Microphones - different types and their uses in performances.

University Degree Creative Arts and Design

Microphone Essay

A microphone is a transducer used to capture acoustic energy and convert it into electrical energy. The electrical energy, also referred to as audio signal, is then sent to a recording device, usually an external interface or a D.A.W. (Digital Audio Workstation). It is then converted back to audio so that it can be played back and edited if necessary. All microphones contain a diaphragm, which is a ‘thin piece of material (such as paper, plastic or aluminium) that vibrates when struck by sound waves’ (Modern Recording Techniques).

The Dynamic microphone works in the following way:

‘When the diaphragm vibrates in response to incoming sound waves, the coil moves backwards and forwards past the magnet. This creates a current in the coil which is channeled from the microphone along wires.’

(Dynamic Microphones)

The diagram below shows this:

(http://www.mediacollege.com/audio/images/mic-dynamic.gif)

Dynamic microphones are mainly used for live on-stage performances as they are robust, can cope with high gain before experiencing feedback and are resistant to moisture. They are also relatively inexpensive, as a very basic one with a 3.5mm jack connection can be purchased for under £10, which are commonly used with basic computer applications such as Skype etc. Another benefit of the Dynamic microphone is that it does not require a power source, such as a battery or phantom power. Therefore this means that a Dynamic microphone is very reliable and is unlikely to fail. The two most used Dynamic microphones are the Shure SM-57 and SM-58. The SM-57 is mainly used for capturing guitar amplifiers and snare drums as it has a large frequency range and can withstand the high gain. The SM-57 is suited for guitar amplifiers as it can pick up the lower frequencies of an electric guitar, but is also very good at picking up the higher frequencies used on the higher frets and even harmonics, as shown in the grid below –

The SM-57 can be placed against a guitar amplifier in a number of ways to pick up the desired audio signal for the user. The most common microphone placements are positioning the SM-57 straight against the centre of the speaker; placing the SM-57 at an offset to the centre of the speaker; positioning the microphone at a 45° angle to the centre of the speaker and positioning the SM-57 at a 45° angle to the offset of the speaker. Another common technique used for recording a guitar amplifier is by using the SM-57 in conjunction with a Condenser microphone that is used for picking up room ambiance. This is often placed a few metres away from the amplifier and about 6 feet from the ground.

The SM-58 is also very popular because it is very robust and, like the SM-57, it can withstand high gain and has a large frequency range. Roger Daltry from ‘The Who’ says:

‘The SM58, well it’s the best microphone any singer in a rock band could ever wish to have, especially in the early days of The Who. I mean we were getting drums smashed over our heads. I’d be slamming my mic into cymbals … we didn’t get our gear free then so it bloody well had to be durable.’

(Cheap and bombproof: the Shure SM58 microphone : Cloudymind)

Another popular Dynamic microphone is the AKG D112. This is used for bass instruments and kick drums. It is ideal for kick drums as it has a frequency range of ...

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(Cheap and bombproof: the Shure SM58 microphone : Cloudymind)

Another popular Dynamic microphone is the AKG D112. This is used for bass instruments and kick drums. It is ideal for kick drums as it has a frequency range of 20-17,000 Hz as shown in the diagram below –

This microphone is used in the AKG Drum Set Big II kit, which also has some Condenser microphones. These are a pair of C 1000 S’s and four C 518 M’s. The pair of C 1000 S’s are used for overheads to capture cymbals and also any resonance from the toms. The C 518 M’s are clip-on microphones and are positioned very closely to the drum skins to pick up a clear, high quality sound. The frequency range for the C 1000 S is 50-20,000 Hz and the frequency range for the C 518 M is 60-20,000 Hz. The specified ranges for both of these microphones mean that they are ideal for capturing a very wide range of frequencies.

The Condenser microphone is a far more delicate when compared to the Dynamic microphone as it is far more sensitive and would distort if dealing with high volumes.

The Condenser microphone works as so:

‘A capacitor has two plates with a voltage between them. In the condenser mic, one of these plates is made of very light material and acts as the diaphragm. The diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance. Specifically, when the plates are closer together, capacitance increases and a charge current occurs. When the plates are further apart, capacitance decreases and a discharge current occurs.’

(Condenser Microphones)

The diagram here shows the process:

(http://www.mediacollege.com/audio/images/mic-condenser.gif)

Condenser microphones require a power source, such as a battery or phantom power. This means that the audio signal they capture is stronger than one captured from a Dynamic microphone. ‘Condensers also tend to be more sensitive and responsive than dynamics, making them well-suited to capturing subtle nuances in a sound.’ (Condenser Microphones)

The Ribbon microphone is the most delicate of the three types of microphone. They are similar to the Dynamic microphone as they are thin and have a diaphragm that is usually made of aluminium.

‘Often, this diaphragm is corrugated along its width and is suspended within a strong field of magnetic flux. Sound-pressure variations between the front and the back of the diaphragm cause it to move and cut across these flux lines, inducing a current in to the ribbon that is proportional to the amplitude and frequency of the acoustic waveform. Since the ribbon generates a small output signal (when compared to the larger output that’s generated by the multiple wire turns of a moving coil), its signal is too low to drive a microphone input stage directly...thus, a step-up transformer must be used to boost the output signal and impedance to an acceptable range.’

(Modern Recording Techniques)

The Ribbon Microphone works by this process:

(http://www.cybergeo.com/music/microphones/ribbon1.gif)

The Ribbon microphone also does not require a power source for it to operate, be it phantom power or a battery. However, it differs from the Dynamic microphone because, like the Condenser, it is very fragile and expensive. The Ribbon microphone is not used as much now because the Condenser microphone can be purchased for a lot less whilst still producing a high quality sound. However, manufacturers such as Audio Engineering Associates (AEA), Coles, and Royer Labs are still producing the Ribbon Microphone starting from £500 each (Ribbon Microphones review | Ribbon Mics).

Polar Patterns are the sensitive areas in a microphone and are usually demonstrated by diagrams. The diagrams below show the best areas for a microphone to pick up audio signal:

Omnidirectional –

This pattern is regarded as a perfect sphere and is ideally used for picking up small vocal groups, for example, a small gospel choir standing around the microphone, as it can pick up a balanced audio signal.

(http://upload.wikimedia.org/wikipedia/commons/thumb/8/8d/Polar_pattern_omnidirectional.png/80px-Polar_pattern_omnidirectional.png)

Cardioid –

This pattern is the most common as it is used in many microphones. It is designed to pick up audio signal from only the front of the microphone.

(http://upload.wikimedia.org/wikipedia/commons/thumb/6/6d/Polar_pattern_cardioid.png/80px-Polar_pattern_cardioid.png)

Supercardioid –

This is like the Cardioid pattern although it is more sensitive at the rear of the microphone. This could be used to pick up room ambiance as well as natural reverb.

(http://upload.wikimedia.org/wikipedia/commons/thumb/a/ae/Polar_pattern_supercardioid.png/80px-Polar_pattern_supercardioid.png)

Hypercardioid –

This is very similar to the Supercardioid pattern although it is even more sensitive towards the rear of the microphone. It is therefore better suited for picking up more room ambiance.

(http://upload.wikimedia.org/wikipedia/commons/thumb/8/80/Polar_pattern_hypercardioid.png/600px-Polar_pattern_hypercardioid.png)

Bi-directional/Figure of 8 –

This pattern can pick up equal audio signal from the front and the back of the microphone. It is very common in the Ribbon microphone as they are designed for picking up audio signal from either side.

(http://upload.wikimedia.org/wikipedia/commons/thumb/7/7b/Polar_pattern_figure_eight.png/80px-Polar_pattern_figure_eight.png)

A Pressure Gradient microphone has a setup where the diaphragm is open on both sides and captures audio signal from both sides, like a Bi-directional/Figure of 8 polar pattern. This can be useful when picking up two different audio signals and the user requires both signals to be sent to one input.

A directional microphone can suffer from the Proximity Effect when placed too close to a sound source, usually within 1 foot. The result of this is ‘an increase in bass response’ (Modern Recording Techniques). This can be intentional by some artists/producers but can be a problem for others. This is solved by using equalisation to reduce the lower frequencies and increase the higher frequencies. The Proximity Effect is also associated with ‘popping’ sounds when a vocalist uses the letters ‘p’ and ‘b’. This usually happens when a directional microphone is used, however it can be fixed by replacing the microphone with an omnidirectional microphone or by adding a pop shield (Modern Recording Techniques).

Auditory Localisation refers to stereo sound and how humans perceive stereo through their ears. Humans recognise stereo through three main areas: Interaural Intensity Differences (IID’s); Interaural Timing Differences (ITD’s) and Head Related Transfer Function (HRTF). The IID refers to dynamics and how loud a human hears a sound in both ears. Naturally, if a sound is produced from the left then the IID is going to be louder in the left ear, quieter in the right ear and vice versa. ITD refers to the difference in timing between the two ears and is similar to IID. When a sound is produced from the right, the right ear will pick up this sound just before the left ear does. The difference in time is usually measured in mili-/nano-seconds as the ITD is usually not noticeable to humans and has to be measured using specific hardware. HRTF is the difference in tone between two ears. Panning can be used to make this noticeable and many commercially produced records will have appropriate panning to give a clear stereo image to the listener.

The stereo field can also be determined by how microphones are placed when recording the audio source. A common placement technique is the ‘XY’ position. This uses a pair of microphones, one placed above at a 90° angle to the other, to capture a wide range of audio as demonstrated in the diagram below –

(http://www.pcmus.com/images/xy-stereo-mic.gif)

The next common microphone placement technique is the AB spaced pair. This uses the ratio rule of 3:1, meaning that the microphones need to be positioned apart three times the distance from the sound source. The diagram below shows the AB spaced pair placement –

(http://www.paia.com/ProdArticles/Images/msmicwr4.gif)

The MS pair uses a bidirectional microphone and a cardioid microphone, facing 90° away from each other in order to pick up a wide stereo field. The diagram below shows a good description of this –

(http://www.tape.com/images/Image1.gif)

The Stereo-sonic microphone placement technique, also known as the Blumlein placement technique, uses two Bi-directional/Figure of 8 pattern microphones to pick up a wide range of sounds. They are similar to the XY placement although they pick up sound from behind as well, which helps to pick up room ambiance and good stereo separation (M-Pulse: Digital Newsletter of M-Audio).

(http://m-pulse.m-audio.com/articles/march2004/images/Blumlein1.gif)

The near co-incident placement has many positions that can be used to obtain different results. Firstly, there is the ORTF position, which is good for capturing localisation and gives a sense of depth. The diagram below shows the placement –

(http://www7.taosnet.com/f10/sbncmic.gif)

The NOS produces a very similar result to the ORTF position, although the placement is slightly different, as indicated in the image below –

(http://www7.taosnet.com/f10/sbdone.jpg)

Finally, the Binaural placement uses a model of a human head between two microphones to produce excellent results for audio that is played through headphones, but does not achieve such good results when played back through monitors.

(http://cache.gizmodo.com/assets/resources/2006/12/binauralmic.jpg)

Bibliography –

Huber, D.M., Runstein, R.E. (2005). Modern Recording Techniques (6th Edition). America: Focal Press.

Locke, A. (2008), Cheap and bombproof: the Shure SM58 microphone : Cloudymind. Retriever December 28, 2008, from http://www.cloudymind.com/2008/03/02/cheap-and-bombproof-the-shure-sm58-microphone/

Holt, A., Owen, R. & D. (n.d.). Dynamic Microphones. Retrieved January 6, 2009, from http://www.mediacollege.com/audio/microphones/dynamic.html

Holt, A., Owen, R.& D. (n.d.). Condenser Microphones. Retrieved January 6, 2009, from http://www.mediacollege.com/audio/microphones/condenser.html

Ribbon Microphones review | Ribbon Mics. (n.d.). Retrieved January 10, 2009, from http://www.recording-microphones.co.uk/ribbon-microphones.shtml

Sontronics. (n.d.). Retrieved January 11, 2009, from http://www.sontronics.com/advice.htm

Glossary: Proximity Effect | Sweetwater.com. (n.d.). Retrieved January 11, 2009, from http://www.sweetwater.com/expert-center/glossary/t--ProximityEffect

Glossary: Pressure-gradient Microphone Sweetwater.com. (n.d.). Retrieved January 12, 2009, from http://www.sweetwater.com/expert-center/glossary/t--Pressure-gradientMicrophone

M-Pulse: Digital Newsletter of M-Audio. (n.d.). Retrieved January 13, 2009, from http://m-pulse.m-audio.com/articles/march2004/micguide.php