Microphones

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8. Microphones, speakers and headphones

Microphones, speakers and headphones are components commonly used as the input and output devices of many circuits. A microphone converts sound waves into electrical signals that closely follow the waveform of the sound being received. This signal is then amplified by the circuit and transformed into a sound by a speaker or headphone. The symbols for these components are shown on 8.1.

8.1 Microphones

There are several different types of microphone: carbon, dynamic, crystal, capacitive (electret). Carbon microphones were one of the first to be invented and were used mainly in telephone applications. But they are very noisy as the carbon granules rattle when the microphone is moved and this type is being replaced by more advanced types.
Dynamic microphones are in wide use and their quality of reproduction is superb. They are used in the recording industry for music and speech where high fidelity is required. Basically they are exactly the same as a speaker, the only difference being the size. But their only limitation is the very low output. The internal structure is shown in figure 8.2. A paper cylinder, onto which fine copper wire is wound, is connected to a membrane which moves under the force of sound pressure created by the sound source. This coil is in a narrow gap with a high magnetic field created by a permanent magnet. When the coil moves in this magnetic field, it produces a voltage identical to the sound causing the movement.

Because of the low resistance (impedance) of a dynamic microphone, it usually needs a transformer so it can be connected to an amplifier (called a pre-amp). This transformer is usually built into the microphone's case, but if is absent, it is necessary to connect the microphone to a preamplifier with low input resistance.

Crystal microphones contain a crystal called a "piezo crystal" that is connected to a small diaphragm. When sound waves hit the diaphragm, the crystal changes shape and it produces a voltage. This voltage is passed to an amplifier.
Recently, electret microphones have improved in quality and taken over from nearly all other types of microphone. They are small, rugged, low in price and produce a very high quality output.
The shape, size and characteristics are shown in 8.3.
The microphone contains a Field Effect Transistor, which means it needs a DC voltage for it to operate. Figure 8.3d shows how an electret mic is connected to a circuit. It needs a "load resistor" to limit the current to the FET and the output is taken across this resistor. That's the technical way of saying the output is taken from the point where the resistor meets the FET.

8.2 Speakers

Speakers vary enormously in size and shape. They can be designed as crystal or capacitive, but most often they are dynamic (called electro-dynamic construction).

The cross-section of an electro-dynamic speaker is shown in 8.4. Ferrite rings (2, 3 and 4) are added to a large permanent magnet (1) which creates a strong magnetic field in the narrow gap between magnets North and South poles. A Cylindrical former is added to the gap and it holds coil (5). The ends of the coil are taken to the outside of the speaker.
The two most important characteristics of a speaker are its resistance (we actually call the resistance of a speaker IMPEDANCE as the value is determined at a frequency of 1kHz and the value is higher than its actual resistance) and its wattage. Common impedances are 4, 8 and 16 ohm, but there are also 1.5, 40 and 80 ohm speakers. Speaker wattages range from a fraction of a watt to hundreds of watts.

When choosing a speaker, it is advisable to choose the largest speaker possible as they are more efficient and produce the least distortion especially in the low frequency range. Speakers should be housed in a large box since it functions as a resonating chamber and this greatly adds to the overall quality of the sound.

8.3 Headphones
There are several types of headphone: crystal and electromagnetic. The electromagnetic type is the most commonly used. They functioning in the same way as speakers, with obvious differences in construction, since they are intended for much lower power. Their main characteristic is their resistance (impedance), from a few ohms to a few thousand ohms.
The cross section of an electromagnetic headphone is shown in 8.5. It consists of a horseshoe magnet with poles that hold two coils. These are connected in series. The diaphragm is a thin steel plate. When current flows through the coils, the diaphragm is pulled towards the coils. This moves the air and the result is heard as a faithful reproduction.

8.4 Examples

The schematic of a very simple FM radio-transmitter is shown in figure 8.6. It uses an electret microphone and transmits on a frequency between 88MHz and 108MHz.
The transistor, coil L, trimmer capacitor Ct, capacitor C3 and resistors R2, R3 and R4 creates an oscillator with a frequency determined by:

In this equation CCB represents the capacitance between the collector and the base. The value of this capacitance depends on the voltage on the base. The higher the voltage, the lower the capacitance and vice versa. The voltage on the base is constant while there is no sound, which means the frequency of the oscillator is constant. When the microphone picks up a sound, it is passed to the base of the transistor via C1. This causes the frequency of the oscillator to change and that's why the circuit is called FREQUENCY MODULATED (FM).
To transmit on a frequency away from any other radio station, a trim cap is included. The transmitter has a range up to 200 metres, depending on the length of the antenna and where it is placed. Ideally, the antenna should be vertical and as high as possible.
The antenna can be as long as 3 metres but 180cm will work very well.
Coil L is made by winding 6 turns of 1mm enameled wire on a 6mm dia drill bit. This coil can be stretched of squashed to adjust the operating frequency of the circuit and the trimmer will fine tune the frequency.

High Fidelity (or Hi-Fi) sound reproduction is the main purpose for using good-quality speakers. They are used in radios, TV's, cassette players, CD players, etc. The speakers are housed in speaker boxes and use at least two speakers. This is because no individual speaker is capable of reproducing the full range of frequencies. A speaker with a large cone is called a "Woofer" and will reproduce the low frequencies. A speaker with a small cone is called a "tweeter" and will reproduce the high frequencies. Together, they will reproduce the full range of between 30Hz and 15kHz.
The difficulty is now to detect the low or high frequency and divert the correct frequency to the particular speaker. This is the job of a cross-over network. In the figure 8.7 an inductor L1 passes the low frequencies to speaker Z1 and capacitor C1 passes the high frequencies to speaker Z2. Z1 reproduces frequencies from 30Hz to 800Hz and Z2 reproduces sounds with frequencies from 800Hz to 15kHz.

Headphones are most commonly used with portable devices, such as radio receivers, cassette players, CD walkmans, mp3 players, etc. Headphones produce a very high quality reproduction. All modern devices have an audio-amplifier. It usually employs an integrated circuit and most of these are designed for 32 ohm headphones. There are also 8 ohm and 16 ohm headphones.
The schematic of a AM portable radio is shown in figure 8.8. It's built around the ZN416 integrated circuit. The output is connected to two serially connected 32 ohm headphones, with overall resistance of 64 ohms.
It is possible to connect the radio receiver in figure 8.8 to amplifier in figure 7.3 to produce a radio with speaker output.

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