Getting it From the Source Microphone Basics
Microphone basics • A microphone converts sound energy into electrical energy • A microphone can use EITHER passive or active electronics to do its job (aka transducer principal) • Most microphones have a diaphragm which reacts to changes in the sound field. It is the “eardrum” of the microphone. • All microphones have a directional characteristic (a.k.a. a pickup or polar pattern)
Transducer principals • The transducer principal determines how the mic will turn sonic energy into electrical energy • Dynamic Mic = passive (no power needed): moving a wire coil through a magnetic field induces current flow • Condenser Mic = active (needs a power source): changing the distance between two electrically charged plates (capacitor) induces current flow
Dynamic Microphones • Diaphragm is attached to a wire coil which is suspended in a magnetic field • The diaphragm moves in reaction to sound waves, setting the coil in motion within the magnetic field inducing electron flow • The diaphragm is heavy, meaning it takes some force to move it • This means a dynamic mic can generally withstand high SPL’s without distorting • However, this also results in less detail in the sound reproduction; • Dynamic microphones tend to be more rugged and less sensitive than condenser microphones.
Condenser Microphones • Condenser is the British word for capacitor. • A capacitor is two electrically charged plates which are slightly separated. • In a condenser microphone, the diaphragm is one side of a capacitor which moves in reaction to changes in a sound field. • Since the two plates are charged, the motion changes the voltage between the two plates and these voltage changes induce electron flow. • Condenser mic’s require some sort of external power source (a battery or “phantom power”) • Because the diaphragm is very light, condenser mic’s can have a highly detailed response and tend to be much more sensitive than a dynamic mic • Condenser mic’s are more delicate than dynamics, are more easily damaged and are more prone to distortion when exposed to extreme SPL’’s, the active electronics will generate some noise (self-noise)
Ribbon Microphones • Ribbon microphones are essentially a special type of dynamic microphone. The diaphragm is usually a thin aluminum ribbon, which has a wire attached on either end, suspended in a magnetic field. Changes in the sound field set the ribbon in motion in the magnetic field inducing current • Because the diaphragm is so light, the sound is very detailed. There is no need for active electronics • A ribbon mic is the only microphone where the transducer principal dictates the directional charateristic • Ribbons are extremely delicate and are easily damaged
Tube microphones • A tube microphone is a type of condenser microphone. • The output of the mic is amplified by a vacuum tube instead of transistors • Tubes generally require a high-voltage power source and most tube microphones will have their own power supply rather than use phantom power. • Tube microphones are fairly delicate due to the glass tube inside. They also tend to be very costly.
Directional Characteristics A Microphone can be designed to react to: • absolute changes in pressure • (i.e. pressure transducer) • the difference between pressure at the front and rear of the diaphragm • (i.e. pressure gradient), • or force & direction of the sound wave • (i.e. velocity) • Ribbon mic’s fall into this category This determines its “pickup pattern”
Review of Polarity • Polarity refers to positive and negative • The high and low pressure of a sound wave is “encoded” into positive and negative electrical voltages. • When a speaker is given a positive voltage it moves outward. Likewise, if a mic diaphragm is pushed inwards, it generates a positive voltage. When it moves outwards, it will generate a negative voltage. • Polarity is also important when considering how directional patterns are achieved
Omnidirectional Microphones • Omni = all; an omni-directional mic is equally sensitive to sounds arriving from any direction • A true omni-directional mic is a pure pressure transducer - it strictly measures changes in pressure without any regard to the direction that the wave is traveling. • The rear of the diaphragm is sealed off so that it can only react to pressure changes on ONE SIDE of the diaphragm.
Bidirectional Microphones • Bidirectional = two directions. • A true bidirectional mic can be a pressure-gradient or velocity transducer, meaning its response to the sound will depend on the direction the sound wave is coming from. • The diaphragm is completely open on both sides so that it can react to pressure changes on either side of the diaphragm • A bidirectional mic uses polarity to encode the direction of the sound wave. • If a sound approaches directly from the side, it will cancel itself out completely. There is a “null” in the pickup pattern of the mic at 90-degrees • This results in a “figure-8” pattern - it is sensitive only to sounds arriving from directly in front or directly behind
Unidirectional/Cardioid • Unidirectional = one direction. The microphone is most sensitive to sounds approaching directly from the front (a.k.a. on-axis). As the direction of the sound source moves around to the rear, there is less and less sensitivity. • Generally a pressure-gradient transducer • The microphone has a null at 180-degrees - it will not respond to sound approaching directly from the rear. • This results in a “heart-shaped” pattern (cardio=heart) • A cardioid is a marriage of an omni and a bidirectional pattern. It has specially designed vents to allow some sound pressure behind the diaphragm. If you cover up these vents, the mic will become omnidirectional.
Other patterns • Super- and Hyper-cardioid mic’s are cardioids that use more bidirectional in the “recipe.” This results in a more narrow pickup in the front and a small pickup lobe in the rear. The nulls are moved to 120 or 110 degrees. • Shotgun - uses an interference tube to get a very narrow forward pickup. The longer the tube, the more narrow the pickup. • Multi-pattern mic’s - often use two cardioid capsules back-to-back. For uni-directional the rear capsule is off; for omni-directional pickup it is turned on; the polarity on the rear capsule can also be reversed to generate a bi-directional pattern.
Frequency response • Microphones all sound different! One reason is difference in frequency response • Few microphones react to all frequencies equally (“flat response”). • A mic’s frequency response is one way of describing how it will react to or “color” the frequency spectrum generated by instrument or voice • Some frequencies may be favored over others; some may not be reproduced at all! Polar response and Off-axis coloration: • A microphone’s frequency response changes as you move off-axis (i.e. to the side or rear of). Moving a sound source to the side of a mic will affect how the mic “colors” that sound source. Proximity effect: • As a sound source gets closer to a directional microphone, the bass frequencies are emphasized, resulting in a “boomy” sound (the Brits call it “Bass tip-up”)
Other microphone specifications • Max SPL • How much sound pressure the mic can handle before it will distort. • Measured in Pa* and/or dB SPL • How much distortion is acceptable is up to the manufacturer; it is expressed as % THD (Total Harmonic Distoriton) • Sensitivity • How large a signal the mic puts out for a given SPL; • the higher the number, the more sensitive the mic is. • Measured in mV/Pa • Self or Equivalent Noise - • condenser mic’s only • How much noise the electronics generate • Measured in dB SPL • Pa - Pascal - measures pressure; 0 dB SPL = 2 x 10-5 Pa 1 Pa = 94dB SPL
DI or Direct Box • Studio microphones generally have balanced low-impedance outputs. • Instruments generally have unbalanced high-impedance outputs. • This signal can overload an input designed for a microphone-level signal, and could have noise problems on a long cable run • A DI converts the high-impedance to a low-impedance and creates a balanced signal out of the unbalanced signal. • DI’s can be passive or active • most active DI’s can use phantom power
Care and feeding of microphones • Never blow into a microphone to test it - gently tap the mic or snap your fingers in front of it • Storing condenser mic’s in open plastic bags inside their cases is a good thing to do • Do not use phantom power on a ribbon mic or on tube mic’s that have their own power supply • Switch off phantom power to microphones before plugging in or un-plugging • Do not drop microphones • treat them gently and they will last a good long time
Amplifiers & Pre-Amplifiers • An amplifier increases the amount of signal. The ability of the amplifier to boost the level of a signal is called gain. • Each point where a signal passes through an amplifier is called a gain stage. This simply means that this is a stage where gain made be added to the signal. • A microphone puts out a very low-level signal. Special ultra-quiet microphone amplifiers are needed to boost this signal to a level that the other equipment in the signal chain can use. • These microphone amplifiers come before the power amplifier (which powers the speakers) in the signal chain, and are sometimes called preamplifiers. This is often the first gain stage in any signal chain.
Summary • The two most common types of microphone: Dynamic and Condenser • Condenser mic’s are generally more sensitive • Dynamic mic’s are generally more rugged • Condenser needs power; Dynamic does not • There are three general pickup patterns: omni-directional, bi-directional (figure-8), and uni-directional (cardioid)