Microphone Basics
Rick Pitchford, CSRTE
Guest Lecture, COM 494
February 2, 2015
Microphone Types:
BODY STYLES:
•hand held
•lavalier
•body
•shotgun
•headset
•boom
•parabolic
•desk/stand
•boundary
images from RCA, Shure Brothers, Inc., Electrovoice, Audio Technica, Gibson and AKG
A Microphone is a Transducer:
A transducer is a device, usually electrical or electronic, that
converts one type of energy to another for the purpose of
measurement or information transfer. Most transducers are
either sensors or actuators. In a broader sense, a transducer is
sometimes defined as any device that converts energy from
one form to another.
Common examples include microphones, loudspeakers,
thermometers, position and pressure sensors, and antennas.
In a microphone, the diaphragm moves in response to
changing air pressure (sound), creating an electrical current.
Although not generally thought of as transducers, photocells,
LEDs (light-emitting diodes), and even common light bulbs
are transducers.
definitions from wikipedia.org and whatis.com
Moving Coil (Dynamic) Transducer:
•durable; resists humidity and
temperature extremes
•no power supply needed
•a good quality dynamic is
expensive
•low output level; closer to noise
floor
•hard to physically overdrive the
mic element with loud sounds
•hard to electrically overdrive
amp input with loud sounds
•identical mechanism as
loudspeaker
image from “First Class Radiotelephone License Handbook”, fourth edition, ©1974, Howard W. Sams Co., Inc.
Condenser Transducer:
•delicate; can be damaged by
physical shock, high humidity
and temperature extremes
•requires battery or external
“phantom” power supply for the
internal amplifier
•brighter sounding than a
dynamic mic
•more sensitive than dynamic
mic of same diaphragm size
•higher output level than
dynamic; can provide better
signal-to-noise ratio
image from “First Class Radiotelephone License Handbook”, fourth edition, ©1974, Howard W. Sams Co., Inc.
Ribbon (Dynamic) Transducer:
•delicate; easily damaged by
shock, humidity and high
sound levels
•desk mount, boom or
handheld body styles
•no power supply needed
•warm, rich sound
•very sensitive
•low output level
•expensive; difficult to find
image from “electronic communication”, third edition, ©1975, McGraw Hill Inc.
Microphone Polar Patterns:
The sensitivity of a
microphone varies depending
on the design of the mic,
orientation of the sound to the
mic and the frequencies
considered. This is displayed
graphically as a polar pattern.
Microphones fit somewhere
on a continuum from
omni-directional through
unidirectional, to
bi-directional. In general,
mics vary more from their
“advertised” patterns as
frequency increases.
omni
uni
bi
images from “ksm44_en.pdf”, Shure Brothers, Inc.
Omnidirectional Mics:
sensitive
sensitive
sensitive
sensitive
Diaphragm
•sensitive from all directions
Sound Waves
•has a sealed housing behind the
diaphragm
•positive pressure from any
direction pushes the diaphragm
in
Sound Waves
•most natural sounding mic
pattern due to uniform frequency
response
Housing
Unidirectional Mics: The Cardioid:Housing
sensitive
sensitive
Diaphragm
null
sensitive
Port
sensitive
•more sensitive to the front
•has a ported housing behind the
diaphragm
Sound Waves
•positive pressure from front pushes the
diaphragm in
•positive pressure from rear enters ports,
equalizes on both sides of the
diaphragm, producing no motion Sound Waves
•frequency response not uniform off axis
•usually exhibit “proximity effect”
More Unidirectional Mics:
As the housing ports become larger, the front sensitivity narrows.
A rear sensitivity node forms as nulls move to the sides of the
mic, producing the supercardioid and then the hypercardioid.
Positive pressure to the rear lobe produces an inverted waveform
compared to the front lobe, indicated by the negative sign.
Housing
supercardioid
sensitive
null
+
-
sensitive
inverted
Diaphragm
null
sensitive
Port
Housing
hypercardioid
sensitive
null
+
sensitive
-
sensitive
inverted
Diaphragm
null
Port
Ribbon Bidirectional Mics:
null
sensitive
+
-
Diaphragm
sensitive
inverted
null
Sound Waves
•sensitive front and rear only
•no housing used around the
diaphragm
•rear entering sounds are
naturally inverted relative to
the front
•frequency response not
uniform for sounds entering
off axis
Sound Waves
Condenser Bidirectional Mics:
Ports
null
sensitive
+
-
sensitive
inverted
Diaphragms
null
•formed by two identical
unidirectional mic capsules
mounted back-to-back
Sound Waves
•capsules are connected together
electrically out of phase so that
positive pressure entering the
rear produces inverted waveform
compared to positive pressure
entering the front, mimicking a
ribbon bidirectional
Sound Waves
Hemispherical Mics:
Diaphragm
sensitive
sensitive
Boundary
null
•omnidirectional mic mounted
near a boundary
•sensitive only on one side of
the boundary
•boundary size determines how
closely the mic pattern emulates
the “ideal”
•natural sounding due to
uniform frequency response
Housing
Interference Tube Mics:
null
sensitive
Slotted
Tube
null
sensitive
sensitive
null
null
sensitive
•primarily sensitive to the front
•a slotted tube is fitted to the front of a
non-ported housing
•sound pressure waves from the front
travel down the tube to the diaphragm
•sound pressure waves from the side enter
the tube through slots and travel down the
tube by varying-length paths
•off axis sounds cancel at the end of the
tube – a process called comb filtering
•polar pattern varies with frequency
Diaphragm
Housing
Comb Filtering:
•the effect of mixing two
identical signals, one delayed
in time from the other
•all frequencies in the delayed
signal are delayed by the same
amount of time
level
•different phase delays are
created for different
frequencies because every
frequency has a unique
wavelength
•the sum of the two signals
exhibits a frequency response
of alternating peaks and nulls
frequency
+
1 msec.
level
time
=
•at 1 msec. all even multiples
of 500 Hz., (1000 Hz., 2000
Hz., 3000 Hz., etc.) will fully
reinforce; all odd multiples
(500 Hz., 1500 Hz., 2500
Hz., etc.) will fully cancel
time
+
1 msec.
time
•at frequencies in between,
the interaction will vary from
reinforcement to cancellation
•1msec = 1 foot additional
acoustic path length
time
level
•the same time delay has the
exact opposite affect on these
two frequencies
500 Hz.
level
level
time
For Example,
level
level
1000 Hz.
time
=
Non-Acoustic Comb Filtering:
First 5-6 weeks of ABC’s Spring, 2009 season live broadcast of
Dancing With the Stars demonstrated severe comb filtering.
Commercial breaks were not affected. The problem may have been
caused by mis-timed, redundant signal paths being summed.
Frequency response of
commercial break
Frequency response of
live program
Taming Acoustic Comb Filtering:
•place the mic in such a
position that reflections are
delayed only slightly;
smaller delays push the first
null point to higher
frequencies
level
•place the mic closer to the
sound source and further
from the reflectors so the
Inverse Square law helps
the direct sound overwhelm
the reflected sounds
frequency
Mic Placement:
Obey the Inverse Square Law!
Doubling the distance from source to transducer quarters the sound
pressure level on the diaphragm.
Halving the distance from source to transducer quadruples the sound
pressure level on the diaphragm.
Proximity Effect:
DIRECTIONAL MICS respond to the Inverse Square law differently
than omnidirectional microphones. Low frequencies are enhanced more
than mid and high frequencies as the source-to-mic spacing decreases.
This is the reason many performers and announcers work directional
microphones closely, giving their voice more low end. Be aware varying
source-to-mic spacing during a performance can drastically change the
timbre of the voice.
Many directional mics include a bass-rolloff switch to equalize this
effect for close-micing situations. Some mics are carefully designed to
minimize it.
~17dB change at
100Hz, from 1/8
inch to 24 inches
image from “us_pro_beta57a_ug.pdf”, Shure Brothers Inc., 2004
Selecting a Mic:
Using a mic for a particular situation
requires these decisions be made about:
•the appropriate body style
•the appropriate transducer type
•the best polar pattern to use
•the proximity of the mic to the source
•handling sound reflections
Body Style Selection:
Hand held:
Boom:
•“universal”
Lavalier:
•spoken voice
•some instruments
•instrument
•studio voice
Parabolic:
•sports sidelines
•surveillance
Desk:
•interview
Boundary:
•complete room coverage
•conferences
Body:
•stage or screen
Shotgun:
•high-noise situations
Headset:
•hands-free uses
•sports commentators
•performers
Transducer Selection:
Moving Coil:
Condenser:
•extremely loud sounds
•sports remotes
•emergency/backup mic
•general purpose
•bad weather situations
•extremely quiet sounds
•high quality recording on a
budget
•covert/concealed applications
•near strong magnetic fields
Ribbon:
•predictable sound levels
•controlled studio locations
•experienced performers
•feeding high-gain, lownoise amplifiers
Polar Pattern Selection:
Consider both the desired and all
UNDESIRED sound sources
when choosing a pattern. Use the
best pattern to attenuate undesired
audio as well as delayed
reflections of desired audio.
Multiple Mics for Mono and Stereo:
Multiple Mics for Mono work:
•large sound source needing coverage
•multiple sound sources
Beware comb filter effects!
Micing in Stereo requires a minimum of
two discreet signals, Left and Right.
Stereo is a spatial effect stemming from
both intensity and time differences of
sounds stimulating left and right ears or
the ear’s representatives, the
microphones.
Consideration must be given to
mono mixes derived from
stereo signals.
Spaced Pair (A-B Pair):
Sound Source
8 ft.
25 ft.
L
(C)
R
•two mics placed in front
of and facing sound source
(center mic, fed to both
channels, is optional)
•spacial (stereo) effect due
to both time and intensity
differences
•mic placement must
follow "3 to 1" rule to
minimize comb filtering
effect in mono
•recreates a sound field in
one dimension; listener
movements will change
the mix
Coincident Pair (X-Y Pair):
Sound Source
R
L
•crossed or head-tohead unidirectional mics
placed in front of and
centered on a sound
source
•spacial (stereo) effect
due to intensity
differences only
•no comb filtering
effects in mono - fully
mono compatible
•recreates an aural
sweet-spot
Coincident Pair Deviations:
Sound Source
6-7”
L
R
Sound Source
ORTF
•cardioid microphones at an angle of 110°
•spaced about 6.5 inches apart
•susceptible to comb filtering
•recreates a head-sized sound field
L
R
+
-
+
-
Blumlein Pair
•developed in 1931
•crossed bi-directional mics placed head-to-head
•not susceptible to comb filtering
•recreates a sweet spot
Mid/Side (M-S):
Sound Source
L+R
+
L
-
-R
MID mic is UNIDIRECTIONAL
facing forward
SIDE mic is BIDIRECTIONAL
facing left/right
•mid mic picks up left plus right
(mono/sum) signal
•side mic picks up left minus right
(difference) signal
•adding side to mid gives double
left signal (L+R) + (L-R) = 2L
•subtracting side from mid gives
double right signal (L+R) - (L-R)
= (L+R) + (-L+R) = 2R
•spacial (stereo) effect due to
intensity differences only
•no comb filtering effects in mono
- fully mono compatible
•recreates sweet spot
Multi-Mic:
HORNS
DRUMS
+
-
+
-
PIANO
STRINGS
•combine multiple AB,
XY, MS, ORTF and/or
Blumlein mic pairs with
or without accent mics
•spacial (stereo) effect due
(hopefully) to intensity
differences only
•mic placement must still
follow "3 to 1" rule to
minimize comb filtering
effect in summed L and R
channels
•aspects of both a sound
field and multiple sweet
spots
GUITARS
Close Mic:
VOCAL 1
VOCAL 2
•one mic (or direct
connection) per source,
each recorded separately
•high acoustic isolation
required between mics to
prevent leakage
•no natural spacial
(stereo) effect artificially created during
the track mixdown
•multi-track mono - not
true stereo
•produces a totally
synthetic sound field
Beyond Stereo:
•consumer multi-track “surround sound” systems are everywhere
•quadraphonic sound of the 1970s fell flat due to competing
incompatible formats, poor results, questionable production
practices and music-only implementation
•Dolby Surround presentation of major films and the emergence
of HiFi home video formats in the 1980s re-awakened consumer
interest
•“5.1” surround systems consist of left, center, right, left and right
surround, and LFE (non-directional, low frequency/effects) channels
•surround sound tracks are usually “built” from dry, close-mic,
multitrack original material recorded in studio/sound stage
•“live” surround recording techniques are emerging based on
established stereo practices
Spaced Surround:
Sound Field
•mics are suspended above audience
space in trapezoid arrangement
•front spaced omnis for left and right
•rear spaced omnis for left/right
surround
•single unidirectional for center
channel and LFE placed somewhere
in front of or within trapezoid
•spacial effect due to both time and
intensity differences
L, R, Ls, Rs
C
•recreates the sound field in two
dimensions; listener can move
around to change aural perspective
Coincident Surround:
Sound Field
•mics are either suspended above or
located in audience space
•left, center, right, left and right
surround unidirectionals are mounted
crossed or within a “dummy head”
•down-facing omni for LFE pickup
•spacial effect due to intensity
differences only
•re-creates specific aural sweet-spot
to locate listener; movement within
the sound field doesn’t change
perspective greatly
L, C, R, Ls, Rs
LFE
•stereo and mono compatible; no
comb filtering
MSM Surround Array:
Sound Field
LF+RF
+
-
L
-R
LR+RR
•three mics pick up front, rear and
side audio
•adding and subtracting side
signal from front and rear produce
Left front, Right front, Left rear
and Right rear
•LFE and Center channels derived
or from additional dedicated mics
•recreates a sweet-spot
MID mics are UNIDIRECTIONAL
•stereo and mono compatible; no
facing forward and rearward
comb filtering
SIDE mic is BIDIRECTIONAL
facing left/right
“Sports Event Capture Array-2”
+
L, R
-
Ls, Rs
•designed for 2-goal sporting events
•first used for 2003 NCAA TV
broadcasts on CBS
•mic arrays mount on backboard,
goal stands, etc., provide front and
rear L/R audio
•two supercardioid mics spaced 11”
apart, face forward, down and
slightly in
•mid-side pair centered between and
facing back toward seating area
•center channel is feed from the
announcers; LFE is derived from the
array via low-pass filters
ESPN Monday Night Football:
•coincident (XY) pair of shotguns
on 50 yd line to pick up crowd on
opposite side for L, R
•coincident (XY) pair at top of
stadium for Ls, Rs
•stereo wireless on umpire for
field ambience; L, R
•6 wireless parabolic mics for
field effects; L, R
•umpire and player wireless,
synthesized to stereo, for field
perspective; L, R
•music to L, R
•all cameras have mounted mics
for shot perspective
•announcers mics to Center only
L, R, Ls, Rs
image from http://www.wstickets.com/_venues/oh/cleveland_browns_stadium.gif
Fox Football:
•coincident (XY) pairs on
opposite 25 yd. lines to pick up
crowd; L, R
•spaced (AB) pair hung from
press box for crowd surround;
Ls, Rs
•umpire and player wireless,
synthesized to stereo, for field
perspective; L, R
•4 wireless parabolics plus
camera mounted mics on field
for shot perspective; L, R as
needed
•music to L, R; sound effects
produced in 5.1
L, R, Ls, Rs
•announcers mics to Center
only
image from http://www.wstickets.com/_venues/oh/cleveland_browns_stadium.gif
World Series •parabolic
on Fox:
mics high behind home
plate, down first and third base lines,
focused on home plate, to L & R
•AB pair at high first and high third
pointed across field to farthest crowd
to L, R
•omnis mounted in bases, pointed
toward outfield, to L, L & R, R
•three or four manned parabolic mics
in the outfield, to L, R
•XY configured shotguns in
centerfield, to Ls, Rs
•“a bunch” of hemispherical mics
and lavs on “anything that makes
noise”
•music to L, R; sound effects
produced in 5.1
•announcers mics to Center only
image from http://www.ticketsreview.com/baseball/cleveland_indians_tickets/progressive-field_7397033.php
NBA:
•five short
shotguns on the
table aimed at each
basket, center
court, and to either
side of the center
•12 boundary mics
on the court
•one boundary mic
below each basket
•music to L, R;
sound effects
produced in 5.1
•announcers mics
to Center only
image from http://www.nba.com/cavaliers/sites/cavaliers/files/elements/
Other Sporting Events:
Golf:
Mixed Martial Arts:
•a stereo mic on a stake at
the tee
•two stereo shotguns on
the greens
•stereo mics for the crowd
in the
•long shotguns with
operators on the fairway
•“bird mics” in the trees
•a cluster of five shotguns:
one picking up the center of
the ring, the other four
splayed to capture the rest of
the circular cage
•shotguns on boom poles
•lavs placed under the pad
around the ring
•mics on the 9 handheld
cameras
“The Two Towers”
percussion
brass
low
strings
high
strings
all mics
•captured near-100 piece
orchestra in a 135’ X 75’
auditorium
•three omnidirectional
mics on a Decca Tree
over conductor’s head;
L, C, R
•two omnidirectional
mics wide and outside of
strings; L, R
4.5’
6’
Decca
Tree
•two omnidirectional
mics very high and rear
facing for Ls, Rs
•various omnidirectional
mics for spot pickup
end