How to Design and Specify

High Quality Loudspeakers

Geoff Hill

Loudspeaker Design made simple!

How to Design and Specify HighQuality Loudspeakers

Part I - Introduction

This starts with the Abstract here we give a brief overview of the book and the methodologyapplied throughout it along with a description of the key programs and tools used to designand specify loudspeakers.

Next is the Aim of this Book The aim of this book is to concentrate mainly on the practical aspectsof loudspeaker design, simulation, measurement and not the theory of moving coil loudspeak-ers only. There is a little theory, I have tried to keep it at a fairly basic algebraic level. Leavingall the really complicated calculations to the individual programs themselves.

In my studies over the years I became aware of relatively few books about the actual process ofdesigning loudspeaker drivers and loudspeaker systems, taking the reader through the wholeprocess required to design loudspeakers

This is closely followed by What this Book is Not about which highlights that this book is NOTintended to supplant the theory or academic methods.

Then we discuss What tools and equipment focuses on which tools and techniques are now avail-able for designing loudspeakers - this has changed significantly even over the past 10 or 20years and many older tools and techniques are now effectively obsolete - however some somelike the Thiele-Small parameters have morphed into mainstream design and specifications butare not always in the way originally intended originally. This chapter outlines key categoriesof software and hardware that I consider as minimal requirements today in 2015/2016

Part II - Basic Theory

Chapter 1 - How Does a Loudspeaker Work? we describe the basic operation of a conventionalmoving coil loudspeaker and we look at the Rice & Kellogg loudspeaker and patent, uponwhich so many of our current loudspeaker designs are based.

Chapter 2 - Frequency Response we start here with a Water Wave in a bath before going on tohow we can produce a Sine wave, then how we calculate the overall level of a signal whichis constantly changing over time (R.M.S.) before we get to how we display and interpret a socalled Frequency Response

Chapter 3 - Resonance we describe what Resonance is, some of the effects it has on a loudspeakerand how we can calculate it from a knowledge of mass and compliance

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ii HOW TO DESIGN AND SPECIFY HIGH QUALITY LOUDSPEAKERS

Chapter 4 - Damping As we saw in the previous chapter that resonance is the tendency of asystem or thing to oscillate or to keep vibrating; more at some frequencies than others. Damp-ing acts to control or stop this tendency. In this chapter we examine both electrical dampingas applied to the measurement of electrical Q and also acoustical damping and the effect thata lack of it can have...

Chapter 5 - in Young’s Modulus we will look at Young’s Modulus which is the ratio of stress(force per area) to strain (the ratio of new size to the original size). We will see how it can bemeasured and find why it is so important to modern modelling techniques

Chapter 6 - Finite Element Analysis describes in very simple terms what Finite Element is aboutand how it achieves the results it does - but does so stripped of the mathematics, it also dis-cusses what is important in any model and how in the authors view it is essential to be mod-elling the most important things or parts first.

Part III - Loudspeaker Models

Chapter 7 - Small Signal Model as the name implies these models are only concerned with thebehaviour when the loudspeaker is working at a low level, or in linear conditions; also thesemodels are only concerned about the low frequency region from D.C up until the loudspeakerstarts to depart from a mass controlled system; quite simply these models are NOT designedto work at higher frequencies where the diaphragm affects the response.

Chapter 8 - in Polynomial Models although these are a relatively old techniques, never lessthese relatively simple underlying equations can effectively model much of a real loudspeakersactual frequency response - so we need to be aware of the factors involved.

Chapter 9 - in Thiele-Small we explore three of Neville Thiele’s key alignments that are com-monly used and examine certain aspects of there performance, we then will then explore howthese are extended into the Electro-Mechanical Parameters that as a designer you can directlymanipulate

Chapter 10 - in Large Signal Domain and Model we discuss the Large Signal Domain and Model-ling as these techniques help us to understand what happens when we use a loudspeaker withreal world signal levels, our previous analysis has all been in the Small Signal Domain. In theLarge Signal Domain and Model this breaks down so we need different techniques.

Part IV - The Design Process

Chapter 11 - in There Is A Job To Do - But exactly what? we get to the heart of both specificationand it’s relationship to design, and we start to consider loudspeaker design in it’s widest sense:Taking the example of our Sub-woofer we begin to break it down to bite sized chunks

Chapter 12 - Common Questions outlines some of the more common questions that can be usedto further define answers that in themselves will give you the information needed to break aloudspeaker design down into smaller component pieces, these can then be looked at eitherindividually or collectively to ensure we are aiming at the correct specification(s). We alsodiscuss the importance of having a Sanity check

Chapter 13 - in Specifying A Loudspeaker Driver we examine the core essential key underlyingsystem specifications and what other parameters need to be included taking into account the

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type of enclosure or system before considering the make up of individual drivers how they arebeing operated.

Chapter 14 - in Project Planning and a Bill of Materials here I strongly recommend producing anoutline plan and how this differs from a project plan and discuss why this is important andhow this can be developed later. A simple example is shown which can then be used to formthe core of such a plan.

Chapter 15 - in Designing a Subwoofer we discuss some of the key project stages required totransform our idea’s into an actual loudspeaker - the key one’s we will consider here are thephysical and mechanical aspects.

Part V - What’s really going on inside a Loudspeaker?

Chapter 16 - in Driver Design we develop the underlying electro-mechanical parameters, theseare drawn from an extension of the Thiele-Small parameters by using idea’s drawn fromGarner and Jackson’s Theoretical Bass Unit Design, from the underlying equations we canthen produce the detailed specifications that we will feed into a FEA model. We also discussthe type of FEA model appropriate.

Chapter 17 - in Magnet before we get into the detail of design, we ask what is a magnet wewill look at magnetic flux lines and how we can concentrate these into useful fields we thendiscuss various modelling techniques and then summarise some common magnetic materialsproperties.

Chapter 18 - in Voice Coil we begin to look in detail at the Voice Coil - in certain respects it is theheart of the motor unit and hence the loudspeaker. Get it wrong the whole loudspeaker willnot function as we desire! So we we look not only at the materials used for voice coils but alsosome of the alternative methods for designing them so that they do meet our requirements andspecifications

Chapter 19 - in Bl(x) we saw previously that the Voice Coil is at the heart of a loudspeaker -However Bl B for Flux Density (T) and l for length of wire (m) is how the voice coil translates theelectrical current into the force that moves the loudspeaker, we also discuss in this chapter thischanges with displacement(x) and Shove or B·l2

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Chapter 20 - in Le(x) In this chapter we review briefly Le the inductive and the semi-inductivebehaviour of a loudspeakers impedance characteristic, helping us to understand the mech-anisms behind the roll off we saw earlier, we also extend this to the changes in this due todisplacement the Le(x) term

Chapter 21 - in Motor Unit If as is arguably the case the Voice Coil is the heart of a loudspeaker.Then the Motor Unit is probably best described as the structure that holds everything together,just as with the voice coil get it wrong and no amount of tweaking with a x-over later willcorrect it but get it right first time and things will go easier. Here we develop a complete motorunit from start to finish

Part VI - FEA, BEM and Integration

Chapter 22 - in Material Specifications we discuss the various types of material specifications,what they describe as well as why they matter, we then reference sources of further informationabout them

iv HOW TO DESIGN AND SPECIFY HIGH QUALITY LOUDSPEAKERS

Chapter 23 - in Mechanical Finite Element Analysis We briefly describe how FEA works in nonmathematical language. We then discuss what is significant in a model and what is not, whatis the level of appropriate complexity which we need to model and only then do we start tobuild a model taking as inputs various parameters we worked out earlier.

Chapter 24 - in Cms(x) Cms is the name we give to the mechanical compliance, it is the easewith which a loudspeaker’s moving parts are moved away from there nominal rest position.However we are more interested in Cms(x) and it’s inverse Kms(x) as these describe how theseparameters change with displacement or movement (x)

Chapter 25 - in Suspension Here we decide the essential requirements that a suspension mustmeet, we build an axis symmetric model of the proposed suspension and model it using a NonLinear static model, exporting the force displacement results into a spreadsheet for visualisa-tion.

Chapter 26 - in Mechanical Simulations In this chapter we will discuss how we will producemechanical simulations rather than acoustical simulations as, Our efficiency and coupling isusually so low that such simplification will make little difference. Programs like ABEC candirectly accept such mechanical data or calculations to produce the acoustic results we needlater anyway. What we will do in this chapter is to go through the process of developing thismechanical model.

Chapter 27 - in PafLS takes a very different approach to loudspeaker simulation; rather thango into the detail of electromechanical simulation, it takes the key Thiele-Small and Electro-Mechanical parameters as Inputs these are feed into a mechanical template that couples to thesimulation engine behind PAFEC, this avoids much design complexity at the cost of limitingthe otherwise infinite choices. However it directly gives us access to frequency response andaxial plots of the simulations.

Part VII - Mechanical Design

Chapter 28 - in Visual and Mechanical Design here we start to translate a design into a finalloudspeaker, phone, or completed system, going beyond the driver and how it works towardsthe final result.

Chapter 29 - in Cabinet we discuss the cabinet or enclosure as this often forms a large part ofthe mechanical structure - in the case of a mobile phone or a tablet it maybe core of the physicalstructure. Within a completed loudspeaker system, the cabinet or enclosure is the single largestitem it certainly has a significant effect on the performance of the whole system.

Chapter 30 - in Chassis here we consider some of the requirements that a chassis needs to meet.We can of course use the same or similar CAD tools for the chassis as we can for the rest of themechanical design. The first critical requirement is to support the motor unit, often this is theheaviest part of the loudspeaker so this needs a combination of strength and accuracy in orderto function correctly.

Part VIII - Measuring a Loudspeaker

Chapter 31 - in Acoustical Measurements We start from why acoustical measurements are differ-ent from electrical or electronic ones and how to make reliable, consistent and accurate AcousticMeasurements . We look at conventional anechoic chambers and wedges before pinning down

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the real underlying requirements for measuring loudspeakers. We consider the accurate meas-urements of loudspeaker drive units together with mounting baffles and the numerous waysthis could go and often does go wrong.

Chapter 32 - in A Universal Loudspeaker Driver Test System we discuss the idea behind a univer-sal loudspeaker driver test system, together with the rational behind AES’s X-223 project, whatthe alternatives are and how we could verify and confirm performance. We then introduce theTetrahedral Test Chambers

Chapter 33 - in Tetrahedral Test Chamber - White Paper I present a copy of a White Paper doc-umenting the performance of a SEAS H1207 Bass/Mid Driver as measured by an IEC Baffleconfigured as ground plane and measurements conducted in a TTC350 and TTC750 test cham-bers

Part IX - Appendices

Appendix A - in Glossary is where many of the terms used throughout the book have beendefined.

Appendix B - in References is where many of the references both physical Books in B.1. andvirtual links to websites in B.2. have been listed all the Websites and Links were working as ofNovember 2015

Appendix C - in the ABEC and VACS Tutorial on ABEC - Acoustic Boundary Element Calculator.We build up an mechanical and acoustic model of our Sub-woofer, the acoustical output is thengraphed using VACS - Visualising Acoustics

Appendix D - in the FEMM Tutorial we build a detailed axis symmetric model of the magneticcircuit of the Sub-Woofer Motor unit, from constructing the geometry, setting up the boundar-ies assigning the materials to calculating the flux and looking for saturation to estimating thevoice coil inductance.

Appendix E - in the HeeksCAD Tutorial we use HeeksCAD to produce and render a rotated 3Dmodel of our Sub-woofer drive unit, we will do this from a core *.dxf file which could havebeen produced by FEMM or a CAD package such as Draftsight. This is then followed by alisting of links to the HeeksCAD Unofficial Tutorials on youtube

Appendix F - in the HOLMImpulse Tutorial we use HOLMImpulse to demonstrate the processof measuring and calibrating a Tetrahedral Test Chamber, HOLMImpulse is a freeware pro-gram for measuring the Impulse, Amplitude including THD+n, Harmonics and Phase versusFrequency.

Appendix G - in the MecWay Tutorial we build a model of our subwoofer driver; enteringnodes and elements then assigning materials, loads and constraints. Ultimately developinga dynamic model which enables us to visually see the physical bending behaviour of a loud-speaker drive unit or parts of one such an analysis brings real understanding to the process ofdesign.

Appendix H - in the Microcap Tutorial Is a Spice simulation tool or more accurately a lumpedparameter modelling tool which we will use for modelling relatively simple electrical crossov-ers and many of the loudspeaker models presented here were produced using MicroCap, thefull version is capable of much much more but that is outside the scope of this book!

vi HOW TO DESIGN AND SPECIFY HIGH QUALITY LOUDSPEAKERS

Appendix I - in the PafLS Tutorial we take a peek behind the curtain that PafLS very carefullylays over PAFEC, now PAFEC is a very powerful FEA engine, but it is a bit frightening to me atleast! As there is just so much that you could do with it - at least inTheory but you almost needto know everything before you start! In this appendix we go through using PafLS to modeland ultimately build our sub-woofer driver.

Appendix J - in SpeakerPro Tutorial we will use some of the tools designed and written originallyby Geoff to aid loudspeaker drive unit design and system design: Included are standaloneprograms, modules, and spreadsheets: BandPASS, BOXCALC, SpeakerPRO, TheoreticalBl &VCoils amongst others.

Appendix K - in Statistical Analysis of Loudspeakers is all about, well numbers so statistical ana-lysis is just using these numbers to tell us more information about a design product or process.So, the first thing is to gather and then record data. Fortunately, pretty well all modern soft-ware is capable of gathering the data by default. In this appendix we will explore some simplestatistical analyses from such data.

Appendix L - in the WinISD Tutorial we go through the steps required to enter a loudspeakersparameters into the program, and then to use it to simulate the amplitude, SPL, impedance andmany other response curves. It is also useful for its database of loudspeakers. It is primarilydesigned for Subwoofer Box and Driver as well as basic X-Over work.

Appendix M is a brief Autobiography of the Author: I am and have always been fascinatedby Loudspeakers and Amplifiers and how together they can reproduce Music. How they dowhat they do, and why is and has been a driving force for me as long as I can remember.

Geoff is an acknowledged expert in loudspeaker design and measurement,

you may well have listened to one of his designs without knowing it!

Geoff demonstrates free, open source and low cost software being applied

to High Quality Loudspeaker Driver Unit Design. Modern computing

power, software and hardware have progressed to the point that such

analyses are now practical for an interested individual or small company.

Geoff show’s the process from initial concept through, specifications and

theoretical simulations into detailed design. Then onto the final parts and

detailed simulations of a loudspeaker driver sufficient to give re-assurance

that a design is practical and will perform as expected.

This book brings together many different strands of modelling from Electro

Magnetic through to Mechanical and Acoustics, so that it can act as a

primer for those specifying loudspeakers, those interested in using them or

those people new to loudspeaker or transducer design.

This book shows some of the techniques used in designing modern

loudspeakers and transducers and does so without getting bogged down in

detailed theoretical discussions and arguments.

This is a practical book, based upon nearly 40 years of designing,

measuring and testing loudspeakers and transducers. These loudspeakers

have been used in Automotive, Consumer Electronics, Hi-Fi, Mobile

Phones, Musical Instruments and Public Address Systems.

For further information www.geoff-hill.com/

Tetrahedral Test Chambers www.hillacoustics.com/