lecture 10 -microphones - nathan e adam – audio | video |...

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


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.


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.


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


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


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


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


Spectral (frequency) response graphs – SM57 & PG81


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?

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?

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?

Cost

Durability


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


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


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.

\

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

lecture 10 -microphones - nathan e adam – audio | video |...

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