AET 1380 Lecture 10 Microphones
Microphones – General Definition
Microphone – an electro-acoustic device that converts acoustic energy into electrical energy (transducer).
Microphones more than anything else create the recorded timbre of an instrument.
The transducer in most mics is called the diaphragmThe diaphragm can also be called the element
Mics can be classified by their transducer type:
The most common microphone transducer types for musical use are:
1) moving coil (referred to in studio as “dynamic”) 2) ribbon, and 3) condenser
Microphones – Internal Characteristics
Mics can can also be classified by their impedance:
High impedance (hi z)mics: electrons move with
difficulty; andLow impedance (lo z) mics: electrons flow freely
Microphones – Internal Characteristics
Impedance is the resistance to the flow of electrons in an electrical circuit
The nature of the sound a microphone picks up is dependent on three external variables….
1) The nature of the sound source
2) The acoustic environment
3) The placement of the microphone in relation to the sound source
Other types of transducers found in microphones (less used in the studio today)
First Microphone (1891)
Carbon Mic – Berliner Microphone The 25 year-old Berliner sold his carbon microphone patent for $50,000 to the fledgling Bell Telephone Company paving the way for it to become one of the world's largest corporations.
Not used in recording today
Other types of transducers found in microphones (less used in the studio today)
First Microphone (1891)
Carbon Mic – Berliner Consists of a metallic cup filled with carbon granules.Movable metallic diaphragm mounted in contact with the granules covers the open end of the cup. Wires attached to the cup and diaphragm are connected to an electrical circuit so that a current flows through the carbon granules.As sound waves hit the diaphragm, they compress the carbon dust, which changes its resistance. By running a current through the carbon, the changing resistance changes the amount of current that flows.
2 types of transducers
Mics that use magnets--Moving Coil (Dynamic) – introduced in 1931 by Western Electric
Ribbon – Introduced in 1932 by RCA
Mics that do not use magnets--Condenser (Capacitor) – introduced in 1916 by EC Wente. Commercially mass produced by Neumann in 1928.
Mics that use magnets (Dynamic Mics)
Moving coil - the diaphragm is attached to a coil of wire set lying inside a magnet.
When sound waves hit the diaphragm the coil moves.
Changes in the magnetic field induce current to flow in the wire.
Mics that use magnets (Dynamic Mics)
Moving coil (Dynamic) mics – more illustrations
Practical examples of moving coil (dynamic) mics– Small diaphragm
Practical examples of moving coil (dynamic) mics– Large diaphragm
*In general, large-diaphragm microphones are more suitable for capturing lower-frequency sounds and small-diaphragm microphones are more suitable for capturing higher-frequency sounds.
Mics that use magnets (Dynamic Mics)
Ribbon Mics A thin metallic ribbon is suspended between two magnets and creates a magnetic field.
Sound waves move the ribbon which changes the output of the current.
Classic ribbon mics
Types of Dynamic MicsRibbon mics – more illustrations
Mics that use magnets (Dynamic Mics)
Ribbon Mics – more illustrations
Practical examples of ribbon mics – Large diaphragm, classic mics
Ribbon Mics
RCA 77-DX spec sheet
Ribbon Mics
Ribbon Mics
Practical examples of modern ribbon mics –small and large diaphragm
Mics that do not use magnets
Condenser (Capacitor) Mics
Condenser mic – how it works
Condenser Microphones – other requirements
1) Contain a built-in amplifierCondenser mics are high impedence by nature so
they must use an built in amplifier (with internal or external power supply) to provide a voltage across the capacitor.
The built-in amplifier can be a vacuum tube or Field Effect Transistor (FET)
Condenser Microphones – other requirements
2) Require phantom power to maintain charge. This power supply is commonly called "phantom power“
3) Typically contain a padReduces incoming dBs
Early Diagrams of Condenser Mic
E. C. Wente of Bell Telephone Laboratories designed the first condenser microphone in 1916, however the first quality condenser wasProduced by the Neumann company in 1928
Practical examples of condenser mics – Large diaphragm
Practical examples of condenser mics – Small diaphragm
Other types of condenser mics
PZM (Pressure Zone Mic) (patented and licensed by Crown)
Actually a special configuration of a condenser mic
Pressure sensing element close to a solid metal plate
Eliminates standing waves, out of phase signals
Other types of transducers found in microphones (less used in the studio today)
Crystal Mics Utilizes piezoelectric crystals.
A voltage develops between two faces of the crystal when pressure is applied to the crystal.
Sound waves vibrate a diaphragm, which in turn varies the pressure on a piezoelectric crystal. This generates a small voltage,which is then amplified.
Used by newscasters and in modern telephones
Other types of transducers found in microphones (less used in the studio today)
Crystal Mic
Microphone CharacteristicsWhat characteristics do we look for in a
microphone?
Frequency response Tells us how mic captures frequencies we are
attempting to record. Measured “on axis”. Ideal is flat reponse, but varies according to mic’s
design Why important? We use Spectral (frequency) response graph to
illustrate Output (dB) v.s. Frequency (Hz)
Microphone Characteristics
Spectral (frequency) response graph – AKG 414
Microphone Characteristics
Spectral (frequency) response graphs – SM57 & PG81
Microphone CharacteristicsWhat characteristics do we look for in a
microphone?
Transient response - how quickly the diaphragm of a mic will react to an acoustic soundwave
A transient is a short duration, high level peak, such as a hand-clap or snare drum hit.
Microphone CharacteristicsWhat characteristics do we look for in a
microphone?
In a mic with quick transient response, the diaphragm is able to snap back to a neutral position quickly and thus is ready to be hit by a new wave and react to it accurately.
Transient response of a condenser mic, is accurate and quick.
Dynamic and ribbon mics are not able to represent the full positive or negative cycles of the waveform as quickly as the condenser.
Transient responses of different microphones
Microphone CharacteristicsWhat characteristics do we look for in a
microphone?
Sensitivity Ability to record quiet sounds at low gain Specifies the gain required to raise mic level to line level…
high requires less gain and low requires more gain Voltage that a mic produces at a given SPL Measured in dB
SPL response – distortion with loud sounds Maximum SPL SPL at which a mics output signal begins to distort Measured in dB Higher SPL is better
Microphone CharacteristicsWhat characteristics do we look for in a
microphone?
Cost
Durability
Microphone Characteristics
Lets go on a microphone date…
Advantages & DisadvantagesMOVING COIL / DYNAMIC
Advantages Most rugged Most durable Least expensive Very common Can take high SPLs
Disadvantages The uniformity of
frequency response and sensitivity does not match that of the ribbon or condenser mics
Advantages & DisadvantagesRIBBON
Advantages Better sensitivity than
coil mic Adds "warmth" to the
tone by accenting lows when close-miked.
Disadvantages Very fragile (older ribbons) Least used Cannot take high SPLs Accenting lows - big bottom Very susceptible to wind
noise. Not suitable for outside use unless very well shielded
Advantages & DisadvantagesCONDENSER
Advantages Best sensitivity
Best transient response
Best frequency response
Very common
Disadvantages Must have a power
supply Batteries Ext. power supply Phantom power
Cannot take high SPLs
Most expensive
Advantages & Disadvantages
After taking into account the advantages and disadvantages, what type of microphones do you think work the best for which instruments?
Vocals, cymbals, snare drum, etc
Every microphone has its own unique directional sensitivity
This is also called the mic’s pick-up pattern or polar pattern
We represent a mic’s directional sensitivity with a…
Polar Response Graph
Directional Sensitivity(Pick-up Pattern/Polar Pattern)Different types of polar patterns in microphones have
different ways of picking up sound
Omni-Directional
Practical examples of condenser mics – Large diaphragm
Bi-Directional
Practical examples of ribbon mics – Large diaphragm, classic mics
Uni-Directional
Practical examples of moving coil (dynamic) mics– Small diaphragm
Practical examples of moving coil (dynamic) mics– Large diaphragm
*In general, large-diaphragm microphones are more suitable for capturing lower-frequency sounds and small-diaphragm microphones are more suitable for capturing higher-frequency sounds.
Specific types of Uni-Directional Patterns
Cardioid
Special types of Uni-Directional Patterns
Hypercardioid Supercardioid
Special types of Uni-Directional Patterns
Least directional to most directional pickup pattern
-Cardioid
-Supercardiod
-Hypercardiod
-Shotgun
Multi-Directional Mics
Mics that are capable of switching from one pick-up pattern to
another
Practical examples of condenser mics – Large diaphragm
Off AxisColoration
X
X
Off axis coloration caused by a sound source that is off to one side or behind a unidirectional mic. Placement on or off axis effects instrument’s tone color. Why?
Life Cycle of Sound
Sound
Direct Sound Reflected Sound
Early Reflections Reverberation
Microphone Issues - Proximity Effect
Reduce Three ways: Move mic away from sound source Use Bass Roll-Off Switch – built-in on many microphones (essentially a high pass filter) Use a different mic
Over-emphasis of low frequencies when the sound source is close to a directional mic
Microphone Issues - Proximity Effect
Both cardioid and bi-directional (Figure 8) polar patterns exhibit Proximity Effect, a substantial low-frequency "warmth" when these mics are used close-up for vocals.
Omni polar pattern does not exhibit Proximity Effect
Microphone Issues -Rumble
Unwanted low frequency vibrations
Reduce three ways: Use a shock mount Use bass roll-off switch Use a different mic
Mic Placement Techniques Close miking – 0-3’ away from sound source
Aids in sound isolation-used to capture direct sound only Increases sound isolation Eliminates poor acoustic environment Adds presence to sound – proximity effect Primarily employed in recordings of 1940’s &
50s? Why?
Accent miking – specifically miking 1 or 2 instruments in ensemble
Mic Placement Techniques
Distant miking –placing mics to achieve a balance between source and room.
Ambient miking – placing mics to make reverb more prominent than direct signal
Both techniques above:Add “live” feel to soundUtilize acoustic environmentAdd spaciousness to soundWere primarily employed in the recordings of the 1920s and 1930s
Stereo Miking
These methods capture a sonic event as a whole, typically using only two or three microphones.
In addition, true-stereo miking conveys: the depth or distance of each instrument the distance of the ensemble from the listener (the
perspective) the spatial sense of the acoustic environment--the
ambience or hall reverberation
Stereo Microphone Techniques
Coincident pair or “XY” pattern
Two directional mics are mounted with grilles nearly touching and diaphragms one above the other, angled apart to aim approximately toward the left and right sides of the ensemble
The greater the angle between microphones, and the narrower the polar pattern, the wider the stereo spread (also see handout)
From www.tape.com
Stereo Microphone Techniques-Variations on the XY Coincident Pair
Near-coincident pair -A stereo miking technique similar to coincident pair, but the mics are set up with some distance in-between them, capsules not touching.
Stereo Microphone Techniques-Variations on the XY Coincident Pair
Blumlein array--uses two bi-directional mics angled 90 degrees apart and facing the left and right sides of the ensemble.
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Stereo Microphone Techniques-Variations on the XY Coincident PairMid-side (MS) recording
method –
With this technique, the stereo spread can be remote-controlled by varying the ratio of the mid signal to the side signal.
One mic facing the middle of the sound source is summed and differenced with another bidirectional mic aiming to the sides.
Needs special summing device
More pictures…
Other Stereo Microphone Techniques
Spaced pair (A-B) technique– two mics placed side by side
Not as much stereo spread
Other Stereo Microphone Techniques
Decca tree technique– three mics placed 8-12’ above conductor
Used for orchestral recording – allows for very wide spread
3:1 Distance RuleWhen using two mics, they must be at least 3 times
farther apart from each other than they are from the sound sources
3:1 Distance Rule ctd
Mic 1 Mic2
Sound Source 1 Sound Source 2
1 foot 1 foot
3 feet
3:1 Distance Rule ctd