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Jeremie HuscenotJanuary 8, 2000
ENSEA conferenceLoudspeaker driver Loudspeaker enclosure
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2Confidential Divison, MMMM dd, yyyy, Reference
What is sound?Air molecules
• “The room contains a huge number of air molecules, but there is still a lot of empty space between the molecules in the room. The molecules are moving in random directions at an average speed of over 1000 miles per hour. Each molecule has about 5 billion collisions per second.
• The molecules are colliding with my eardrums. The collisions on one side of my eardrum are precisely balanced by collisions on the other side. The eustachian tube equalizes the pressure on either side of the eardrum as long as pressure varies slowly. My eardrums don't move, and I hear nothing.
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3Confidential Divison, MMMM dd, yyyy, Reference
What is sound?Pressure
Sound is typically described as a small rapid variation in pressure. This is one part of the story, but there is another effect
A variation of the average molecular velocity (propagation)
http://www.kettering.edu/~drussell/demos.html
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4Confidential Divison, MMMM dd, yyyy, Reference
Why do we need a loudspeaker ?
• A driver is made to set air in motion
• Several technologies available– Electro-dynamic– Electrostatic– Piezoelectric– Ribbon– Distributed Mode Loudspeaker– …
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5Confidential Divison, MMMM dd, yyyy, Reference
HistoricBeginning of the story
• 1874 - Ernst W. Siemens was the first to describe the "dynamic" or moving-coil transducer, with a circular coil of wire in a magnetic field and supported so that it could move axially (not use for audibletransmission)
• Alexander Graham Bell patented the first electrical loudspeaker.
• The modern design of moving-coil drivers was established by Oliver Lodge in 1898
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6Confidential Divison, MMMM dd, yyyy, Reference
HistoricApplications
• 1926 - Vitaphone sound system for motion pictures used a new speaker developed at Bell Labs.
• In the 1930s, loudspeaker manufacturers began to combine two and three bandpasses worth of drivers in order to increase frequency response and sound pressure level. (Bell Labs)
Multi-way loudspeaker
• In 1937, "The Shearer Horn System for Theatres“ (a two-way system) was introduced by Metro-Goldwyn-Mayer (the first film industry standard)
• At the 1939 New York World's Fair, a very large two-way public address system.
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7Confidential Divison, MMMM dd, yyyy, Reference
HistoricAltec Lansing
• 1941 - Altec Lansing Corp. was formed when Altec bought Lansing.
• Altec introduced their coaxial Duplex driver in 1943
• 1945 put on the market the 2-way "Voice of the Theater" speaker system with improved horns and magnet drivers.
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8Confidential Divison, MMMM dd, yyyy, Reference
HistoricNext evolutions
• Fundamentals were set
• Continuous developments in design and materials led to significant audible improvements
• The most notable improvements in modern speakers are – cone materials– introduction of higher temperature adhesives– improved permanent magnet materials– improved measurement techniques– computer aided design– finite element analysis.
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9Confidential Divison, MMMM dd, yyyy, Reference
Electro-dynamic loudspeaker driver
A driver is composed of
• a moving mass• a motor• a suspension
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10Confidential Divison, MMMM dd, yyyy, Reference
Motor
Is composed of the
• the pole• the voice coil• front and rear plates• the magnet
To set the motion
PoleVoice coil
Front plate
Rear plate
MagnetAir gap
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11Confidential Divison, MMMM dd, yyyy, Reference
Motor
• How?– Permanent magnetic field: B– AC signal in the voice coil: i– Force perpendicular to B and I: F
• Main parameter: Bl factor– Measure the “strength” of the motor (Magnetic field x length of
the voice coil) Unit: Tesla-meter
• Non-linearity given by the geometry of the pole and the front piece (air gap), length of the voice coil,…
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12Confidential Divison, MMMM dd, yyyy, Reference
Bl non-linearityAir gap geometry influence
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13Confidential Divison, MMMM dd, yyyy, Reference
Bl non-linearityLength of the voice coil influence
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14Confidential Divison, MMMM dd, yyyy, Reference
MembranePiston mode
• Low limit given by the driver’s resonance frequency
• High limit given by the radiation impedance
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15Confidential Divison, MMMM dd, yyyy, Reference
Membrane
But the truth is out there:
• Resonance modes: The membrane is not infinitely rigid
• Directivity: all drivers become more directive with frequencies, according to the ratio λ (wavelength) / D (driver diameter)
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16Confidential Divison, MMMM dd, yyyy, Reference
Membrane non-linearity
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17Confidential Divison, MMMM dd, yyyy, Reference
Membrane directivity
• The directivity becomes significant when λ(wavelength) ≈ ½ membrane circumferenceλ ≈ 3,1416 x membrane radius.
• Examples:380mm driver diameter: λ ≈ 0,60 m f ≈ 570 Hz. 20mm driver diameterλ ≈ 0,031 mf ≈ 10 800 Hz.
Tweeter directivity (with enclosure)
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18Confidential Divison, MMMM dd, yyyy, Reference
Suspension
Is composed of
• The surround– To center the membrane in the pole piece – To damp the membrane’s resonances
• The Spider– Compliance of the system (1/rigidity)
• Non-linearity given by geometry and the material of the surround and the spider
SurroundSpider
Spider
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19Confidential Divison, MMMM dd, yyyy, Reference
Transformers
• A driver is composed of two transformers, to convert an electrical signal to a acoustic signal
– 1st transformer: Electric Mechanic– 2nd transformer: Mechanic Acoustics
• The motor connects the electrical and the mechanical “worlds”. The parameter is the “strength” of the motor: Bl
• The membrane connects the mechanical and the acoustic “worlds”. The parameter is the “projected” area of the driver diaphragm: Sd
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20Confidential Divison, MMMM dd, yyyy, Reference
Thiele and Small parameters
• Almost all the parameter needed to describe a loudspeaker driver were set by others scientists.
• In the 60’s, Thiele and Small collected all these parameters and defined a complete methodology to deduce them from the impedance measurement.
• These parameters are called T&S parameters.
• To deduce all the T&S parameters, two impedance measurements with two specific conditions are needed.
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21Confidential Divison, MMMM dd, yyyy, Reference
Driver Impedance measurement
• Low frequencies: – Impedance module = driver resistance at DC– Phase is about 0 deg
• When frequency is increasing (below the resonance frequency)
– Impedance is increasing– Phase angle is positive
• At fs = mechanical resonance frequency
– Impedance is maximal– Phase equal zero
Impedance curve
fs : resonance peak
Frequency response
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22Confidential Divison, MMMM dd, yyyy, Reference
Driver Impedance measurement
• Above the resonance frequency– Impedance is decreasing– Phase angle becomes negative
• High frequency– Impedance is increasing
effect of the voice coil – Phase angle is positive
Impedance curve
fs : resonance peak
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23Confidential Divison, MMMM dd, yyyy, Reference
T&S parametersSmall signal • Small signal means linearity assumption between input and output
• Re - DC resistance of the voice coil, measured in ohms.
• Le - Voice coil inductance measured in millihenries (mH) (Frequency dependent, usually measured at 1 kHz).
• Bl - The product of magnet field strength in the voice coil gap and the length of wire in the magnetic field, in tesla-metres (T·m).
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24Confidential Divison, MMMM dd, yyyy, Reference
T&S parametersSmall signal • Mms - Mass of the diaphragm, including acoustic load, in kilograms.
• Cms - Compliance of the driver's suspension, in metres per newton (the reciprocal of its 'stiffness').
• Rms - The mechanical resistance of a driver's suspension (ie, 'lossiness') in N·s/m
• Sd - Projected area of the driver diaphragm, in square metres.
Mind that Mms = Mmd + air mass in front of the membrane
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25Confidential Divison, MMMM dd, yyyy, Reference
‘Useful’ T&S parameters
• Fs – Resonance frequency of the driver
• Qes – Electrical Q of the driver at Fs
• Qms – Mechanical Q of the driver at Fs
• Qts – Total Q of the driver at Fs
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26Confidential Divison, MMMM dd, yyyy, Reference
‘Useful’ T&S parameters
• Vas – Volume of air (in cubic metres) which, when acted upon by a piston of area Sd, has the same compliance as the driver's suspension. To get Vas in litres, multiply the result of the equation below by 1000.
Where ρ is the density of air (1.184 kg/m3 at 25 °C), and c is the speed of sound (346.1 m/s at 25 °C).
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27Confidential Divison, MMMM dd, yyyy, Reference
Electrical analogyDriver
Front wave of the driverBack wave of the driver
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28Confidential Divison, MMMM dd, yyyy, Reference
Acoustic Short circuit
• Front and back waves must be separated, especially when the driver is omni-directional
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29Confidential Divison, MMMM dd, yyyy, Reference
First ideaA baffle
• Easy to built • Not “really” optimized for a real living room
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30Confidential Divison, MMMM dd, yyyy, Reference
EnclosureClosed box
• Back waves enclosed in a specific volume
Electrical analogy: volume ≡ capacitor
High pass filter (1st order)Volume dimensions dependent
Back volumeDriver
Box
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31Confidential Divison, MMMM dd, yyyy, Reference
EnclosureVented box
• Closed box architecture + a tube connects the inside of the box to the outside
Electrical analogy: volume ≡ capacitortube ≡ induction coil
High pass filter (2nd order)Volume and vent dimensions dependent
Back volumeDriver
Box
“Vent”“Bass Pipe”
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32Confidential Divison, MMMM dd, yyyy, Reference
Closed box VS vented box
• Simulation with WinISD: Green (vented box) and yellow (closed box)– Additional resonator– 2nd order versus 1st order
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33Confidential Divison, MMMM dd, yyyy, Reference
A little bit of theory…
fc proportional to1/√(LC)
From Loudspeaker and Headphones handbook by John Borwick
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34Confidential Divison, MMMM dd, yyyy, Reference
Electrical AnalogyBass reflex enclosure
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35Confidential Divison, MMMM dd, yyyy, Reference
EnclosurePassive radiator• Instead of using a tube to create an resonator, another driver is used
Volume in series and passive driver acted as a resonator
High pass filter (2nd order)Volume and passive radiator parameters dependent
Driver Volume
Box
Passive “driver”/radiator
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36Confidential Divison, MMMM dd, yyyy, Reference
Enclosure effect
• Vibrations
• Diffraction
• Standing waves
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37Confidential Divison, MMMM dd, yyyy, Reference
Vibrations
• Vibrations create noise• Small vibrations on a large surface area
B&W matrix
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38Confidential Divison, MMMM dd, yyyy, Reference
Diffraction
• Edges create secondary sound sources that interfere with the primary sound source (driver)
• Influence the directivity of the driver
B&W enclosure
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39Confidential Divison, MMMM dd, yyyy, Reference
Standing waves
• Made of two waves (incident and reflected)
Enclosure top view
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40Confidential Divison, MMMM dd, yyyy, Reference
Multi-way loudspeaker
• Objective: get a flat and broad frequency range (measured at 1 meter with a 1 Watt signal)
• Audible range 20Hz – 20kHz impossible to get this frequency range with a single driver.
• Different drivers for different applications
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41Confidential Divison, MMMM dd, yyyy, Reference
Multi-way loudspeakerDifferent drivers for different applications
1”≈2.5cm
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42Confidential Divison, MMMM dd, yyyy, Reference
Different type of loudspeaker
Woofer (Peerless)Tweeter (Timphany)
membrane
Front plate
Mid-range
Full-range (Visaton)Subwoofer (Alpine)
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43Confidential Divison, MMMM dd, yyyy, Reference
Electrical crossover2-way system
Tweeter
Woofer
Enclosure
SPL
frequency
fc
Used to separate the high frequencies form the bass frequencies
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44Confidential Divison, MMMM dd, yyyy, Reference
Electrical crossover
• Is ART… ☺It is the last step of the development of a loudspeaker.Could be seen as the loudspeaker conductor!
• At this step, the objective is to make a loudspeaker to sound goodWith the best loudspeakers ever made and a “wrong” electrical crossover, sound quality won’t be good.
• An electrical crossover is made for a specific loudspeaker driverElectrical and Acoustic part need to be closely evaluated to get good performance
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45Confidential Divison, MMMM dd, yyyy, Reference
Analog crossover
• Between amplifier and loudspeaker
• Crossover in parallel with the driver, each driver is separately filtered (gives flexibility)
• Filter types: RLC network– High Pass, Low Pass, Band-pass
• Alignment:– Butterworth, Bessel, …
• Around fc both drivers are playing at the same frequency and the same time
Interferences
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46Confidential Divison, MMMM dd, yyyy, Reference
Digital crossover
• Between pre-amplifier and amplifier
• “Almost” everything is possible– phase, frequency response, … independently adjustable
• But it is expensive compare to analogue solution
• Other advantage:– Less inter-modulation distortion in the amplifier.
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47Confidential Divison, MMMM dd, yyyy, Reference
A hifi loudspeaker
Model: QuartetRange: GeneseBrand: Triangle
Tweeter
Mid-range
Woofers Vents
Woofers volumes
Mid-range volume
Cross-section of an hi-fi loudspeaker
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Bibliographyweb site• In English
– Acoustic phenomena• http://www.kettering.edu/~drussell/demos.html
– Loudspeaker design• Linkwitz: http://www.linkwitzlab.com/
– A lot of thing…• Art Ludwig: http://www.silcom.com/~aludwig/
– Headphones:• Headwise
• In French– Loudspeaker driver:
• http://hyperbol.free.fr/Sommaire/sommaire.htm• http://www.subaudio.org/hautparleur.html#hp_electro20HP/cine
hp.htm– A lot of thing:
• http://www.petoindominique.fr/php/01-table.php
• DIY websites• Wikipedia
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BibliographyBooks
• Loudspeaker and Headphones handbook by John Borwick
• Loudspeaker design cookbook by Vance Dickason(Enceintes acoustiques et haut-parleurs)
• Testing loudspeakers by Joseph d'Appolito(Le haut-parleur: manipulation et measures electro-acoustiques)