Microphones

How Microphones Work Sound is created when a vibrating object (such as a guitar string, drum skin etc..) causes the air around it to vibrate within the frequency range of human hearing (20Hz – 20KHz). When this vibration reaches our ears, our eardrums vibrate accordingly and our brains perceive this as sound. A microphone converts the sound energy into electrical energy using a diaphragm. The overall result is an electrical signal that rises and falls in voltage analogous to the rise and fall in air pressure caused by the original sound.

Types There are various types of microphones, all of which have different methods of construction and are appropriate for different settings. Some common microphone types are listed below: Condenser / Capacitor – Uses two thin sheets of metal which are held a small distance apart. A standing electrical charge builds up between them. When the diaphragm vibrates the electrical charge is disturbed and electricity flows between the two. Capacitor microphones do require a power source which usually comes from the mixing desk or recording device. Dynamic – Has a diaphragm attached to a metal coil, which is placed inside a magnet. Sound energy makes the diaphragm vibrate which causes the coil to move rapidly within the magnetic field. This is known as electromagnetism and creates an electrical signal which can then be amplified. Ribbon – These are similar to dynamic microphones in that they use electromagnetism to induce an electric current, but there construction is slightly different. They have a corrugated metal ribbon suspended in a magnetic field. The ribbon is electrically connected to the microphone's output, and its vibration within the magnetic field generates the electrical signal. They are noted for there excellent frequency response and their natural sound. How Microphones Work - Video

Polar patterns The polar pattern of the microphone dictates from where sound is picked up by the diaphragm. There are several different polar patterns: Cardioid – Cardioid mic’s are most sensitive to sound directly in front of the diaphragm. They are also called ‘directional mic’s’ Omni directional – These microphones pick up sound equally from all directions. Figure of 8 – This poplar pattern picks up sound mainly from the front and the back. This polar pattern is ideal for broadcasting, such as an across table interview.

Polar patterns Cardioid Polar pattern – sensitive to sound directly in front of the mic Omni directional – Picks up sound from all around Figure of 8 – Picks up sound from the front and the back Polar patterns explained - Video

Uses Dynamic Less sensitive are therefore well suited to high level sound. Ideal for Kick, snare and tom drums. Amplified instruments such as guitar and bass. Loud singers Close mic’d trumpets, sax or any instrument with a high sound pressure level Capacitor More sensitive and therefore ideal for acoustic or lower level sound pressure levels. Ideal for stereo or ambient recording. Drum overheads Acoustic instruments (that aren’t too loud!) Better capture of the frequency range, and therefore can give a better quality sound. Some have a sensitivity switch allowing you to reduce the sensitivity of the microphone by 10dB

The Proximity effect When recording an instrument or vocal you need to be aware of the proximity of the microphone to the sound source. If the microphone is too close to the sound source, bass frequencies will be emphasised and thus create a booming, bass heavy sound. This is usually undesirable unless creating a special effect.

Plosives and Sibilance When recording a singer, P’s and S’s are often emphasised creating plosives and sibilance respectively. This can be minimised by using a pop shield. A pop shield will reduce the amount of fast moving air reaching the diaphragm and hence reduce the effect of ‘popping’ and sibilance.

Close mic’ing When you place a microphone close to the sound source, this is known as close mic’ing. It has the advantage of cutting out the sound of other instruments and gives a very focused, clear sound with little room reverb. If sound from a nearby instrument is picked up in a close mic’ing recording it is known as ‘spill’. Spill can be reduced by using an acoustic barrier such as a self standing board covered in a soft absorbent material. To avoid spill, unwanted sounds should be at a distance of five times the proximity between the microphone and the instrument being recorded. Example of close mic'ing an acoustic guitar

Ambient mic’ing Description If you are recording an ensemble in a room that has a good acoustic (such as a concert hall) then you may choose to use ambient mic’ing techniques instead of close mic’ing. When recording using ambient mic’ing, you would usually use a stereo pair of mic’s, and place them in such a way that you capture the natural acoustic of the room in which the ensemble are performing. This is particularly useful when recording an acoustic ensemble such as a choir, or chamber group. Methods A/B or spaced pair – two mics (usually omni- directional) face the performers with a set distance between them. Often engineers will use the 3:1 rule to space the mic’s. This means the mic’s will be three times the distance apart, that the mic’s are from the performers. So if the mic’s are 1 meter away from the performers, the mics will have a distance of 3 meters between them. X/Y or coincident pair – Two identical directional microphones are placed with their heads as close together as possible (without touching) and facing inwards at 90 degrees. The output signals are panned equally left and right.

Exam Question 250 words / 16 marks Explain how a microphone works and the different types of polar patterns available. Comment on which type of microphone is best suited to different recording situations.