How Do Microphones Work?
Microphones are a kind of transducer. A transducer is a device that converts energy from one form to another. Microphones convert acoustical energy into electrical energy - sound waves to audio signal.
Different microphone types convert energy in different ways. But all have the diaphragm in common. A diaphragm is a thin piece of material that vibrates when it is hit by sound waves. It is often located in the head of the microphone.
When the diaphragm vibrates, other components in the microphone vibrate as well. The vibrations are converted into an electrical current which becomes an audio signal.
This is converted back into acoustical energy (sound waves) through amplifiers.
Types of Microphones
The differences of the number of different microphones that are in common use can be divided into two areas: the type of conversion technology they use and the type of application they are designed for.
The type of conversion technology they use is the technical method the microphone uses to convert sound into an electrical signal. The most common technologies are dynamic, condenser, ribbon and crystal. Each have advantages and disadvantages and are generally more suited for certain types of application.
The type of application they are designed for is the use of the microphone. Some are designed for general use and can be effectively used in different situations; others are specially designed and are only useful for their intended purpose. Characteristics that influence this include directional properties, frequency response and impedance.
Mic Level and Line Level
The electrical current generated ny microphones is small and is often referred to as mic level. It is typically measured in millivolts. Before the signal can be used it needs to be amplified, usually to line level which is typically 0.5 - 2 volts.Line level is the standard signal strength used by audio processing equipment and common domestic equipment such as CD players and tape machines.
The amplification can be achieved in four ways. Some microphones have built in amplifiers that boost the signal to a high mic level or line level. The microphone can be fed through a small boosting amplifier (a line amp). Sound mixers have small amplifiers in each channel; attenuators can accommodate microphones of varying levels and adjust the to an even line level. The audio signal is fed through a power amplifier, which are specially designed to boost signals enough to be fed through loudspeakers.
Dynamic microphones are versatile and ideal for general purpose use. They have a simple design and are relatively sturdy and resilient to rough handling. They are better suited to handling high volume levels. They have no internal amplifier and do not require batteries or external power.
The dynamic microphone uses a wire coil and magnet to create the audio signal. The diaphragm is attached to the coil. When it vibrates the coil moves backwards and forwards past the magnet. This creates a current in the coil which is channeled from the microphone along wires.
Condenser microphones require power from a battery or external power source. The resulting audio signal is stronger signal than from a dynamic. They use a capacitor to convert acoustical energy into eletrical energy. They also tend to be more sensitive and responsive than dynamics.
This makes them well suited for capturing subtle nuances in a sound. However they are not ideal for high volume work as the high sensitivity causes the signal to distort.
The directionality of a microphone is the sensitivity to sound from various directions around the microphone. Some can pick up sound from all directions and others can pick up sound from a particular combination of directions.
The types of directionality can be divided into three main categories: omnidirectional, unidirectional and bidirectional.
The directionality is often shown in graphs called polar patterns. These show the direction of sensitivity to sound is based around a microphone.
Omnidirectional microphones pick up sound from all directions.
This directionality is used: to capture ambient noise, in situations where sound is coming from all directions, in situations where the microphone position needs to be fixed while the sound source is moving. Omnisound is general and often unfocused which means if a sound is the focus of the recording it may be overpowered by other noise.
Cardioid microphones pick up sound from mainly the front but slightly at the sides as well.
This polar pattern can be used for: emphasising sound from the direction the microphone is pointing while laving some latitude for mic movement and ambient noise. Cardioid microphones are very versitile and are ideal for general use. Handheld microphones are often cardioid.
Hypercardioid is an exaggerated version of the cardioid pattern. It is very directional and eliminates most sound from the sides and back.
This polar pattern can be used for: isolating sound from a subject or direction when there is a lot of ambient noise, picking up sound from a subject at a distance. Unidirectional sound can sometimes be a little unnatural due to the removal of ambient noise. The microphone also needs to remain pointed at the subject otherwise all audio would be lost.
Bidirectional microphones use a figure 8 to pick up sound equally from two directions.
There are alot of different uses for this polar pattern. An example would be an interview with two people facing each other.
This is the way a microphone responds to different frequencies. All microphones have frequencies that are exaggerated and others that are attenuated.
This can be shown in frequency response charts. The x axis of the chart shows frequency in Hz and the y axis shows response in dB. A higher value means that frequency will be exaggerated and a low value means the frequency will be reduced.
Frequency Response Contin.
The ideal response curve (shown on frequency response charts) is a flat frequency response. This would mean that the microphone is equally sensitive to all frequencies and none would be exaggerated or reduced. This would mean that the recording is a more accurate representation of the original sound. It produces the purest audio. However, perfectly flat response is not possible.
In some situations tailored frequency response is more useful than flat response. Some microphones are developed to pick up the right frequencies for certain sounds such as the human voice.
Response patterns that emphasise the wrong frequency should be avoided. For example using a vocal microphone on a kick drum would not emphasise the low frequencies needed.
Frequency response is often quoted between two figures. This is an easy way to see the frequencies a microphone is capable of capturing effectively. This range makes no reference to the response curve of the microphone.
Condensers tend to have flatter frequency responces than dynamic microphones. This would usually mean that the condenser is more desireable if accurate sound is the prime consideration.
Phantom power is a way of distributing a direct current (DC) through studio cables to provide power for equipment, such as microphones. The voltage that is supplied is generally between 12 and 48 volts; 40 volts is the most common. Individual microphones draw as much of this current as needed.
Balanced audio signals that are connected to a 3 pin XLR have audio signal travelling on the two wires - they are usually connected to pin 2, which is the positive charge, and pin 3, which is the negative charge. Pin 1 is connected to the sheild; the sheild is earthed. Audio signals are alternating currents (AC) whereas phantom power is a direct current (DC). The phantom power is transmitted simultaneously on both pins 2 and 3 with the shield being the ground. Because the DC signal on the 'hot' and 'cold' pins is identical it is registered by equipment as 'common mode' noise and rejected by the equipment.
Phantom power is a way of using a single cable to transmit both currents.