Microphone Basics

Rick Pitchford, CSRTE

Guest Lecture, COM 494

February 2, 2015

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

Microphone Types:

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

•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

Moving Coil (Dynamic) Transducer:

image from “First Class Radiotelephone License Handbook”, fourth edition, ©1974, Howard W. Sams Co., Inc.

•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.

Condenser Transducer:

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.

images from “ksm44_en.pdf”, Shure Brothers, Inc.

omni

uni

bi

Diaphragm

Housing

•sensitive from all directions

•has a sealed housing behind the diaphragm

•positive pressure from any direction pushes the diaphragm in

•most natural sounding mic pattern due to uniform frequency response

Sound Waves

Sound Waves

Omnidirectional Mics:sensitive

sensitive

sensitive

sensitive

•more sensitive to the front

•has a ported housing behind the diaphragm

•positive pressure from front pushes the diaphragm in

•positive pressure from rear enters ports, equalizes on both sides of the diaphragm, producing no motion

•frequency response not uniform off axis

•usually exhibit “proximity effect”

Housing

Diaphragm

Port

Sound Waves

Sound Waves

Unidirectional Mics: The Cardioid:sensitive

sensitive

sensitive

sensitive

null

More Unidirectional Mics:

supercardioid

hypercardioid

Housing

Diaphragm

Port

Housing

Diaphragm

Port

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.

+ -

+ -

sensitive

null

null

sensitive

sensitiveinverted

sensitive

null

null

sensitive

sensitiveinverted

•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

Diaphragm

Sound Waves

Sound Waves

Ribbon Bidirectional Mics:

+ -null

null

sensitivesensitiveinverted

•formed by two identical unidirectional mic capsules mounted back-to-back

•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

Sound Waves

Diaphragms

PortsCondenser Bidirectional Mics:

+ -null

null

sensitivesensitiveinverted

•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

Diaphragm Housing

Boundary

Hemispherical Mics:sensitive

null

sensitive

Interference Tube Mics:

sensitive

sensitive

sensitive

null

sensitive

null

null

null

•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

Housing

Diaphragm

Slotted Tube

•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

•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

leve

lComb Filtering:

•the same time delay has the exact opposite affect on these two frequencies

•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

•at frequencies in between, the interaction will vary from reinforcement to cancellation

•1msec = 1 foot additional acoustic path length

leve

l

time

leve

l

time

time

leve

l

1 msec.

1000 Hz.

leve

lle

vel

time

time

1 msec.

time

leve

l

500 Hz.For Example,

+

=

+

=

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:

frequency

leve

l

•place the mic in such a position that reflections are delayed only slightly; smaller delays push the first null point to higher frequencies

•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

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:

image from “us_pro_beta57a_ug.pdf”, Shure Brothers Inc., 2004

~17dB change at 100Hz, from 1/8 inch to 24 inches

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.

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:

•“universal”Lavalier:

•spoken voice•some instruments

Body:•stage or screen

Shotgun:•high-noise situations

Headset:•hands-free uses•sports commentators•performers

Boom:•instrument•studio voice

Parabolic:•sports sidelines•surveillance

Desk:•interview

Boundary:•complete room coverage•conferences

Transducer Selection:

Moving Coil:•extremely loud sounds•sports remotes•emergency/backup mic•general purpose•bad weather situations

Condenser: •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, low-noise 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 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.

Multiple Mics for Mono and Stereo:

•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

Sound Source

8 ft. 25 ft.

Spaced Pair (A-B Pair):

L (C) R

•crossed or head-to-head 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

Sound Source

Coincident Pair (X-Y Pair):

R L

+ +- -

ORTF•cardioid microphones at an angle of 110°•spaced about 6.5 inches apart •susceptible to comb filtering•recreates a head-sized sound field

6-7”

Coincident Pair Deviations:

Blumlein Pair •developed in 1931

•crossed bi-directional mics placed head-to-head•not susceptible to comb filtering

•recreates a sweet spot

Sound Source

Sound SourceL R

L R

Sound Source

•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

MID mic is UNIDIRECTIONAL facing forward

SIDE mic is BIDIRECTIONAL facing left/right

L+R

L -R

Mid/Side (M-S):

+ -

•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

DRUMS

HORNS

Multi-Mic:

PIANO

+ +- -

STRINGS

•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

DRUMS HORNS

GU

ITA

RS

VOCAL 1

VOCAL 2KEYBOARDS

Close Mic:

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

•recreates the sound field in two dimensions; listener can move around to change aural perspective

L, R, Ls, Rs C

Coincident Surround: •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

•stereo and mono compatible; no comb filteringL, C, R, Ls, Rs LFE

Sound Field

MSM Surround Array:

LF+RF

L -R

MID mics are UNIDIRECTIONAL facing forward and rearward

SIDE mic is BIDIRECTIONAL facing left/right

+ -

LR+RR

Sound Field •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

•stereo and mono compatible; no comb filtering

“Sports Event Capture Array-2”•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 filtersL, R Ls, Rs

+ -

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, Rsimage 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

•announcers mics to Center onlyL, R, Ls, Rs

image from http://www.wstickets.com/_venues/oh/cleveland_browns_stadium.gif

World Series on Fox:

image from http://www.ticketsreview.com/baseball/cleveland_indians_tickets/progressive-field_7397033.php

•parabolic 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

•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

NBA:

image from http://www.nba.com/cavaliers/sites/cavaliers/files/elements/

Other Sporting Events:Golf:•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

Mixed Martial Arts:•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”

highstrings

woodwinds

lowstringswoo

dwin

ds

percussion brass

•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

•two omnidirectional mics very high and rear facing for Ls, Rs

•various omnidirectional mics for spot pickup

4.5’ Decca 6’ Treeall mics

end