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Loudspeakers for Music Recording and Reproduction, by Newell and Holland

There are a number of cookbook and do-it-yourself guides to building loudspeakers, but few aimed at experienced builders and those already in the industry. And no, you can’t find everything on the Internet, and separating the gold from the dross of PR-speak “white papers” in the AES Journal pre-prints is a major project in itself.

If you are interested in high-efficiency, high-headroom loudspeakers and how they are used in modern studios, this is the book to get. More broadly, if you are interested the key technical parameters that affect the subjective listening experience, this is one of the few books that has a rigorous and comprehensive coverage of the subject, backed by the two authors’ decades of experience in professional audio. Dr. Keith Holland is a lecturer in electro-acoustics at the Institute of Sound and Vibration Research at the University of Southampton, and Philip Newell is an international consultant on acoustic design with over 40 years of experience designing hundreds of recording studios (this is his fifth book on professional audio).

In addition to loudspeakers - which are covered by an introduction on acoustics, and moving on to separate chapters for the main types of loudspeaker design, loudspeaker cabinets, horns, and crossovers, there are chapters on amplifiers and cables, loudspeaker behavior in rooms, interfacing loudspeakers with the rest of the studio, subjective and objective assessment (with particular attention to the effects of phase delay at low frequencies and the midrange), challenges in low frequency design and the effects of these choices on transient response, and a final chapter on surround sound.

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This is a contrarian view of loudspeaker dispersion. I’ll start by saying dispersion isn’t directly audible, unlike frequency response variations, energy storage in the time domain, or nonlinear (IM and harmonic) distortion. That isn’t to say dispersion characteristics aren’t audible - they are, but not in the way usually described in the literature.

Let’s start with a visual metaphor - imagine a multicolored mirror-ball, with red representing 20~100 Hz, yellow 100~300 Hz, green 300 Hz ~ 3 kHz, and blue 3~20 kHz. This mirror-ball is a visual representation of the loudspeaker, with most of its energy coming from the front. The front portion of the mirror-ball is close to white, and the rearward portion is deep red. Along the side, there are twinkly fine-grained yellow, green, and blue colors - these are diffraction artifacts, many of them visible only over a few degrees. Move just a little bit, and you’ll see a rainbow of colors shimmering off the cabinet edges, as well as interference fringes between the drivers.

When you sit in front of the mirror-ball, it is close to white (representing flat response) but there are twinkly multicolored artifacts coming from the edges of the loudspeaker enclosure (diffraction). As you walk around the loudspeaker there are a rainbow of twinkly multicolor diffractions artifacts, until you reach the dead center of the rear of the speaker and see a red glow from the center surrounded by the multicolor diffraction artifacts coming from the edges.

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Designers who freely share their research and projects on the web are a boon to mankind, or at least a boon to those with shared interests. John Krutke is such a person, with his Zaph|Audio site. John is a speaker designer with a good scientific habits: he theorizes about a possible new design, simulates it, builds it, carefully tests it, and compares the results with the original theory and simulations. Sadly, a lot of designers skip many of these steps. There are a dozen complete speaker designs on the site plus tests of drivers and other speaker-related projects.

John is much more towards the objective end of the subjective-objective spectrum than I am, and comes dangerously close to saying that “if I can’t measure it, you can’t hear it”, but his techniques seem good and he certainly clarifies speaker design. Check it out: Zaph|Audio.

The best musical sound is live music. Despite legendary “world-class” playback systems, the details and nuances are lost in the recording and reproduction process. Sadly, most people hardly, if ever, have heard live music. One of the few ways they can hear it is by the buskers who play in parks or subway stations, looking for a tip. Their performances are usually just adequate, but the novelty of hearing them live does make them interesting.

Gene Weingarten, in the Washington Post, wrote a fascinating article a few days ago that described an experiment sponsored by the Washington Post to see how people would respond, not just to live music, but to great live music. The Post got Joshua Bell, one of the worlds best violinists, to play, on his Stradivarius, some of the best classical solo violin pieces in the lobby of the L’Enfant Metro (subway) station in Washington D.C. He was dressed in casual clothes and a Washington Nationals baseball cap, and had his violin case open to accept tips. He played for 43 minutes during the morning rush hour, recorded by a hidden video camera set up by the Post. 1,097 people walked by. What do think happened?

If you thought that a crowd gathered, people clapped, or the news media called, think again. By far the most people just passed by, simply ignoring him, despite the loudness of his playing in the indoor lobby. It took three and a half minutes before someone made a donation. It took six minutes before someone stopped to listen. Quoting Weingarten:

“Things never got much better. In the three-quarters of an hour that Joshua Bell played, seven people stopped what they were doing to hang around and take in the performance, at least for a minute. Twenty-seven gave money, most of them on the run — for a total of $32 and change. That leaves the 1,070 people who hurried by, oblivious, many only three feet away, few even turning to look.”

To be fair, this was at the peak of the rush hour on a Friday morning before a holiday, and virtually all the people were going to government jobs. However, the obliviousness of most of the people was stunning. The race or gender of passers-by didn’t seem to make a difference, except for one thing: “Every single time a child walked past, he or she tried to stop and watch. And every single time, a parent scooted the kid away.” Ran Prieur comments on this as a kind of training of people for the Matrix (see his comments for April 8 and 10). What happened between the time of the curious kids and the “mature” parents? Maybe we need to go back to out childhood innocence to enjoy the music better?

In any case, this is a great article. It may require a free registration, and if the Washington Post takes it down, I’ve saved a copy.

Lynn Olson has started a good thread on diyAudio.com, called “Beyond the Ariel“. It has gotten some good participation, and Lynn’s comments and insights are enlightening. I’ve been thinking about large driver dipoles after hearing Robert Bastanis’ speakers two years ago at the Rocky Mountain Audio Fest. In any case, check out the thread if you are interested in these kinds of speakers.

If you plan on designing or building speakers, especially open-baffle dipole types or subwoofers, you need to check out Seigfreid Linkwitz’s vast Linkwitz Lab site. He is a retired microwave engineer from Hewlett-Packard, and applies an engineer’s insight and detail to his designs without getting into the fatal “meter-reader only” mindset that many professional audio engineers fall into. Although he sells both products and design services, he doesn’t hold back in putting up his design details onto his website. It is now so large that he sells a CDROM of the entire site, which is especially helpful for people like me with slow dial-up internet access. I could go on and on about all the interesting parts of the site, but the best thing is to peruse it yourself.

I’ve knocked out a little web page to show some of the MLSSA measurements I’ve made over the years, with interpretations of the time, frequency, and waterfall displays. There’s a fair variety here: A Celestion SL-6, Klipsch Chorus before and after time-alignment, Quads solo and stacked, a Martin-Logan CLS II, two different audiophile drivers used in very expensive American high-end speakers, my own Ariels, and the 12″ Tone Tubby AlNiCo guitar speaker.

In addition to the data, there’s comments about weighting first-arrival vs overall room frequency-response curves, how to find resonances that are 20dB or more below the main signal, and some unusual distortions in the time domain. These are the kinds of things that are undetectable on 1/3 octave real-time analyzers, or swept sinewave measurements - it’s why I tell people to invest in a MLS-capable measurement system with a highspeed sound card, so you can see, not guess.

This was originally a two-part review, with Lynn Olson commenting on Geddes’ book and on horns in general. Lynn has requested that I remove his part of the review and the associated comments. Here is my take on the book:

I recently finished reading Earl Geddes’ Audio Transducers, and thought I would add my two cents. I agree with Lynn on the fact that horn loudspeaker design is sadly lacking in the theoretical rigor that is applied to conventional direct driver speakers. Earl Geddes has brought modern techniques to the subject, both better analytical work and the use of “T-matrices”. I couldn’t verify every equation, but based on my engineering background, his work seems sound.

Geddes is an iconoclast, and introduces some interesting new ideas (at least to me) as well as puts down some “conventional wisdom”. Examples of the former are: the appropriate choice of coordinate system to get separable coordinates, his “Acoustic Lever”, the effect of the horn mouth on reflections, and his analysis of room reflections. Examples of the latter include: the neglect of horn system efficiency (”Loading essentially became a non-issue with the almost unlimited power capability available today.”), and the non-use of time-domain testing (e.g. waterfall plots, etc.). Because of this, Geddes’ book does not replace existing textbooks and theory of speaker design, but serves as a powerful stimulant for others in the field to re-think their analyses, especially regarding horn theory.

In physics, people generally fall into either the “experimental” category or the “theoretical” category. Albert Einstein and Richard Feymann are examples of theoreticians and Michael Faraday and Ernest Lawrence were experimentalists. I fall into the experimentalist category, so was especially looking forward to Geddes’ chapter on measurements. He presents a concept of taking frequency response measurements at various locations and by mathematical manipulation, generating the terms for his T-Matrix models. A similar plan is laid out for determining the coefficients for the nonlinear model. However, I would like to have seen actual tests and results, so that the loop between modeling and reality is closed. Although a good concept, I get the feeling that the software for doing this data reduction has never been done, leaving this as essentially a hand-waving exercise.

So, with the few reservations noted above, I recommend any speaker designer read Earl’s book, especially horn designers!

- John Atwood

Updated May 22, 2007

Now it’s time to get down to the nuts and bolts of how speaker drivers really operate, and where they depart from the ideal. Despite the physical simplicity of a classic Rice & Kellog direct-radiator, the conversion from electricity to sound has three steps:

1) Electric power to magnetic force - the voice coil

2) Magnetic force to mechanical acceleration - the magnet and pole piece

3) Mechanical acceleration to acoustic radiation, via direct radiation and/or augmented by an enclosure or a waveguide (horn).

There are characteristic departures from the ideal at every step of the way, which we’ll be looking at in the following paragraphs. This is a good point to draw your attention to marketing claims of “removing” this or that coloration - as you’ll see, complete removal is impossible (contradicted by physics), and techniques to reduce problems in one area quite often make problems in another area much worse.

There are other ways to accomplish the electric-to-acoustic transformation - electrostats, piezoelectric transducers, or even ionized and modulated air - but these have their own set of problems, which are even more intractable than the problems of the conventional direct-radiator.

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