vt =V dc +V a
Where
Vt= Instantaneous voltage
Vdc= D.C bias applied to the condenser plate
Va= Audio signal corresponding to the sound pressure variations
The output of the microphone is connected to the amplifier through a capacitance C₁ which blocks d.c. and allows a.c. only.
Characteristics :
• Sensitivity: Low (about 3 mV).
• Frequency Response: 30 Hz to 9 kHz.
• Directivity: Omni Directional.
• Output Impedance: High, 100 ΜΩ
Advantages:
1. Absence of internal noise,
2. No matching transformer is required. 3. Free from distortion.
4. Good high-frequency response.
Disadvantages:
1. High external d.c. voltage is required.
2. Bulky in size and costly.
3. Lack of portability and ruggedness
CRYSTAL MICROPHONE
Principle: Crystal microphone works on the Principle of Piezoelectric effect. Generally, Rochella salt is used in crystal microphones since it gives greater output voltage than the other piezoelectric materials. Crystal microphone is a pressure-operated type microphone.
Construction: Constructional details of the basic microphone. A diaphragm used is very light and stiff, it is made of duralumin. It is connected to the crystal structure in such a way that it exerts pressure on the crystal. This crystal of opposite polarity known as bimorph is used. Bimorph is formed by joining two crystals which are given with a conductive coating on the upper and lower surfaces. When the diaphragm exerts pressure one of the crystals will be in tension and the other in compression. The whole unit is encased in a protective case.
A protective cover is provided in front of the diaphragm.
Working: When sound waves strike the diaphragm it will move to and fro. Thus varying pressure is applied to the bimorph crystal unit through a connecting pin. A varying voltage is produced across the plates of crystals. This voltage is proportional to sound pressure variations. More crystals are connected in such a way that the potential differences are added and a high output is obtained.
Characteristic:
• Sensitivity: 50 mV.
• Signal to Noise Ratio: High, 40 dB.
• Directivity: Omni-directional.
• Output Impedance: High about 1 ΜΩ.
Advantages:
1. These are extremely robust.
2. Small in size and inexpensive.
3. Very low distortion.
4. It requires no external battery or matching transformer.
5. Easy maintenance.
Disadvantages: These are affected by temperature and moisture.
Applications:
1. Used in Broadcasting stations.
2. Used in recording equipment
3. Used in high-quality public addressing system
4. Used in hearing aids
5. Used in sound level meters.
