Panning for loudspeaker gold
Mar 1, 1998 12:00 PM, Ted Uzzle
Mark R. Gander, ed., Loudspeakers vol. 3: Systems and Crossover Networks,Audio Engineering Society, 1996, 456 pp., paper, $38 (AES members), $50(non-members).
Mark R. Gander, ed., Loudspeakers vol. 4: Transducers, Measurement andEvaluation, Audio Engineering Society, 1996, 494 pp., paper, $38 (AESmembers), $50 (non-members).
It's supposed to work something like this. Engineering societiescontinuously publish papers from the forefront of research. Every fewyears, maybe five or ten, someone publishes a book incorporating theup-to-date research; those who buy the major books in their field needn'tkeep decades of back issues.
Somehow, it doesn't seem to work that way in audio. Maybe our business isso idiosyncratic that many engineers don't see the need to know what othershave published on the subject of their research or product development. Ifyou were to take a poll at an AES convention and ask ten audio engineers,"Who is the greatest expert in your particular area of work?" eight out often of them will answer, "Me." Our industry is charmingly decentralized anddemocratic but depressingly reluctant to learn from what's going onelsewhere.
The Audio Engineering Society has published the journal of record for audiotechnology since 1953. For several decades, the society has had anaggressive book publishing program in which key articles from the Jour.Audio Eng. Soc. are reprinted in anthologies. For those without completesets of back issues or without the inclination to root through hundreds ofbound issues to find pertinent publications, these anthologies are atreasure trove.
The first two loudspeaker anthologies collected articles from the Journalon all sorts of loudspeaker topics, from 1953 to 1977 (volume one) and from1978 to 1983 (volume two). The years 1984 to 1991 were so rich inloudspeaker papers that they are represented by the new volumes three andfour in the series. Many author names from earlier volumes are representedmultiple times: Earl Geddes, Don Keele, Richard Small, Stanley Lipshitz,Daniel R. von Recklinghausen (papers from before he became editor of theJournal), and John Vanderkooy.
Volume three includes papers on loudspeaker systems and crossover networks.The section on loudspeaker systems includes 23 papers, reprinted infacsimile from their first appearance in the Journal. A group of papersrefines and extends the earlier work of A. N. Thiele. Written by W. J. J.Hoge, Don Keele, Richard Normandin, and Richard Small, they explore newalignments and the effect of parameter variation from manufacturingprocesses.
Much attention is paid to cabinet effects-the behavior of loudspeaker soundinside the cabinet and diffraction effects at the edges of cabinets.There's also a paper on the effects of filling a loudspeaker cabinet withsuch material as fiberglass. Another group of papers deals with feedbackcircuits applied to loudspeakers to increase linearity, especially at thefrequency extremes.
A key paper in this section, "An introduction to band-pass loudspeakersystems" by Earl Geddes, details a new loudspeaker design technique whichhas become important in the areas of domestic stereo, home theater, andauto sound. It permits a comparatively small loudspeaker, driven bycomparatively low power, to produce low frequencies at high levels. Thoseinterested in loudspeaker technology couldn't hope for a much more usefulintroduction to this technology. There's another paper by Geddes in volumefour, on acoustic waveguide theory, another innovative area this researcherhas studied.
There are also a number of papers studying unknown behaviors of theloudspeakers we all use. An especially interesting one by Otala andHuttunen presents measurements of the peak-current requirements ofloudspeakers. There's a surprise. A loudspeaker can draw almost seven timesas much amplifier current as an 8 V resistor, although the current demandmay last only a few hundred microseconds. Sine waves and square waves ofthe same root-mean-square values draw hugely different instantaneouscurrents. Figures five through eight in this paper are a hair-raisinginsight for any designer wanting to develop a practical amplifier.
The second half of volume three is devoted to papers on crossover networks.This section is all math, and it's for crossover designers only. An impishreviewer with the acquaintance of a number of crossover designers mightwonder how many real-world crossover designers can actually understand thisstuff, and whether it would really help their work if they could, but nevermind. There are two or three actual circuit diagrams here, accompanied bylots of diagrams of the complex plane. Those not quite familiar withcomplex impedance might get little from this section.
There's a most interesting 1984 paper from Vanderkooy and Lipshitz. Knowingthat multiway loudspeakers are inherently non-minimum phase, how canhigh-slope, minimum-phase networks be applied to make these systems minimumphase? Well, pretty clearly they can't, and for that reason designers havespent lots of effort applying signal-delay technology to improve thissituation. The authors show, at length, that signal delays, in conjunctionwith minimum-phase crossovers (or their twin siblings, equalizers) cannotrender non-minimum phase loudspeaker systems into minimum-phase deviceswith flat magnitude response.
Perhaps no loudspeaker topic has had as much hooey written about it asphase behavior. This is one of those areas where there seems to be a gapbetween the extensive theoretical work published in the present anthologyand elsewhere, and the more popular explanations that tend to appear in thetrade and consumer press.
These crossover papers include several on the use of computers to assistthe design of networks. As more complex design techniques have come intouse, it has become impossible for the crossover designer to make thecalculations over and over to optimize the design. Repetition ofcalculations with variables changed by a set interval: that's whatcomputers were made for, and it has become difficult to use the newresearch into crossovers in practical designs without them.
Volume four, the second new one, reprints papers on transducers andmeasurement and evaluation.The transducer section of this volume includes a paper by Gander on powercompression in moving-coil loudspeakers. Increase the driving voltagefeeding a loudspeaker voice coil by 10 dB, and theoretically the powertransferred from amplifier to loudspeaker will be increased by 10 dB, andthe acoustic output of the loudspeaker will be increased by 10 dB (assumingall levels are within the operating range of the device). Alas, it doesn'thappen that way. The voice coils in loudspeakers heat up as they operate athigher electrical powers. This reduces the current drawn from theamplifier, and thus, the sound output. The author notes that heat transferaway from the voice coil is more important (a better management method)than the heat resistance of the coil itself. This paper is among the mostuseful published in any loudspeaker anthology.
Another paper important to all who use loudspeakers is included here:Clifford Henricksen's paper on heat transfer in loudspeakers. As Gander inthe paper mentioned above studied the effect of heat on loudspeakerperformance, Henricksen looks at the physics of heat inside theloudspeaker. Resistive heat can be moved away from a voice coil byconduction through the air in the gap, conduction through the movingstructure, radiation and forced convection. Henricksen describes all theseand summarizes with an equivalent circuit.
Much attention is paid in this section of the anthology to thefinite-element method of analysis and prediction of loudspeaker behaviors,as well as a paper on the Volterra expansion, and modal analysis, which hasbeen widely used in vibration analysis but not so widely found in the studyof loudspeakers. There's some new work on horn theory and onstretched-membrane loudspeakers. There's a couple of papers here on eddycurrent effects on loudspeaker impedance.
Some innovative concepts in loudspeaker design are described here,including the ribbon tweeter, and, spectacularly, a plane-wave tube (ineffect) with holes along its length, placed so the tube becomes anend-fired array, with unusually sharp directivity control.
The second part of volume four is devoted to the measurement and evaluationof loudspeakers.
There's no question about which parts here are the most noteworthy: thereare three papers by Floyd Toole. The first, "Subjective measurements ofloudspeaker sound quality and listener performance," is reprinted from theJanuary-February 1985 issue. It's about the deep problems in subjectivelistening tests of hi-fi loudspeakers meant for reproduction of music. Hereis also reprinted Toole's papers from April and May 1986, "Loudspeakermeasurements and their relationships to listener preferences," parts I andII. Part I is devoted to a study of 50 years of literature on this topic.It is drawn almost entirely from such psychological journals as HighFidelity, Stereo Review, Hi-Fi News, Wireless World and such, the journalsof record where all the perceptual psychologists publish their best work.Your reviewer is not mocking Dr. Toole: if he's to review the literature,then he must go where the literature is. Your reviewer is mocking 50 yearsof cockamamie pseudo-psychology, which has led exactly nowhere, and nowonder.
Part II of this paper gets to the meat. The author devised listening teststhat produced reasonably consistent results from listeners. That in itselfis quite an achievement. Then he compares these subjective results withobjective measurements and finds those most often preferred are thosesuperior in certain measurements.
This section on measurement also includes papers on objective,laboratory-type measurements, of course. These include holography, thelaser velocity transducer and other very new devices.
These anthologies are indispensable to anyone who would know the directionsof modern research on loudspeaker technology. There are no review articles;there are no books illustrating such a diversity of approach and result.These two new volumes, plus the two previous ones, are a necessary part ofthe technical libraries of those who design loudspeakers, and some of thepapers are of great importance to those who use loudspeakers.