Principle: Ribbon microphone works on the principle of electromagnetic induction. It is a velocity or pressure gradient microphone.
The diaphragm is exposed to sound waves on both sides of it.
Construction: The constructional details of the ribbon microphone are shown in Fig. 4.6 along with its polar characteristics. It has a light corrugated metallic ribbon suspended between the magnetic pole pieces N and S.
The reason for using corrugated ribbon is to increase the effective length of the ribbon, thereby increasing the impedance and also making it more strong. The structure is mounted in a circular bottle.
Working: When the sound waves strike the ribbon on both the front and back sides of the ribbon, it vibrates. The vibrating ribbon creates a magnetic flux. Hence emf is induced in the microphone. The variation of sound pressure in any direction may vibrate the ribbon. But the induced emf is directly proportional to the component of sound velocity in the direction perpendicular to the ribbon. The induced emf is very small and can be stopped by a transformer usually enclosed in the microphone casing itself.
Characteristics :
• Sensitivity: Low about 30 µV
• Frequency Response: 20 Hz to 16 kHz
• Signal to Noise Ratio: Low (10 dB)
• Directivity: Bidirectional
• Output Impedance: Low 25 Ω
Advantages:
1. Used by singers for introducing bass quality into voice.
2. No Bias supply is required.
3. Excellent frequency response (flat over audio range).
4. Avoids noise from sides.
5. No popping problem.
Disadvantages:
1. The speaker or performer must remain in a relatively fixed position.
2. It is highly noisy when used outdoors.
Applications:
1. Used in studio work.
2. Used in public address system.
3. Used in orchestral musical programs.
DYNAMIC MICROPHONES(MOVING COIL)
Principle: It is also known as a dynamic microphone. When sound pressure variations move a coil placed in a magnetic field, there is a change of magnetic flux passes through the coil. An emf is, therefore, induced on the coil, and this emf forms the output of the microphone.
Construction: The constructional details of the moving coil microphone. It consists of a strong E-shaped permanent magnet. Its center pole acts as the South pole and either side acts as the North pole. The magnet produces a constant magnetic field. A thin aluminum diaphragm is used which is very light in weight and fixed to the body of the magnet with the help of springs. A coil made of aluminum ribbon is kept between the pole pieces. The whole unit is encased and a hole is provided to make it free from atmospheric effects.
Working: When sound waves strike the diaphragm, it vibrates thus making the coil move back and forth in the magnetic field. The moving coil cuts the magnetic lines of flux and hence voltage is induced into the coil. The induced emf is proportional to the frequency and pressure of the sound waves striking the diaphragm.
Characteristics :
• Sensitivity: About 30 µV
• Frequency Response: 60 Hz to 8 kHz
• Signal to Noise Ratio: 30 dB
• Directivity: Omni-directional
• Output Impedance: Low, 25 Ω
• Distortion: 25%
Advantages:
1. The output can be sent to a long distance without much attenuation when a step-up transformer is connected to the output.
2. No DC voltage is required.
3. It can withstand extreme temperature variations and humidity.
Disadvantages:
1. Impedance matching transformer is required.
2. Poor sensitivity.
Applications:
1. Used to pick up sound from large orchestra.
2. Used in broadcasting.
3. Used as a probe microphone for measuring fairly intense sound fields.
4. Used in applications where long cables are required.
LOUDSPEAKERS
A loudspeaker is a transducer that converts electrical signals of audio frequency into sound waves of the same frequency. In principle loud speaker is the converse of the microphone and the two instruments perform complementary functions in the sound reproducing system. A good loudspeaker should be able to reproduce all sounds equally well irrespective of their amplitude, frequency, and waveforms. The circuit symbol of the loudspeaker.
PMMC LOUDSPEAKER
These loudspeakers are most widely used because of their simplicity compactness and uniform response. It is a direct radiating loudspeaker. These loudspeakers can also be called Dynamic Loud Speakers.
Principle: A voice coil made with copper carrying the audio signal moves in and out of the strong magnetic field produced by the permanent magnet. The vibrating coil makes the diaphragm vibrate and produce variations in air resulting in sound waves.
It consists of a permanent magnet made of high-grade magnetic material like alnico. It consists of a central pole that acts as a south pole and two adjacent poles that act as a north pole formed by the yoke. It provides a strong magnetic field in the narrow air gap. A voice coil consists of a few forms of aluminum or copper wire wound on a Bakelite or Bica suspended in the air gap between magnetic poles. At the same time, it is attached to the diaphragm. The diaphragm is made of paper and provided in the form of a cone of the required size. Corrugations provided near the outer rim permit the cone to move easily. The inner edge of the cone is attached to the coil and the outer edge is fixed to the speaker frame for external connection.
Spider is a flexible corrugated disk, connected to the coil at one end and connected to the frame on the other end. Spider keeps the coil in the center of the air gap between pole pieces without allowing it to touch the pole pieces and it allows the diaphragm to move back and forth but not sideways. Taking lead wires from the voice coil to the outside of the cone surface completes the construction. A dust cap is provided to protect the voice coil from dust.
Working: A permanent magnet produces a strong magnetic field. When an audio signal current passes through the voice coil. magnetic field will be generated around the voice coil whose polarity is continuously varying. The voice coil vibrates due to the interaction of the permanent magnetic field and the magnetic field produced by the voice coil. The diaphragm attached to the voice coil also vibrates and causes compression and rarefaction cycles in the air. The sound energy produced depends on the shape and area of the cone.
Specifications:
• Efficiency: 5 to 10%
• Signal Noise Ratio: 30 DB
• Distortion: 5 to 10%
• Directivity: Omni-Directional
•Impedance: 20 to 32 Ω
• Power Handling Capacity: Few mw to 25 w
Applications:
1. Widely used in radio and T.V. receivers.
2. Used in record players, cassette players, CD players
3. Used in low-power audio amplifiers.
