MODULE2 Microphones
-
Upload
vishal-nair -
Category
Documents
-
view
34 -
download
5
description
Transcript of 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
-
Classification Based Upon Construction
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
-
Classification Based Upon Construction
Condenser microphone
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
-
Classification Based Upon Construction
Condenser microphone
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.
Classification Based Upon Construction
Ribbon microphone
-
Classification Based Upon Construction
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.
Stereo Miking Techniques
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
Stereo Miking Techniques
Near Coincident or OSS (Optimal Stereo Sound)
NOS
ORTF
-
Stereo Miking Techniques
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.