Sound Recording Theory

Sound Recording
Theory
Class 5
the loudspeaker
The Loudspeaker
A transducer which transforms electrical
energy to mechanical and eventually acoustic
energy
l  Three main types:
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Moving coil loudspeaker – by far the most
commonly used
Ribbon loudspeaker
Electrostatic loudspeakers
The Moving Coil Loudspeaker
Moving Coil
Loudspeaker
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Also called a Dynamic Loudspeaker
Applies the same basic principles of the dynamic microphone
A coil (called the voice-coil) through which the electric audio signal
flows
Placed in a socket within a permanent magnet
Connected to a cone diaphragm (made from paper or other
material)
Through a suspension system (spiders, surround, air pressure)
A multi-way speaker system is made out of several speakers
(drivers), each optimized to a certain frequency range of the
spectrum, with the audio signal being divided by electronic circuitry
(a crossover network)
Ribbon Loudspeaker
Similar in concept to the
ribbon microphone
l  Seldom used
l  Usually used for tweeter or
midrange drivers.
l  Example: Edition 626R
ribbon loudspeakers (see
photo): approx $4500 for pair
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Electrostatic Loudspeaker
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Same principle as the condenser
microphone; they need a power
supply to polarize the plates
Not very efficient in the low
frequencies, therefore requires a
subwoofer
However, its bass inefficiency
changes in different power levels,
therefore a “smart” crossover is
necessary
Ex. Final Sound Model 500
($4500), see photo
Mostly used for classical music
Resonant Frequency
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Each substance, object, and space have their own
natural vibrating frequency (or resonant frequency)
In a loudspeaker, Resonant frequency is the
frequency at which it has maximum efficiency.
With a higher mass of the mobile section (a larger
membrane and moving coil), the resonant frequency
is lower.
With a stiffer suspension the resonant frequency is
higher.
For a flat response we prefer to have the resonant
frequency outside the human hearing range
Transient response, damping/
compliance
Transient Response: the accuracy in
reproducing sounds with a short duration and
a short attack (transients – or percussive
sounds)
l  Damping: some form of acoustical,
mechanical or electrical resistance. Damping
improves transient response and makes the
resonant frequency higher
l  Compliance: the ease at which a speaker’s
diaphragm moves (inversely-proportional to
damping
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Damping / compliance
Radiator types
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Direct radiator: a loudspeaker driver, with its
diaphragm directly coupled to the air in front of it
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Indirect radiator: coupled to the surrounding air by
way of a horn
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Largely inefficient, 5% efficiency at most
Through the acoustic transformation of the horn, indirect
radiators are more efficient
The dispersion pattern has a narrower angle, especially in
the highs
Passive Radiator: an unpowered loudspeaker
cone, also called a drone cone or slave cone (no
audio signal goes through it, but it allows lower
frequencies out of a speaker acoustically)
Indirect radiators
A compression driver: a specialized high
or mid frequency loudspeaker with a small
domed diaphragm usually attached to a
directional horn which constricts the volume
of air (the compression) – very efficient, but
good ones are very expensive.
l  Folded Horn – for low frequency, a large
horn is necessary; for space considerations
the horn is folded inside the speaker
enclosure
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A Direct Radiator
A Compression Driver
Gemini GX350 Powered 2-Way Loudspeaker
Folded Horn
JBL D30085 Hartsfield Circa 1957
La Scala Floorstanding Loudspeaker
Crossover
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Passive crossover: commonly used, made of simple low and
high pass filters, placed between the power amp and the
speakers.
Active crossover : placed between the line-level preamp and
the amplifiers; it requires each speaker driver to have its own
power-amp.
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They may be external to the loudspeaker enclosure, or internal in
powered systems
All crossover networks introduce some level of phase shifting
Active crossovers are more tweakable but introduce more
electronic noise and phase shifting.
Passive crossover
Active crossover
External Active Crossover
SL-AX-1101H (1-ch, 1-way subwoofer electronic crossover)
Powered Speakers
2 or 3 way speakers integrate an active filter
with 2 or 3 amplifiers in the speaker’s
cabinet.
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are:
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Consistency, because the amplifier and
loudspeaker are perfectly matched
No crosstalk between channels
They utilize internal crossovers (passive or
active)
Powered Speakers
Recommendations
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A good current review of high end studio monitors:
http://www.junodownload.com/plus/2011/06/07/10-best-studio-monitors/
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Adam A7X Monitors – great highs
Mackie HR824 MK2 – full sound
Genelec 8030A Bi-Amplified Monitoring System - great for its size (near field)
Sonodyne SM 50Ak - not bad on bass, great mids (near field)
Fostex PM2 MKII – great for larger studios
Focal CMS 40 – sound great, need a sub
KRK Rokit RP8 G2 – great for electronic music
Tannoy Reveal 501A – excellent for the price (near field)\
M Audio Studiophile CX5 – Also sound great for the price (high freq response)
Speaker enclosures
Infinite baffle
l  Sealed baffle
l  Acoustic Suspension
l  Open baffle
l  Bass-reflex
l  Passive radiator
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Infinite Baffle / Sealed Baffle
A complete acoustical separation between
the front and rear of the speaker, so the outof-phase signals cannot interact
l  Theoretically, this would be achieved by
mounting the driver on an infinite wall (hence
the name)
l  Practically, it is created by ceiling or wall
mounted speakers, or a sealed encasing of
the speaker driver
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Infinite Baffle
Acoustic Suspension
Edgar Villchur, 1954
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Developed by Edgar Villchur, 1954 to solve the problem of the high
bass distortions in all (sealed enclosure) speakers of the day
Speakers had to be 14 feet tall to produce a reliable 40 Hz tone
He got rid of the mechanical suspension (springs) and used the air
pressure in the sealed baffle as an air spring in his AR-1 acoustic
suspension system
The enclosure had to be small to have sufficient air pressure
Air turned out to be a much more linear spring, and the response in
the low frequencies was very flat
In his AR-3 design Villchur included a tweeter and solved the highfrequency problem of his design (1958)
Nowadays, practically all sealed baffles use acoustic suspension
Acoustic Suspension
Villchur AR-3, 1958
Open Baffle
A speaker without an enclosure – phase
cancellations – most strongly off axis, and
much more noticeable in the low frequencies
l  Different levels of separation between front
and rear can be created to regulate the bass
response:
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Mounting a speaker on a panel
A partially open resonance box– like in the back
of a guitar amp (but not bass amps)
Open Baffle
Traynor YCV 50 blue, guitar tube amp
Nola, Viper Reference with sealed woofer and open tweeters
Bass Reflex
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Small sealed enclosures are limited in producing low
frequencies (below 60 Hz)
In vented / ported designs, the otherwise sealed enclosure has
venting holes in the front or back.
The ports allow the enclosure to resonate below the woofer’s
roll-off (its low end), and gives an equalization bass-boost
+: This brings the frequency response to a lower frequency
-: The open ports reduce the amount of damping and therefore
the transient response.
-: open ports in the rear are problematic if the speaker is close
to a wall, because of increased bass response
Bass Reflex
M-Audio EX66
Passive Radiator
They are made of a "weighted diaphragm"
and a "spring"
l  The weight and springiness of the diaphragm
determines its resonance frequency
l  They are tuned to resonate below the
woofer’s roll-off (like the venting ports)
l  Unlike the port, the acoustic suspension is
maintained and damping is usually better.
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Passive Radiator
Thiel CS 2.3
Cone / Dome Driver
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Most woofers are designed as cone radiators
In tweeters, cone drivers are much too small and have a narrow
dispersion angle
Most tweeters used dome (metal or soft)
Dome tweeters allow a larger voice coil and have a wider
dispersion angle
A typical one inch dome tweeter begins to be directive at about
8000 Hz, below this frequency it is quite wide (like a point
source).
Speaker Specifications
Frequency / amplitude response
l  Time coherence / phase response
l  Imaging / soundstage
l  Power rating / Power Handling
l  Sensitivity
l  Dispersion Patterns
l  Impedance characteristics
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Frequency / amplitude
Response
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output or volume of the loudspeaker over the entire
frequency range (ex. 50 Hz -22 kHz +- 3 dB)
flat frequency response – a speaker that uniformly
reproduces all parts of the frequency spectrum (with
variations up to 3 dB)
Several points to consider
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Flat frequency response with a microphone in front of the
tweeter, does not mean that from the listening angle it will
still be flat
Most speakers today have a flat on-axis response
Off-axis response is therefore more important
Time Coherence
The acoustical alignment of the sounds
propagating from the different drivers
l  In most speakers, the voice coils of the
different speakers are aligned differently
l  If they are not well aligned, transient
response, clarity of detail, and as a result
spatial coherence will be compromised
l  Good speakers are “time-coherent”
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Phase Response
The electric waves that reach the drivers may
be in phase or out of phase
l  Although similar to time alignment, it is the
result of the electronic circuitry – the
crossover
l  Good crossovers have a high phase
response
l  Problems of time coherence and phase
response can be solved acoustically (as in
the previous slide) or electronically by
introducing electronic delays in the signal
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Imaging / Soundstage
The ability of the speakers to recreate a clear
sense of space
l  Some use the terms as follows:
l  Imaging – the spatial coherence on the L-R
continuum
l  Soundstage – the ability to recreate a sense
of depth (front-rear)
l  Imaging is strongly reliant on time coherence
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Power Handling
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The continuous electrical power a loudspeaker can withstand
without degradation
Measures in RMS (Root Mean Square) value – a statistical
measure of a varying amplitude
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Calculated as the square root of the mean of all the squared values
The reason RMS is used and not simple mean, is because an RMS
value is more weighted towards the mean, because it gives a better
indication of loudness (less affected by sudden peaks)
It is generally considered as the continuous power level that a
speaker can take
Measured in Watts
Ex. 60W RMS
It is important to match the power amp with the loudspeakers.
Counterintuitively, under-matching power amps are more
dangerous to the speaker, because they are more likely to be
overloaded and introduce damaging distortion.
Sensitivity
Speakers are in general very inefficient in
energy conservation
l  Efficiency is measured as the ratio of output
power (acoustical) to input power (electrical)
l  Example: 90 dB SPL for 1 W @ 1 m (in a free
field)
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Sensitivity example
Yamaha NS-10m:
l  90 dB SPL for 1 W at 1 m
l  The power handling is 60 W
l  Find the maximum dB SPL we can get with
these speakers at 1 m and at 2 m.
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Exercise answer
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10 log (60 W / 1 W)
10 log 60
Log 60 = log 2 + log3 +
log 10 = 0.3 + 0.48 + 1
= 1.78
10 X 1.78 = 17.8 dB
90 + 17.8 = 107.8 or
108 dB SPL at 1 m
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20 log (2 m / 1 m)
20 log 2
20 X 0.3 = 6 dB SPL
108 - 6 = 102 dB SPL
at 2 m
Dispersion Pattern
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ideally, a loudspeaker should disperse sound widely
and evenly throughout the listening area enabling all
listeners to hear the same sound quality.
problems arise in directionality at the crossover point
between drivers.
Dependant on placement on the baffle and
crossover design
A solution would be speakers with wide band range
which do not need steep slope crossovers
Crossover design – the simpler the better
Normally in a two way nearfield studio monitor, we
are looking for a low crossover frequency (around 2
kHz); 3Khz or more are usually indicative of steeper
filters, and of off-axis coloration
Dispersion Pattern
The Studio Monitor
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Studio Monitors – perhaps the second most
important part of mixing (after room acoustics)
The response, texture, and quality of the sound is
affected by:
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The drive units construction
The speaker enclosure
The acoustics of the room
Studio monitors are expected to be flat in a
sufficiently wide listening sound field area
Monitor Uses
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The monitoring demands are different throughout the industry:
broadcasting, television, film, video
Broadcast: the control rooms are usually small and simulate the
home environment
Film: monitor in a theatre system
Recording Studio: varied, often have 3 separate monitoring
systems; this is a result of the many possible listening mediums
(car, home, headphones, etc.)
l  Some engineers prefer to work with nearfield monitors, some
with the main monitors
l  The main monitors will be more influenced by the room acoustics
l  The nearfield, are closer to car or boombox mixes
Room Characteristics
(control room)
Normally between “dead” and moderately live
l  Depends on:
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Shape
Building materials
The equipment’s build and material (on absorption
and reflection)
Monitor Positioning (Main)
Monitor Positioning (Nearfield)
Monitor positioning
Traditionally, the main monitors are
positioned at a 60o angle to the listener
l  They are elevated to the listening plane, with
the tweeters at the ear level or above (above
the vision window when necessary, but
pointing toward the listener)
l  In room designs that require the console to
be close to the monitors, the horizontal
angle of coverage (the dispersion pattern)
must be larger
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Nearfield Monitor Positioning
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About a meter away from the listener
They must be positioned on solid stands
At least 60 cms away from any wall, especially if
they have a rear venting port
Away from any reflecting surface (walls, mirrors,
tables)
Far from interfering devices (hums, radio, crt
monitors)
If your nearfield monitor stands close to a sidewall,
you are likely to have midrange coloration
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you may have to point the monitor straight in a 90o angle
(and therefore loose highs)
You can pad the sidewall
Sound source directivity
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A sound source may be:
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Point source
Line source
Planar source
3D source
Point source (ideal)
(Theoretical), an infinitely
small source which
radiates all frequencies
equally in all directions
(like a natural source).
l  Some manufacturers tried
to emulate it (see the
Hemisphere picture)
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(Ideal) Point source
SPL falls at a rate of 6dB for each doubling of
distance
l  The sound emanates in a spherical shape
l  The surface area of the sphere is
proportional to the distance (radius) squared
(4πr2)
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Room Acoustics, Point Source
dB SPL is dispersed equally in all angles
SPL is increased by 6dB
Room Acoustics, Point Source
SPL is increased by 12 dB
SPL is increased by 18 dB
Room Acoustics, loudspeaker
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Since the dispersion pattern of a loudspeaker gets
narrower with higher frequencies, being positioned
near a wall the loudspeaker’s sound will be
reinforced in the following manner:
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Bass – from any direction
Midrange – side walls, bottom (desk), and top (ceiling)
Highs – front only; however, tweeters have a strong off-axis
attenuation due to their limited dispersion angle
Line Source
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A group of point sources arranged in a
line (A highway could be considered a
line source)
Radiates a sound in a cylindrical pattern
SPL falls at a rate of 3dB for each
doubling of distance
The result is that highway noise reaches
farther than a point source sound.
This is also why in big concerts piling of
speakers on top of each other works
well, especially when they are horizontal
(aligning the sources)
Line array systems
A-line AL 10
l  L-acoustics System Kudo
l  Examples: http://
srforums.prosoundweb.com
/index.php/m/265687/0/
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The Loudspeaker as sound
source
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While point source is considered ideal, in reality
loudspeakers are complex 3D sources (the cone
and dome shape)
Some are planar sources (flat diaphragms)
Planar and 3D sources have a complex relationship
with the room acoustics, because each part of the
sound source has a different angle and distance
from the reflecting surfaces.
Moreover, multiple drivers complicate the sound
source by providing two or more separate sources,
each being acoustically affected by the room (+
complex phase relations).
Coaxial speaker system
Coaxial Speaker System
A tweeter is mounted inside the woofer
l  This design attempts to approximate a single
point source
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Loudspeaker impedance
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Normally 4,8, 15, or 16 Ohms
Loudspeaker impedance varies with frequency,
therefore the amplifier signal cannot be accurate
throughout the spectrum; the speaker cables
introduce an additional impedance factor.
Amplifier’s impedance are usually very low, to allow
the loudspeaker to have higher impedance (and not
bring the amplifier to overload and clip)
In powered monitors, impedance matching is not an
issue because they have their own dedicated (and
matched) amplifiers
Loudspeaker Impedance –
some points
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8 ohm nominal impedance specifications – meaning approximate (because
the impedance is different across the frequency range)
Impedance can actually vary quite a bit (somewhere between 5 – 20 Ohm
easily)
Lower impedance is the problem (the speaker being driven more than
expected)
Adding more speakers decreases impedance (two speakers of 6 Ohm
means 3 Ohm)
Too low an impedance will cause an amplifier to clip – this can either burn
the amplifier or the speaker (because the DC produced by clipping)
Watch out carefully when working with speakers and power amps on stage.
It is easy to lose track of how many speakers are going on an amplifier.
Learn the impedance frequency response curve!
See article:http://www.churchsoundcheck.com/imp1.html
Line source loudspeakers
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http://www.wisdomaudio.com/sage_seriesline-source-advantages.php