Microphones and Miking Techniques

• A microphone is a transducer, in that it converts acoustic energy (variations in air pressure) first into mechanical energy (movement of diaphragm), and then into electrical energy.

• Because microphones are concerned with electrical power and sound waves, they are appropriately called electro-acoustic transducers.

Condenser Microphones

• Condenser microphones operate on an electrostatic principle • Microphones using the variable capacitor principle are

universally referred to in the industry as condenser microphones.• A capacitor is an electrical device that is capable of storing

electrical charges • A capacitor is an electrical device that is capable of storing

electrical charges • The amount of charge that a capacitor can store is determined

by its value of capacitance and the applied voltage across its plates according to the formula Q=CV

• Where

– Q is the charge in Coulombs – C is the capacitance in Farads V is the voltage in Volts

• The capacitance of the capsule is determined by the composition of the surface area of the plates (which are fixed values), the dielectric or substance between the plates (which in this case is air and is fixed), and the distance between the plates (which is a variable).

• As the diaphragm (moving plate) vibrates as a result of variation in air pressure, the distance between it and the other plate of the capacitor varies and so does the device’s capacitance. When the distance between the plates decreases, the capacitance increases.

• The main disadvantage of a condenser microphone is that it needs to be powered.

• It is unsuitable for use in live sound reinforcement due to its overly sensitive response picks up leakage from across the stage.

• limits its usage to a controlled acoustic environment such as a studio is its excellent sensitivity to even small pressure variations allowing it to capture the ambience as well as other tiny details of sound in a musical performance.

Dynamic microphone • The dynamic microphone is one of the most widely used

types. It is also referred to as the moving coil microphone.

• the dynamic pickup system operates by electromagnetic induction in order to generate an output signal.

• When an electrically conductive metal is made to cut across the flux lines of a magnetic field, a current of specific magnitude and direction is generated withing that metal.

• The motion of the coil in the fixed magnetic field in the gap causes the wires of the voice coil to cut across the flux in the gap. This in turn produces an induced current across the coil that is analogous to the pressure acting on the diaphragm.

• The output would be an electrical equivalent of the acoustic sound wave hitting the diaphragm.

• The signal from the dynamic element can be used directly, without the need for additional circuitry. This design is extremely rugged, has good sensitivity when close to the sound source and can handle the loudest possible sound pressure levels without distortion

Peizo Electric Microphones

• Piezo means pressure, and hence piezo electric translated means pressure electric.

• If pressure is applied to some crystalline substances such as Rochelle salts, quartz, and man-made ceramics like Barium Titanate, a voltage potential appears across the faces of the crystal or ceramic.

• When the wafer or crystal is bent or twisted at an audio frequency rate, the wave shape of the voltage generated across the faces of the crystal will be the same as that of the audio signal.

Ribbon Microphones • In contrast to the moving-coil design, some dynamic

microphones use a metallic membrane, which functions as both the diaphragm and as the moving conductor.

• Electrically conductive ribbon placed between the poles of a magnet to produce a voltage by electromagnetic induction.

• Extremely low mass of a ribbon diaphragm does offer a potentially excellent transient response.

Microphone Polar Patterns

• An important characteristic of a microphone is its response to sound arriving from different directions. This is also called polar pattern.

• Omni directional Mics : The omni directional microphone has equal response at all angles. Its “coverage” or pickup angle is a full 360 degrees. This type of microphone can be used if more room ambience is desired. For example, when using an “omni”, the balance of direct and ambient sound depends on the distance of the microphone from the instrument, and can be adjusted to the desired effect.

Carbon Granule Microphones

• Carbon granules are loosely packed in contact with the diaphragm in such a way that when the diaphragm moves it moves the granules.

• This movement of the granules causes the closely packed and less closely packed which affects the resistance of the granules and therefore a DC current flowing through he granules will be modulated.

Pressure Gradient Operation

• Bidirectional Microphones• Unidirectional Type Microphone

• Cardioid : This has full sensitivity at 0 degrees (on-axis) and is least sensitive at 180 degrees (off-axis).

• Supercardioid : front pickup angle115 and is least

sensitive direction is at 125 degrees. • Hypercardioid : front pickup angle 105 and is least

sensitive direction is at 110 degrees.

• Microphone Placement is broken down into 2 broad categories:

1. Mono miking 2. Stereo miking

In the study of acoustics there are three basic ways in which sound is altered by its environment.

1. Reflection 2. Refraction 3. Diffraction Reflection : • Echo• Reverberation

• Standing waves

Mono Microphone Techniques

• Distant Microphone Placement • Close Microphone Placement • Accent Miking • Ambient Miking

• “3 ft or more” away from source. (often further)• Captures direct and reflected sound• Pro’s

– Useful for picking up an ensemble– Natural tonal balance (at distance roughly equal to

size of sound source)– Capture’s room acoustics naturally.

• Con’s– Room acoustics must be good– Phase cancellation can be a problem

Distant Microphone Placement

Close Microphone PlacementClose Microphone Placement

• 1 inch to 3 feet• “present” sound quality• Excludes acoustic environment (isolation)• Tonal balance can be a problem

– Instruments don’t radiate all frequencies in all directions

• Leakage should be avoided– Physical distance between sources– Directional microphones– Gobo (acoustic barrier)

Accent MikingAccent Miking • A not too close miking technique used to highlight an

instrument in an ensemble which is being picked by distant mikes.

• The accent mike will add more volume and presence to the highlighted instrument when mixed together with the main mic.

Ambient Microphone PlacementAmbient Microphone Placement

• Large distance from source - reverberant signal more prominent than direct.

• Control over natural reverb level in stereo or surround applications

• Audience reaction• Natural room acoustics

Stereo Miking TechniquesStereo Miking Techniques

• The use of two (or more) microphones to obtain a coherent stereo image.

• Five main techniques:– Spaced Pair– X/Y– ORTF– M/S– Decca tree

AB or Spaced Pair AB or Spaced Pair

• Two mic’s (same make and model), placed in front of an instrument/ensemble.

• Spacing ranges from a couple of feet to over 30 feet.• Typically, omni pickup pattern is used.• Interaural time and interaural amplitude cues create stereo

image.• Potential for phase problems, especially when mixed to mono.• In placing AB mics use the 3:1 Rule : the distance between the

to mics should be at least 3 times the distance between the mics and the source.

X/YX/Y• Two directional microphones (same make and model)

placed as close together as possible without touching. (coincident technique)

• Generally at angle of 90° to 135°.• Midpoint faces towards middle of source.• Directionally comes entirely from amplitude differences.• No phase problems.• Cardiod or bi-directional• Bidirectional gives the “Blumlein” pattern, which can yield

excellent ambient results.

ORTF(Office for Radio and TV of France)

• Near coincident technique.• Two directional microphones (same…), symmetrically

angled outwards at 110°, about 17 cm apart.– simulates physical ear placement

• Combines directional amplitude differences and interaural arrival time differences.

• Best when used at a distance from ensemble. • Named after French National Broadcasting Organization.

(Office of Radiodiffusion—Television Française)

NOS stereo

• Two directional microphones (same…), symmetrically angled outwards at 110°, about 17 cm apart.

• The NOS stereo technique is more useful at shorter distances, for example on piano, small ensembles or used for creating stereo on a instrument section in a classical orchestra.

Mid-side technique• Mid-side technique, using a cardiod pattern pointed at the sound

source, and a bi-directional microphone oriented towards the sides (90° and 270°)

• The mid pickup captures the source, while the side pickup captures ambience.

• Offers excellent control of source-to-ambient balance, even after recording.

• Excellent source pickup for chamber music.• No phase problems when mixed to mono. (coincident)• Requires a decoder/transformer matrix:

– Software decoder, or– The side pickup is sent to two channels, with the phase

reversed on one channel.– M + S = X ; M - S = Y