ENSEA Conference loudspeaker

Jeremie HuscenotJanuary 8, 2000

ENSEA conferenceLoudspeaker driver Loudspeaker enclosure

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.


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


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– …


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


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.


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.


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.


Electro-dynamic loudspeaker driver

A driver is composed of

• a moving mass• a motor• a suspension


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


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,…


Bl non-linearityAir gap geometry influence


Bl non-linearityLength of the voice coil influence


MembranePiston mode

• Low limit given by the driver’s resonance frequency

• High limit given by the radiation impedance


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)


Membrane non-linearity


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)


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


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


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.


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


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


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).


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


‘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


‘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).


Electrical analogyDriver

Front wave of the driverBack wave of the driver


Acoustic Short circuit

• Front and back waves must be separated, especially when the driver is omni-directional


First ideaA baffle

• Easy to built • Not “really” optimized for a real living room


EnclosureClosed box

• Back waves enclosed in a specific volume

Electrical analogy: volume ≡ capacitor

High pass filter (1st order)Volume dimensions dependent

Back volumeDriver

Box


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”


Closed box VS vented box

• Simulation with WinISD: Green (vented box) and yellow (closed box)– Additional resonator– 2nd order versus 1st order


A little bit of theory…

fc proportional to1/√(LC)

From Loudspeaker and Headphones handbook by John Borwick


Electrical AnalogyBass reflex enclosure


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


Enclosure effect

• Vibrations

• Diffraction

• Standing waves


Vibrations

• Vibrations create noise• Small vibrations on a large surface area

B&W matrix


Diffraction

• Edges create secondary sound sources that interfere with the primary sound source (driver)

• Influence the directivity of the driver

B&W enclosure


Standing waves

• Made of two waves (incident and reflected)

Enclosure top view


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


Multi-way loudspeakerDifferent drivers for different applications

1”≈2.5cm


Different type of loudspeaker

Woofer (Peerless)Tweeter (Timphany)

membrane

Front plate

Mid-range

Full-range (Visaton)Subwoofer (Alpine)


Electrical crossover2-way system

Tweeter

Woofer

Enclosure

SPL

frequency

fc

Used to separate the high frequencies form the bass frequencies


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


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


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.


A hifi loudspeaker

Model: QuartetRange: GeneseBrand: Triangle

Tweeter

Mid-range

Woofers Vents

Woofers volumes

Mid-range volume

Cross-section of an hi-fi loudspeaker

Confidential Divison, MMMM dd, yyyy, Reference


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


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)

Documents

ENSEA Conference loudspeaker