|Radio and Electronics (DED Philippinen, 66 p.)|
As we said already: at the beginning of each modern communicationsystem the sound has to be converted into an electric signal and at the end again the electric signal has to be translated back into sound. This task is carried out by devices which are called in general TRANSDUCERS.
Whereby the transducers which translate sound into an electric signal are called MICROPHONES and those which translate an electric signal back to sound are called EARPHONES or LOUDSPEAKERS.
Until today there was not found a way of translating sound directly into an electric signal it must be always gone the roundabout via mechanical forces.
Hold a piece of paper sensitively in your hand, position it directly in front of your mouth and start talking loudly against the paper. You will experience, that the paper is moved (vibrated) to an for by the sound. Result: sound can move by light pieces of material which show a big area vertically to the direction of the movement of soundwaves. Sound waves are exerting forces on to these light and flat pieces which we will call from now on a DIAPHRAGM. On the other hand the leather on top of a drum is another type of diaphragm. This diaphragm is able to produce sound if it is moved to and for (if it is oscillating). These considerations make it very clear, why we will find at the beginning and at the end of our communication system always a diaphragm, as shown in fig. 8.
CARBON MICROPHON is a very old type but still in use when a cheap microphon is desired and fidelity is not so important. CARBON GRANULATES change their resistance, if they are pressed together by an external force. The inner hollow part of the microphon is filled with this type of carbon particles. At the front of the microphon is fixed a very thin sheet of metal which is here the diaphragm, and at the backside is fixed a second metalplate which stands here as an electrode to give contact to the carbon granulates.
When exposed to sound the diaphragm is moved by the air oscillations, and the pressure on the carbon granulates changes according to the frequency of the air-oscillations. Therefore the overall resistance of the carbon granulates changes according to the frequency of the sound.
Is working like a variable capacitor. The diaphragm is made from metal and stands for one plate of the capacitor. It is positioned very near to a second metalsheet with a lot of holes in it a few tens of millimeter inside of the microphone. This second metalplate stands for the second plate of the capacitor. If the diaphragm is hit by soundwaves it moves to and for, and by doing so, the distance between the tow plates changes. As well know from physics, the change of the distances lets also change the capacity of the capacitor. So the whole microphone stands for a capacitor which changes its capacity according to the sound waves hitting the diaphragm.
Here a coil is fixed to a diaphragm made from insulating material (like cardboard):
This coil is positioned free within the gaps of a strong permanent magnet.
If the diaphragm is moved by soundwaves, the coil is moving to and for as well.
This movement causes induction of a voltage in the coil and so this microphon is producing a voltage depending on the frequency of the sound waves.
Here is used the so-called PIEZO EFFECT. If a crystal is exerted to pressure there will appear a voltage across its edges. The force to press is produced again by diaphragms, now positioned in front and behind the crystal. If the diaphragms are moved to and for by air-pressures the microphone generates a low voltage which has the same frequency as the sound wave have it.
Actually this type is working just the opposite way of the electrodynamic microphone. A coil fixed to a diaphragm is suspended within the field of a permanent magnet. If a changing current is passed through the coil, there will arise a force which will tend to move the coil to and for. As the diaphragm is connected to the coil it will be moved to and for according to the frequency of the current, and by doing so it will produce sound of this frequency. Most of the earphones and loudspeakers used nowadays work on this principle.
ELECTROSTATIC PRINCIPLE electric charges exert forces on each other.
If one plate of a capacitor is fixed on the housing of the loudspeaker and the other plate is fixed to the diaphragm, the diaphragm can be moved, by the forces of the electric charges brought on the plates by an ac-current. This kind of loudspeaker is found very rarely.
PIEZO ELECTRIC PRINCIPLE
Special crystals are not only able to produce voltage if they are under pressure, but they are able to produce forces if there is a voltage connected to their edges.
This principle is not very common up to now in connection with radios, but it is more and more used to produce special sounds for example in computers.
With the devices explained during the last chapter, you are able now to understand how a normal telephon circuit connected by wires is functioning. Just imagine in the circuit shown above is used a mocrophon of the CARBON TYPE and an earphone working on the electrodynamic principle.
You can imagine for sure, that a railway engine for example cannot oscillate (to move to and for) ten times a second, while a leaf of a tree can do that easily. The difference between both is obviously their mass. This consideration shows: mechanical devices are very limited in the range of oscillations they can follow. This problem forces the designer of a communicationsystem first to find out what range of oscillations will be required within that system. From now on we will call the oscillations FREQUENCIES and the range required will be called the FREQUENCY SPECTRUM. During the last chapter we have been talking about the translation of sound into electric signals. When we are deciding which material should be used for the diaphragm. It is obviously very important to know the highest and the lowest frequency of sound. This frequency range is called SPECTRUM OF AUDIO FREQUENCIES. If we connect a loudspeaker to a Low-frequency-generator and if we listen to the sound produced by the speaker we will find, that we start to hear sound at a minimum frequency of about 50 Hz and most of us will not hear any sound anymore, if the frequency reaches values above 18 kHz. Therefore the audio frequency spectrum is defined as the range between 50 and 20 kHz.
As we already could see during the experiment described above, we can produce a much wider range of frequencies than the range we can listen to. Our ears are able to receive soundwaves within special limits, the range of audible (hearable) waves is called also a BANDWIDTH. We can say our ears have a bandwidth of 50 to 20000 Hz. We will come across these terms several times while dealing with radiotechnology.
The graph shown in fig 17. explains again what a bandwidth is, and it shows too how different the bandwidths are for different sophisticated communicationsystems. Keep in mind: Even though the bandwidth of a telephon system is very narrow in comparison with bandwidth of the audio frequencies we are able to understand the partner at the other end of the communication line.
1. What is the meaning of the term COMMUNICATION actually?
2. What is the difference between a telephon and a radio system?
3. How are the devices called which are translating sound waves into electric signals?
4. How are the four different types of microphones functioning?
5. Which different types of loudspeakers do You know?
6. What is the meaning of the terms fidelity and distortion?
7. What is the meaning of the terms Spectrum and bandwidth?
8. Applying your knowledge of Ohms Law try to describe how the circuit shown in fig. 16 manages to produce the