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 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.
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
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"
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