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INTRODUCTION TO LOUDSPEAKER DESIGN
Reviewed by Dennis Colin

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Introduction to Loudspeaker Design, by John L. Murphy. Available as part #BKTA1, for $24.95 plus s/h from Old Colony Sound Lab, PO Box 876, Peterborough, NH 03458, (603) 924-6371, FAX (603) 924-9467,
E-mail custserv@audioXpress.com. Published by True Audio. 166 pp.

I highly recommend this book to anyone entering the difficult, but rewarding, area of speaker design. I found it a pleasure to read. With a lively writing style, Mr. Murphy accurately presents the necessary physics fundamentals (he is a physicist), while requiring no more than basic math understanding on the part of the reader.

But the book is mostly about the practical considerations and tradeoffs to involved in designing multi-way dynamic driver systems. Although neither a design cookbook, electroacoustics text, nor musical perception treatise, it nicely covers the basics of those areas needed for a good intuitive feel for the various phenomenon taking place in loudspeaker systems. The book is well illustrated with diagrams, graphs, and handy charts, and contains many practical test procedures.

CHAPTER SUMMARIES
Chapter 1, Audio Basics-includes a brief history of audio and speakers; the basics of the audible frequency range, SPL, and so on; pitch perception; and an introduction to the frequency-range division of multi-way speakers.

Chapter 2, Loudspeaker Basics-covers   

speakers system components and theory of enclosure types, including the dipole, sealed box, vented box, isobaric, and bandpass systems. Although the author presents some mathematical relationships, his emphasis is on the significant variables (box volume, resonance frequency, Q) and their effect-pro and con-on each enclosure type. He also describes frequency-response rolloffs, excursion response, volume velocity, phase/transient/group delay, impedance, and power handling (thermal and mechanical), and discusses proper damping. The Chapter concludes with accurate and understandable definitions of Thiele/Small parameters, along with an explanation of the tradeoffs between efficiency, box size, and low frequency extension.

Chapter 3, Advanced Loudspeaker Topics-contains nicely illustrated explanations of spatial loading; cabinet diffraction loss; cavity effect (room and car cabin gain); point, line, and plain sources; and enclosure losses; as well as an interesting description of the use of a stethoscope to hear very sensitive spurious noises such as leaks, buzzes, resonances, port noise, and so forth. The Chapter also includes Olson's classic diffraction responses of various-shaped baffles, and basic circuits to compensate the typical 6 dB response step.

Chapter 4, Enclosure Design and construction-covers the basics of good material selection and construction techniques (sealing and bracing for example), and includes charts relating the three box dimensions, based on suggested "golden ratio" proportions

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to box volume.

Chapter 5, Crossover Design-contains a comprehensive description of the types of crossovers widely used (Butterworth, Linkwitz-Riley, first-order parallel, quasi-first-order-series) and their relative pros and cons. The Chapter also describes impedance compensators -both

the Zobel (driver inductance compensation) and resonance compensation. Also included is a table of recommended upper frequency limits vs. driver diameter, attenuators for (usually) tweeters, and crossover component recommendations regarding quality. Driver/crossover interactions such as non-flat impedance and inter-driver phase effects are also mentioned.

As I mentioned, this is not a cookbook; formulas for crossover components are not given, nor are they necessary. Many references, such as Vance Dickason's excellent works, already cover this.

I would like to comment on one area: Mr. Murphy mentions the lack of perfect amplitude response summing in for some standard crossover types, for example, the 3dB summed peak of (even an ideal) second-order Butterworth crossover (with drivers in opposite polarities, necessary to avoid a deep notch). I would like to have seen a mention of how "tweaking" " crossover elements either real time or with simulation, can often flatten these aberrations by adding some experimentally optimized phase shift, and so forth. I realize that this is really nit-picking, but reviewers are supposed to do that! Actually, this chapter (as well as a whole book) very well suit the purpose of introducing the reader to these very intricate topics.

Chapter 6, Driver Parameter Measurement-shows how to measure F (s), Q(ts), Q(ms), Q(es), and V(as) using a signal generator, 10k Ohm 1% resister, AC voltmeter, and a sealed test box.

Chapter 7, Frequently Asked Questions-includes very practical questions,

ABOUT THE AUTHOR
John L. Murphy, B.S., M.S., AES, IEEE, ASA, is a physicist with over 20 years experience in the design of recording consoles, electronics for guitars and electric bass, and loudspeakers for pro audio, hi-fi, musical instruments, and autosound. As an Air Force captain, he served as a space systems software analyst. In the audio industry he is probably best known for his recent WinSpeakerz and MacSpeakerz application software.
page 56    Speaker Builder 1/99

on topics such as vented-box port variables, driver placement, impedance compensation, driver protection, phase response, and musical instruments and sound reinforcement considerations.

APPENDICES
In addition to 12 technical references and a list of physical constants, conversion factors, and loudspeaker system relationships, the appendices include a section called "Box Types. " Each and of the 18 types shown includes drawings, list of box variables, and a basic graph showing rolloff slope(s). The box types range from "2nd Order Closed Box Highpass" to "6th-order Symmetric Bandpass Triple Chamber Isobaric" (whew!)

Overall, I found the book easy and interesting to read, with its emphasis on real-world practical situations. The subject matter is treated sometimes with humor. For example, when the author mentions the present impossibility of a 0.25ft3 Speaker with 100 dB 1W/1m SPL at 20 Hz. Or, in response to a question: "What? You say you're having trouble getting 20 Hz out of your piccolo? Just smack the bass player with it on the downbeat! It's an acoustic jungle out there! Speaker designer's beware!"

APPLICATIONS
I think that the more experienced readers and authors of Speaker Builder probably know most of the subject matter covered in this book. But probably not all of it, especially some tradeoffs aspects of crossover interactions. Even for the experienced designer, the book contains many useful and handy tables, charts, and other data.

However, for those interested in learning about the fundamentals of speaker design and operation, this book provides an excellent introduction. And even though the emphasis is not on detailed design methods, those covered are accurately patterned after Thiele's and Small's comprehensive work. Even if you don't know a woofer from a tweeter, but can solder, cut wood, and do basic arithmetic, this book will give you sufficient knowledge and intuitive grasp to select the type of system you need, and, with some software and/or experience, successfully design a complete speaker system.

In conclusion, I cannot imagine a better "Introduction to Loudspeaker Design" than this book. And the low price of $24.95 represents a small fraction of the cost of one serious audio mistake, which this book should help prevent.

 

 

 

 

 

 

 

 

 

 

 

 

 

page 57   Speaker Builder 1/99

Speaker Builder,
The Loudspeaker Journal

Reprinted with permission from
Speaker Builder
Volume 20, Issue 1, 1999, p.p. 56-57,
of Speaker Builder magazine

Copyrightę1999 by Audio Amateur Corporation,
P.O. Box 876, Peterborough, NH 03458, USA,
All rights reserved.


Introduction to Loudspeaker Design, Front Cover

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Introduction to Loudspeaker Design
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