Part 5 - Electroacoustic Devices - Handout

1Part 5a

Electroacoustic Devices:Microphones

ECE 211 - Broadcast Engineering & Acoustics (Electroacoustic Devices) 2

Microphone

z An elctroacoustic device that converts variations in sound pressure into electrical signals

z An electroacoustic transducer (sound to electricity)

2ECE 211 - Broadcast Engineering & Acoustics (Electroacoustic Devices) 3

Microphones According to Construction

z Dynamicz Condenserz Carbonz Crystal (Ceramic)z Ribbon


Dynamic mics

z Composed of a thin vibrating membrane (diaphragm) with a coil attached to it immersed in permanent magnetic field

z When the diaphragm moves in response to sound, the coil also moves in the magnetic field, thus inducing a current proportional to the sound

z Also called an electrodynamic mic


Condenser mics

z Has a moving plate (diaphragm) and a fixed, charged backplate which forms a capacitor

z When the diaphragm moves, the spacing between the two plates change, thus varying the electric field

z A condenser micrequires a polarizing voltage, sometimes from an outside source (phantom power)


Carbon mics

z Used in old telephone receivers

z Has a capsule with a diaphragm at one end, filled with carbon granules

z When sound strikes the diaphragm, it compresses the carbon granules, thereby changing the capsules resistance, and the current passing through it


Crystal mics

z Uses the piezoelectric effect

z Has a piezoelectric crystal attached to a diaphragm

z Vibrations from the diaphragm deform the crystal, generating a proportional amount of electric current


Ribbon mics

z A modification of the dynamic microphone

z Has a thin metal ribbon (diaphragm) immersed in a permanent magnetic field

z As the ribbon moves in response to sound, it disturbs the magnetic field, inducing a current through the ribbon

z Has a better response but a lower output


Directionality

z A microphone's sensitivity to sound relative to the direction or angle from which the sound arrives. Also called a pickup pattern.

z There are a number of different directional patterns found in microphone design. These are typically plotted in a polar pattern to graphically display the directionality of the microphone.

z The polar pattern shows the variation in sensitivity 360 around the microphone, assuming that the microphone is in the center and that 0 represents the front of the microphone.


Microphones According to Directionality

z Omnidirectionalz Cardioidz Supercardioidz Hypercardioidz Lobarz Bidirectional


Omnidirectional

z Picks up equally in all directions.z Traits of omnidirectional pickup

z All-around pickupz Most pickup of room reverberationz Not much isolation unless you

mike closez Low sensitivity to pops (explosive

breath sounds)z No up-close bass boost (proximity

effect)z Extended low-frequency response

in condenser mics. Great for pipe organ or bass drum in an orchestra or symphonic band.

z Lower cost in general


Cardioid

z Heart-shaped pattern that offers maximum rejection (null) at the rear of the microphone.

z A unidirectional microphonez Traits of cardioid pickup

z Selective pickupz Rejection of room acoustics,

background noise, and leakagez Good isolation--good separation

between recorded tracksz Up-close bass boost (except in

mics that have holes in the handle)

z Broad-angle pickup of sources in front of the mic

z Maximum rejection of sound approaching the rear of the mic


Supercardioid

z Another unidirectional micz Has a narrower pickup pattern

than cardioid, but also has some rear pickup. Note that there are two nulls of maximum sound rejection.

z Traits of supercardioid pickupz Maximum difference between

front hemisphere and rear hemisphere pickup

z More isolation than a cardioidz Less reverb pickup than a

cardioid


Hypercardioid

z Another unidirectional micz Has a narrower pickup pattern

than supercardioid, but also has more rear pickup than supercardioid. Note that there are two nulls.

z Traits of hypercardioid pickupz Maximum side rejection in a

unidirectional micz Maximum isolation - maximum

rejection of reverberation, leakage, feedback, and background noise


Lobar

z A very directional micz Has the narrowest pickup pattern,

but at the cost of sidelobes and a rear lobe.

z Traits of lobar pickupz Maximum on-axis pickup in a

unidirectional micz Used in applications such as

distant mic pickup stage plays, bird calls


Bidirectional

z Also called figure of eightz Picks up mainly in two directions

(in front of and behind the mic) and rejects sound from the sides.

z Traits of bidirectional pickupz Front and rear pickup, with side

sounds rejected (for across-table interviews or two-part vocal groups, for example)


Microphones According to Applicationz Microphones are generally used for either of

two applications:z Instrumentz Vocal

z Instrument microphones are usually constructed for specific musical instruments, with a response curve and polar pattern tailored for maximum effect.

z Vocal microphones are probably the most widespread, with the response suited for voice pickup.


z Vocal microphones can still be subdivided into three types:z Handheld the most common type. Mics which are

held to the mouth.z Lavalier also called clip-on mics. Used in

applications where mic pickup needs to be unobstrusive.

z Pressure-Zone Mics (PZM) also called boundary mics, used in surface pickup applications (table top, podium, etc.)

Microphones According to Application

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Part 5b

Electroacoustic Devices:Loudspeakers


Loudspeaker

z An electroacoustic device that converts electrical impulses into sound energy

z Loudspeakers work in the reverse manner as microphones do, and would therefore have several types according to how its construction. However, we would focus only on the dynamic type of loudspeakers, which is the most common type.

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Parts of a loudspeaker


Exploded View of a Loudspeaker

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Types of Loudspeakers according to Frequency Response

z Wooferz Speakers designed to reproduce low frequencies.

Generally are large in diameter because of the large wavelengths it must reproduce

z Tweeterz Designed to reproduce high frequencies and have

small diameters due to the wavelengths involvedz Midrange

z Also called a squaker, reproduces frequencies between bass and treble tones


Baffles

z Loudspeakers are inefficient radiators in free space

z Baffles are enclosures in which speakers are mounted to in order to increase the efficiency

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Why a loudspeaker isinefficient in free spacez When air from the high-pressure side of the

cone mixes with air from the low-pressure side, sound cancellation occurs.

z At high frequencies, the sound is directional, so little mixing occurs, but for frequencies at which the wavelength is long compared to the diameter of the speaker, the waves can curve back around the cone so that the out-of-phase waves mix.

z One of the basic requirements of a speaker enclosure is to block this unwanted mixing of out-of-phase waves.


Types of baffles

z Flat or Infinitez Closed Boxz Ported Boxz Labyrinth and Transmission Linesz Horns

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Flat or Infinite

z The most basic type of baffle, used to prevent phase-cancellation of the high- and low-pressure sides of the speaker.

z Most commonly found in cars, with the trunk space used as a resonating chamber.


Closed-box

z Literally a closed-boxz When a speaker is installed in a

closed box, the air in the box acts as a spring against the cone, particularly if the box is small.

z The smaller the box, the stiffer the air spring.

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Ported-box

z Also called a bass-reflex due to the added bass response from a similarly-sized closed-box speaker.

z A modification on the closed-box, with an added hole (port) that acts another piston (called a Helmholtz resonator).

z At specific frequencies, the air inside the port moves in-phase with the cone, providing more punch.


Labyrinths and Transmission Lines

z A labyrinth is a tuned pipe with the driver at one end and a port at the other end.

z At the frequency where the length of the pipe is equal to l/4 of the sound, the air at the mouth of the pipe is at minimum velocity but maximum pressure.

z Transmission lines are stuffed labyrinths.

z Transmission lines have obvious disadvantagesz they are largez require a complicated structurez can be unpredictable

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Horns

z Acts as an acoustical transformer.z It matches the high acoustical impedance at the driver

to the low impedance of the room air by its smooth rate of increased cross-sectional area from the driver cone to the horn mouth

z Because of its impedance-matching characteristics, a horn offers much higher efficiency than other types of speaker enclosures.

z Disadvantagesz The size required for a bass horn is tremendous; some are 30

feet long.z Because of their size and complexity, horns are found chiefly

in theater sound systems or other high-level applications.


Crossover Networks

z Due to the size-frequency relationship of speaker, no one speaker can faithfully reproduce the entire spectrum.

z Several speakers are needed, with the corresponding frequency range, to accomplish this.

z In order for the speakers to be efficient, they need to be bandpassed to their respective ranges.

z Crossover networks are simply filters that tune the response for a specific speaker.

z Crossovers can either be passive (composed of capacitors, inductors, placed in the baffle) or active (uses active components, placed before the amplifier)

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z Two-way systemsz Composed of a woofer and a tweeter, with a low-pass and a

high-pass filter.z Has the crossover point at a frequency common to the two

speakers.z Also called bi-amped systems when active filters are used,

with one amplifier for the woofer and another for the tweeter.z Three-way systems

z Uses a woofer with a low-pass filter, a midrange with a bandpass filter, and a tweeter with a high-pass filter.

z Has two crossover points, one for the woofer-midrange, an for the midrange-tweeter.

z In tri-amped systems, uses three separate amplifiers

Crossover Networks

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Part 5 - Electroacoustic Devices - Handout