module2 microphones

Department of Electronics

and Media Technology

14MT2003 Audio Engineering

MODULE 2: MICROPHONES

MODULE 2: MICROPHONES

Objectives:

What is an Input Transducer

Classification of Microphones

How to interpret Microphones Specifications?

Use of right Miking Techniques

Microphone accessories

Miking different musical instruments

Transducer

A transducer is a device that converts a signal in one form of energy to another

form of energy

Microphones are input transducers that convert acoustical energy into electrical energy.

Electromagnetic transducers

Transducer Types:

Passive Transducers: Convert acoustical energy directly into electrical energy (and vice-versa) without the need for any external power feed. Includes Dynamic, magnetic and piezoelectric microphones, as well as condenser microphones using dc polarization

Active Transducers: Convert electrical energy from an external source synchronously with the sound vibrations they receive. Carbon microphones and RF-condenser microphones make use of this principle.

Velocity Transducers: Magnetic microphones operating on the basis of the law of induction. Their output voltage is proportional not to the extent, but to the velocity of the diaphragm displacement.

Classification of microphone types

Classification of Microphones

Based upon Construction:

Dynamic microphone

Condenser microphone

Ribbon microphone

Electret condenser

Based upon Polar Pattern:

Omni Directional

Uni Directional/ Cardiod

Bidirectional/ Figure of Eight

Hyper-Cardioid

Super-Cardioid

Shot Gun

Microphone Fundamentals

Pressure transducers:

Pressure gradient transducers:

Only the front face of a pressure transducer is exposed to the sound field. Diaphragm responds equally to all sound pressure fluctuations occurring at its surface. Omnidirectional by nature.

They respond to the momentary sound pressure difference occurring between two points A and B, which are a slight distance apart in the sound field. TF = TFo cos V

TFo = field transmission factor (sensitivity) with sound arriving perpendicularly to the diaphragm. V = angle between the perpendicular to the diaphragm and the direction of sound incidence.

Classification Based Upon Construction

Dynamic microphone

It consists of a rigid diaphragm,

typically 20 30 mm in diameter,

which is suspended in front of a

magnet

Cylindrical former is attached to the

diaphragm on to which is wound a

coil of very fine-gauge wire

When the diaphragm is made to

vibrate by sound waves the coil in

turn moves to and fro in the magnets

gap, and an alternating current flows

in the coil, producing the electrical

output

Operates on the Principle of variable magnetic inductance


Dynamic microphone

Features:

Robust

Big and Heavy

Better Freq Response in Presence range

Rapid fall-off in response above 8 or 10 kHz

Applications:

Vocals

Pick up bass drum

Outdoor Sound reinforcement



The capacitor consists of a flexible

diaphragm and a rigid back plate,

separated by an insulator

The 48 volts DC phantom power

charges the capacitor

When sound waves move the

diaphragm the capacitance varies,

and thus the voltage across the

capacitor varies proportionally

Operates on the Principle of variable capacitance



Features:

Light Weight

Higher sensitivity

High Level Output

Better immunity to interference

Applications:

Vocals

Instruments pick-up

Indoor/ Studio Recording

Consists of a long thin strip of conductive

metal foil, pleated to give it rigidity and

spring

Opposing magnetic poles create a magnetic

field across the ribbon

The electrical output of the ribbon is very

small, and a transformer is built into the

microphone which steps up the output.


Ribbon microphone


Ribbon microphone

Features:

Capable of very high-quality results

Low-frequency resonance at around 40 Hz

Smooth High Freq response

(but roll-off above 14 kHz)

Applications:

Miking Acoustic instruments

Miking Classical ensemble

Directional Responses/ Polar Patterns

The polar pattern of each microphone indicates how sensitive it is to sound

coming from different directions.

The polar diagram is a form of two-dimensional contour map, showing the

magnitude of the microphones output at different angles of incidence of a

sound wave.

Large membrane microphone Side Fire

Small Membrane microphone End Fire

Omnidirectional Pattern

Picks up sound equally from all directions.

Achieved by leaving the microphone diaphragm open at the front, but completely

enclosing it at the rear.

High frequencies are picked up less well to the rear and sides of the mic.

Most immune to handling and wind noise since they are only sensitive to absolute

sound pressure

Omnidirectional Pattern

Typical polar diagram of an omnidirectional microphone at a number of frequencies

Applications:

To record a choir

Lapel/ Lavallier/ Collar Microphones

Advantages:

Extended and open bass response

Omni will always sound more natural and open

Less proximity effect

Disadvantage:

More prone to Feedback

Figure of eight or bidirectional pattern

A microphone with a figure of eight polar pattern picks up the sound from in front

of the microphone and from the rear but not the side

Microphones with this Figure of Eight polar pattern are typically ribbon or Large

Diaphragm Microphones

The output produced by a sound which is picked up by the rear lobe of the

microphone will be 180 out of phase compared with an identical sound picked up

by the front lobe

Figure of eight or bidirectional pattern

Typical polar diagram of a bidirectional microphone at a number of frequencies

Applications:

Record a dialogue.

Used in Midside stereo recording

Advantages:

Reduces unwanted room ambience

Maintains natural sound

Excellent for acoustic instrument miking

Disadvantage:

High Cost and delicate

Prone to Proximity effect

Cardioid or Unidirectional pattern

Cardioid may be considered theoretically as a product of omni and figure of eight

response

A cardioid microphone has the most sensitivity at the front and is least sensitive at

the back

It isolates from unwanted ambient sound

Much more resistant to feedback than omnidirectional microphones

Cardioid or Unidirectional pattern

Typical polar diagram of a Cardioid microphone at a number of frequencies

Applications:

Loud on stage Live Sound

Stereo Miking

Instrument miking

Advantages:

Immunity to feedback

Ideal for handheld application

Disadvantage:

Suffers from proximity effect

Prone to Handling, pop & wind noise

Hypercardioid Pattern

It is described mathematically by the formula 0.5 +cos. It is a combination of an

omni attenuated by 6 dB, and a figure-eight.

It is very directional and eliminates most sound from the sides and rear.

The hypercardioid has the highest direct-to-reverberant ratio of the patterns.

Good for excluding unwanted sounds such as excessive room ambience or unwanted

noise.

Applications:

Isolating the sound from a subject or direction

when there is a lot of ambient noise

Picking up sound from a subject at a distance

Advantages:

Good Isolation

Disadvantage:

By removing all the ambient noise,

unidirectional sound can sometimes be a little unnatural

Need to be careful to keep the sound consistent

Hypercardioid Pattern

Super-cardioid Pattern

Supercardioid microphones offer a narrower pickup than cardioids and a greater

rejection of ambient sound

They also have some pickup directly at the rear

They are the most resistant to feedback.

Shot gun polar Pattern

They offer very narrow pick up in the front to a significant distance.

They also have some pickup directly at the rear and sides

Used for recording on-location sounds in movies (Boom Mic).

Comparison of Polar patterns

Classification according to Ease Of Use

Handheld microphones

Most common & convenient type of Microphone

Used by the Stage artists, Presenters etc.

Can be used for Vocal as well as Instruments

Rugged design

Lavalier/ Collar Microphone

Allows Hands-free Operation

Used for TV, Theatre or public speaking applications

Used as wired or wireless

Headworn Microphone

Allows Hands-free Operation

Used for Sports & Speech

applications

Used as wired or wireless

Boundary Microphone

Used in Theatres & Conference rooms

Can be used in less-than-ideal acoustic spaces

Gooseneck Microphone

Directional Microphones used for Speech

Used Conference systems & Podiums

Large Diaphragm Microphone

Majorly Large Diaphragm are Condenser Microphones

High sensitivity and a good high-frequency response

Fixed on Stands & Shock mounts

Boom Mic

Used for ENG

Used for Film Sound

Can pick up sound from

long distance

USB Microphone

Easy to use Plug & Play

Suitable for Home recording

Built in A to D convertor

Wireless Microphones

Electromagnetic waves are the carrier medium for information in wireless systems

Every WMS has to operate on one specific frequency

Transmitter and receiver of a system need to work on the same frequency

Two transmitters cannot be used with one receiver at the same time

Two systems operating on close frequencies at the same time can cause

interference problems

Parabolic Microphones

Hydrophone

Used underwater for recording or listening to underwater sound.

Based on a piezoelectric transducer that generates electricity when

subjected to a pressure change.

Miking Techniques

Mono Microphone Techniques

Distant Miking

Close Miking

Ascent Miking

Ambient Miking

Stereo Miking Techniques

Spaced Pair

Coincident Pair

Near Coincident Pair

Mono Microphone Technique

Distant Microphone Technique

Positioning of one or more mics at 3 feet or more from the sound source

Picks up a tonally balanced sound from the instrument or ensemble and also picks up the acoustic environment ie reflected sound

Close Miking Technique:

The mic is placed 1" to 3' from the source.

Only direct, on - axis sound is captured.

Creates a tight present sound quality which effectively excludes the acoustic environment.

Miking too close may colour the recorded tone quality of a source.

Common technique when close miking is to search for the instrument's "sweet spot"

Accent 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 Miking An ambient mike is placed at such a distance

that the reverberent or room sound is more prominent than the direct signals.

Restore natural reverb to a live recording

Used to pick up audience reaction in a live concert.

In a studio, used to add the studio rooms acoustic back in to a close miked recording

Mono Microphone Technique

Stereo Miking Techniques AB or Spaced Pair

The two mics (Omni or cardioid) are placed quite

far from each other to preserve a L/R spread or

soundstage.

Works on the arrival time differences between the

two mics to obtain the stereo image

In placing AB mics use the 3:1 Rule

The AB stereo method can give an exaggerated

stereo spread and can suffer from a perceived hole

in the center effect.

The sound can be warm and ambient but off

centre.

Sources can seem diffuse i.e not properly located.

The stereo image is obtained by intensity

differences produced by the sound source

on each mic.

Two cardioid mics (Top angled L ,bottom

R) set at an angle of between 90 and 135

degrees.

The angle increases the intensity

differences and widens the stereo image.

2 omni mics can be used for more

ambiance.


XY or Coincident Pair

The stereo image is obtained by intensity

differences produced by the soundsource

on each mic.

Utilizes a cardioid and a bidirectional

mic.

The side picks up ambient sound while

the mid picks up the direct sound.

May also by done on three channels of a

console with the 3rd channel containing

the reverse phase of the bidirectional mic.

Stereo Miking Techniques MS miking Technique

Works on the principle of IID as well as

ITD

Two Techniques: NOS and ORTF

Microphone pair is separated by a

distance similar to that between the 2

ears

Uses the best features of AB and XY to

produce a soundstage with sharply

focused images and an accurate stero

spread


Near Coincident or OSS (Optimal Stereo Sound)

NOS

ORTF


Dummy head microphone

Microphone Specifications

Pick up patterns

Frequency Response

Proximity effect

Transcient response

Output level or Sensitivity

Overload

Impedence

Pick up patterns

The frequency response of microphone is a measure of the consistency with which it translates a given sound pressure level into a given audio signal level at

different frequencies.

Good quality microphones have uniform frequency response off axis also.

Presence peak can help to increase the intelligibility of sound.

On-axis and Diffused field responses vary especially in the higher frequency range.

Frequency Response

On-and off-axis responses of DPA 4011 measured in 30 cm

Frequency Response

Proximity effect

Proximity effect is an increase in low frequency response when a microphone is very close to the sound source.

It is an inherent characteristics of directional microphones.

Sometimes causes preamplifier overload and cause distortion.

A low cut filter usually is an effective way to cure this problem.

Transient Response

Measure of microphones ability to render very sharp, fast musical attacks and signal peaks.

Sensitivity

The ratio of the analog output voltage or digital output value to the input pressure

Typically measured with a 1 kHz sine wave at a 94 dB SPL, or 1 pascal (Pa) pressure.

In the data sheets the sensitivity always applies to the frequency 1 kHz, unless otherwise noted.

Reference: 94 dBSPL 1 pascal = 10 bars = 10 dynes/cm2

Auditory threshold: 0 dBSPL 0.00002 Pa = 0.0002 dyne/cm2

The bar found in some non-European specifications refers to 74 dBSPL (20 dB less than 1 Pa) and the sensitivity or the sensitivity factor is not expressed

as transfer factor in the usual form of "mV/Pa" as open circuit voltage rating.

Overload

Distortion in sound is often blamed on microphone overload.

Overloading typically takes place in the amplifier stage and not in microphone stage.

A good quality microphone should be able to withstand SPL of 140dBSPL or more without overloading.

With condenser mic overload point will be reduced if the battery power is reduced.

Relation of microphone sensitivity to overload: Eg: Mic sensitivity is -47dB SPL at 94dB SPL

The output level then would be -47 + (140-94) = -1dBV

This will surely overload the mixers preamplifier stage unless a PAD is used to drop the

signal level.

Impedence

Impedance is an electronics term which measures the amount of opposition a device has to an AC current ( measured in ohms) Often Mics with a hard-wired cable and 1/4" plug are high impedance

Mics with separate balanced audio cable and XLR connector are low impedance

This is a measurement of the AC resistance looking back into the microphone Low (50-1,000 ohms)

Medium (5,000-15,000 ohms)

High (20,000+ ohms)

There is a limit to how much cable should be used between a high-impedance microphone and its input. Losses will be evident in high frequencies and output voltage levels above a distance of 20feet.

Low-impedance microphones and cable, microphone cables can be almost any practical length, with no serious losses of any kind.

Low impedance microphones have better noise immunity.

Microphone Specifications

SM57

Behringer XM1800s

Behringer C2

Samson CO2

Behringer B2-Pro

Microphone Accessories

Windscreens

Blimp Windshield

Dead Cat

POP Filters

Microphone cables (Balanced and Unbalanced)

Boom Pole

Mic Stand

Handy Pistol Grip Mounts

Reflection Filters

Shock mounts

Windscreens

Blimp Windshield/ Dead Cat

POP Filter

Microphone Cables

Boom Pole

Mic Stand

Handy Pistol Grip Mounts

Reflection Filters

Shock Mounts

Miking Musical Instruments

Acoustic Guitar

Timbre and position

Large diaphragm condensers

The guitar's sweet spot

Close-miking with miniature condensers

Instrument clip mics for live situations

Miking a drum kit Overhead

Hi-hat

Snare drum

Bass drum

Toms

Trumpet

The trumpet can generate sound with very high levels and also compresses and accelerates the speed of sound.

Choose a microphone that can handle high levels comfortably

For a well balanced sound, position the microphone 30 to 50 cm from the bell, slightly off axis.

Violin Supercardioid microphone directionality pattern can be used

The position will always be a compromise trying to capture the whole instrument with a narrow pick up pattern.

Miking Flute and Recorder

Approx. 5-10 cm away from the instrument, aim the mic halfway between the mouthpiece and the left hand.

Omni directional microphone may be an advantage, due to their lower sensitivity to wind and pop.

Due to its polar character, the flute can also be spot-miked behind and slightly above the head of the player, pointing at the finger holes.

module2 microphones

Documents