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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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I missed the Rocky Mountain Audio Fest for the first time this year, since I spent the month of October in Vietnam. IMHO, this is the best high-end audio show in North America, so was sad to miss it, but Lynn Olson (who lives near Denver) went and he and others have made some good observations on the show.  A good overview is given on the following diyAudio thread: Rmaf 2008. Lynn expects to post more here on ClariSonus, but to whet your appetite, here are a few tidbits.

There were quite a few new speakers, many with really good sound. For example, here is the new RAAL omnidirectional speaker with ribbon tweeters, designed by Aleksandar Radisavljević from Serbia (click on the pictures for a full-sized display):

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This post comes a little late, since I’ve just spent two weeks in Morocco on a Country Walkers’ walking tour. Morocco is very interesting, with a rich culture and really friendly people. The weekend before flying to Morocco, I attended a “Tube Tasting” at Oswald’s Mill in eastern Pennsylvania. Here is the mill itself:

It is a 18th century grain mill built right into the house, and has been refurbished over the last ten years by Jonathan Weiss. Jonathan’s “Tube Tastings” have been an invitation-only event since 2002, where people into vintage, exotic, and exceptional home-built equipment can set-up and compare their projects. Jonathan is also a world-class cook, and cooked all the food for the gathering - and the food was fantastic!

The main listening room takes up nearly all of the third floor. The open beams, various vintage paraphernalia (verging on steam-punk), and the 3rd-to-4th floor opening (see the large windows on the picture to the left) gave plenty of room for the speakers to breath. The window openings in the two-foot thick stone walls made perfect turntable mounts.

The impressions given in this article can only give a glimpse of the totality of the tasting. The official 2008 Tasting page is not yet up (although 2003 through 2007 tastings can be seen here), but there is a pretty complete review of the 2008 Tasting at the 6 Moons site, with many tasty pictures. If I leave anyone’s equipment out of the following text, it is due to my inability to listen to everything plus my fading memory of the event three weeks ago.

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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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Here are three terrific dipole sites, must-reads for anyone contemplating a jump into the wild blue yonder. Fair warning: like direct-heated triodes, once you hear a speaker with no box coloration - at all - it’s hard to go back.

Linkwitz Lab, by Siegfried Linkwitz, the grand-daddy dipole site of them all. The site is huge, and comprehensive. The starting point for dipole research - and Siegfried’s speakers are a lot better than the “famous name” speakers.

Music and Design, by John Kreskovsky. This site is a good complement to SL’s site, lots of interesting analysis of polar patterns, interactions of crossover slope, baffle step, and MTM’s on open-baffle designs - things I haven’t seen elsewhere. I haven’t heard John’s speakers, but the engineering looks solid - always a good sign.

Dipole Plus, a German site. I don’t read German, but the graphs make the point, and the Links section is an excellent resource. Fun pix too.

The Edge, a free Windows program that simulates the response of an open baffle. Note this doesn’t simulate the floor reflection, so if you’d like to see that as well, you should construct a mirror-image baffle “under” the floor. Tolvan Data also has Basta!, a program that simulates open baffles, vented and closed-box systems, and active and passive crossovers.

When I met Siegfried Linkwitz at the CES and heard his Beethoven dipoles for the first time, I was deeply impressed. Superbly spacious sound, beautifully balanced, powerful, dynamic, with much of the effortlessness and ease of an electrostatic, but none of the hard-to-set-up crankiness and questionable midbass of electrostats.

But the electronics - hmm - multi-amping with six or eight 200-watt amps, and a plenty of op-amps with the mandatory 6 dB/octave frequency contouring demanded by the 1/f transition of the flat baffle. A lot of wattage of the amplifiers disappeared into that 1/f transition, and it worked the Scan-Speak drivers hard. Part of the reason for all the 24 dB/octave active crossovers was to separate the frequency bands, keep the excursion for each set of drivers in the reasonable range, and keep overall system IM distortion at acceptable levels.

Liked the sound - a lot - but that implementation, not for Mr. Triode here. That was several years ago at the CES. Then I heard the Bastani Apollo’s at the 2005 RMAF. Hello, efficient dipoles driven by a 20-watt amplifier - and that played LOUD, in horn territory, way louder than anything Scan-Speak makes. And Bob, unlike Siegfried, broke all the rules - the big, heavily-treated Alnico-magnet 12″ driver is operating far outside the piston band, there’s no highpass crossover at 200 Hz to “protect” it from big bad bass, and the baffle seemed a lot narrower than expected. Gary Pimm measured a -3 dB 220 Hz rolloff from the 15-inch-wide baffle - that seemed a lot smaller than theory demanded.

I thought about what I’d heard for some time afterward. I did a little math calculating the path-length differences and adding them together again as vector sums, and was surprised to find the -3 dB point is actually somewhere between 1/6th and 1/8th of a wavelength for an open baffle. So the required baffle in order to meet a subwoofer wasn’t as big as I thought.

About a year ago I had an insight and saw something that I think Linkwitz may have missed. Driver efficiency, frequency response, and power-handling drop by 6 dB/octave for a dipole compared to a monopole (below the 1/f frequency, set by the baffle size). This is intuitively obvious.

Linkwitz solves this by applying a compensating 6 dB/oct lift (an integrator) at all frequencies below the 1/f, and splitting the frequency bands so no set of drivers needs to handle much more than a couple of octaves, thus limiting the overall boost necessary in any single frequency band.

But … there’s another way to solve this. You can build a cascade of drivers, doubling the driver count as you go downward, octave by octave. Say the 1/f frequency is -3 dB down at 200 Hz. You have one fullrange driver, two drivers working in tandem at 200 Hz and below, four drivers working together at 100 Hz and below, and so on. By spacing the crossovers closer together, or further apart, the degree of driver overlap can be controlled, giving an option for room compensation in the midbass region.
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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.

In the course of writing a series of posts about a proposed dipole speaker, I had a few thoughts about vibration control, a topic near and dear to speaker and turntable designers everywhere.

First, the “mainstream” approach of using every known method to get emitted vibrations as low as possible. Thanks to a post by jzagaza at diyAudio, we have some very interesting links to Fujitsu’s advanced audio research project at:

http://www.timedomain.co.jp/tech/tech_e.html

http://www.timedomain.co.jp/tech/theory/td_theoryA4_eng.pdf

And a rare Stereophile review that actually describes the real-world results of this project - compared to something completely different, an Avante-Garde all-horn system!

http://stereophile.com/standloudspeakers/107fuj/

By the way, the Fujitsu Ten Eclipse TD712z speaker is about as close to an ideal monopole - in terms of extremely low diffraction and fully decoupled vibration control - as any speaker I’ve ever seen. This seems to represent the limiting case for the “mainstream” method of rigorous isolation and mechanical sinking of vibration from the driver. The physically small size of the single-driver speaker also limits unwanted emissions from the cabinet, something that is quite troublesome in big audiophile (and dipole!) speakers.

And for the other view - instead trying to completely get rid of vibration, over at Mother of Tone, they’re aiming for “consonant” vibration, using selected woods and varnishes:

http://www.mother-of-tone.com/mother.htm

http://www.mother-of-tone.com/lacquer.htm

This isn’t as ridiculous as it sounds. Thom Mackris and I auditioned identical compressed-carbon (not carbon-fibre) record mats with, and without, a woodworking varnish in a direct A-B test. The difference was equivalant to the difference between a $500 and $5000 tone-arm, or a $200 and a $2000 phono cartridge. More musical, as you’d expect, but also much more lifelike and a remarkable absence of “mechanical” or metallic coloration. Instead of adding a “musical” coloration, it removed quite a bit of harsh and grainy “mechanical” coloration, and revealed much more of the musical dynamics, tone colors, and timbre. The impression of space was also quite a lot better, remarkably so, and it didn’t sound like a coloration to me, more like the removal of a number of resonances.

Are the “absolute” and “consonant” approaches in conflict? Not as much as it appears. Yes, it’s good to get coloration as low as possible, but there’s always going to be a residue. That residue might as well be musically consonant, so it can subjectively disappear. The Mother of Tone approach using solid woods and selected lacquers (not C37, by the way, but conventional wood lacquers) seems like a direct and cost-effective way to build agreeable-sounding speaker systems using commonly available materials.

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.

As mentioned some time earlier, the most important function of a high-pass crossover is to physically protect a tweeter from damage - try testing a tweeter without a series cap to protect it and you’ll discover why this is true. So even the cheapest no-name speakers use a cap to protect the tweeter - customers do notice if the treble goes missing.

In a high-quality application, the highpass filter does more than physical protection: it minimizes IM distortion by reducing out-of-band excursion. Obviously, a tweeter can’t move the quarter-inch or more than woofers can - 1mm or less excursion is about it for a tweeter.

But there’s more to it than that: direct-radiator drivers (operating in the flat-response piston region) are constant-acceleration devices - that is, the acceleration is constant regardless of frequency. At frequencies below Fs (fundamental resonance), they change over to constant-excursion devices; this is the familiar 12dB/octave rolloff seen in direct-radiators. (The rolloff curve is steeper for resistive-vent, transmission lines, and vented-boxes.)

There are implications for the constant-acceleration operation in the flat-response region; put another way, the excursion increases at a 12dB/octave rate as the frequency goes downward, and then flattens out below the Fs frequency.

This means a 6dB/octave crossover (a single cap) still allows the excursion to increase below the nominal crossover point; in fact, the excursion increases until Fs is reached.

For example, think about a typical 1″ dome tweeter with a Fs of 800 Hz; if you use a 1st-order/single cap crossover, the excursion below a nominal 3.2 kHz crossover actually increases below the crossover frequency until the 800 Hz Fs is reached.

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