WOOFERS AND TWEETERS
Sound recording and reproduction is required to produce good sound output. A single loudspeaker cannot give a good response to entire audio signals. Systems that satisfy this requirement to the maximum possible extent are described as Hi-Fi (High Fidelity) systems. To have a uniform response for all audio frequency ranges one speaker may not serve the purpose Hence a set of speakers form a Hi-Fi system. A typical Hi-Fi system contains a set of loudspeakers called Woofer, Tweeter, and squawker
Woofer: A woofer is a relatively large loudspeaker that is used to reproduce sounds of relatively low frequency in the range of up to 1 kHz in a high-fidelity (HiFi) sound reproduction system.
Tweeter: A Tweeter is a relatively small loudspeaker that is used to reproduce relatively high frequencies in the range of 2,000 to 20,000 Hz in audio systems.
NEED OF HORN TYPE LOUDSPEAKER
The cone-type loudspeakers are suffered from the impedance mismatch between the diaphragm and air trapped near it so the efficiency of a cone-type loudspeaker is up to 5%. To increase the efficiency, loudspeakers must vibrate large amounts of air. This needs a relatively larger vibrating surface such as cones in loudspeakers. The larger the cone surface, the greater the output. As the cone size increases the high frequency response of the loudspeaker will become reduced. Hence to increase the efficiency of the speaker the impedance must be matched between the diaphragm and the air trapped near it. This is possible with a horn. It couples the small voice coil area to a large area of air.
WORKING PRINCIPLE OF HORN-TYPE LOUDSPEAKER
The horn is a tapered enclosure whose diameter increases from a small value at the diaphragm side called the throat to a large value at the radiating side called the mouth. An air chamber is trapped between the throat and the diaphragm. The horn acts as an acoustic transformer and transfers the sound waves from the diaphragm to the air. In horn loud speakers power is not radiate directly from the diaphragm but indirectly through the horn. Hence horn type loudspeakers are also called indirect radiating speakers. It also helps in improving the efficiency of the speaker from 5% to 40%.
Construction: The horn loudspeaker consists of a driving unit and a horn. The driving unit is nothing but a direct radiator i.e., PMMC loudspeaker. It consists of a permanent magnet, a thin aluminum diaphragm, a voice coil, and an air chamber trapped between the throat and diaphragm. The horn is a tube tapered in such a way that its diameter increases from a small value at one end (throat) to a large value at the other end (mouth). Because of its varying cross-sectional nature, power delivered from the diaphragm is increased greatly. Hence it acts as an acoustic transformer.
Working: When the audio signal current flows through the voice coil the driver unit converts them into sound waves. The diaphragm vibrates and vibrations are coupled to the throat of the horn. The horn couples these vibrations to a large area of air through its transformer action. The result is that higher sound levels are produced for a given amount of energy fed into the voice coil. From its work, it is known that it has an unidirectional radiation pattern.
HEADPHONES
Headphones or earphones are small loudspeakers designed suitably to be worn on the head. The headphones offer two important advantages for the listeners. They are:
1. Privacy.
2. Free from the effect of room acoustics.
The use of headphones avoids disturbance to nearby people. The headphones give low power output and they are made to fit to the ear.
There are two types of headphones. They are :
1. Electromagnetic type.
2. Crystal type.
MAGNETIC HEADPHONES
Principle: It works on electromagnetic principle i.e., converting electrical energy into mechanical energy.
Construction: Construction depends on typical magnetic headphones.
A curved permanent magnet is connected with two permanent magnets. These two permanent magnets are cut as two poles.
(N and S) of the curved magnet. Thus a U-shaped permanent magnet was formed with these two poles. A coil of an insulated copper wire is wounded on each pole. The two coils C1 and C2 thus obtained are connected in series so that the same signal current passes through the two coils. A soft iron diaphragm is held in place close to the pole ends. The edge of the diaphragm rests on the case and is held fixed by the cap.
Working: When no signal is applied to the coils, the permanent magnet is in position 1 as shown in Fig. 4.13. When the audio frequency current passes through the coils, the face of attraction on the diaphragm will be increased for one-half cycle and decreased during the other half cycle. Thus the diaphragm moves between position 2 and 3 in accordance with the applied a.c. signal. The moving diaphragm in turn moves the air adjacent to it, thus producing sound waves.
Applications:
1. Telephone exchange
2. Aeroplane equipment
3. Medical electronic equipment
4. Communication receiver
5. Sound recording equipment
6. Tape recorders
CRYSTAL HEADPHONES
Principle: Crystal headphones work on the principle of the piezoelectric effect. The piezoelectric effect is the effect that occurs when certain crystalline materials are applied with electrical energy, vibrations will be produced at the opposite faces of the crystalline materials.
Constructions: A small crystal slice is arranged in a small plastic container with one side opening. Two connecting leads were taken outside from the two plates of crystalline material to make an electrical connection of the signal voltage. The opening end of the crystal headphone is attached to the ear so the sound can be heard.
Working: When the AF signal is applied, it sets the opposite faces of the crystal to vibrate according to the amplitude and frequency. Thus mechanical vibrations are produced in the air surrounding the crystal element resulting in sound reproduction.
Advantages:
1. Light in weight.
2. Operates with low voltage.
Disadvantages: These headphones will have limited sound output levels.
Applications: These headphones are operated in conjunction with tape recorders, stereo amplifiers, etc.
