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William Cumpiano's
String Instrument
Newsletter #10

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Acoustic Guitar Pickups II

William R. Cumpiano 1998, All Rights Reserved

Dear fellow guitar nuts;

This newsletter includes a wrap up of the subject of acoustic guitar pickups begun in newsletter #9 (I commented on "contact" and "strip transducer" pickups. The wrap up discusses internal guitar microphones. Afterwards I address several issues which subscribers have asked me to comment on.

So up to know we have gone to the next level of acoustic guitar sound goodness: buffered strip pickups are feeding the guitar's intimate string sound AND wood vibrations to our ears. But that is NOT the whole story. THE REST OF THE STORY is ... soundbox air resonances. This is the final frontier, the whole enchilada. The thin, flexible walls of the soundbox vibrate, driven by resonances mimicking, in their way, the resonances in the vibrating strings. This energy is fed into the bucket of isolated air in the guitar box, causing it to be alternately puffed in and out rapidly at the soundhole. This creates a phenomenon called "point source:" A high pressure plug of air at the soundhole fluttering resonantly in an unfathomably complex way; in turn causing the expansion at the soundhole of concentric domes of rarefied and compressed air which travel outwards at the speed of sound. This sound-producing mechanism, which few except acoustic scientists know about, is critical to the adequate production of the guitar's lower frequency range. By the way: snag a tiny down feather on the tip of a pin and place it right in front of the soundhole. Now pop your low E string forcefully: the feather shoots across the room as if jet-propelled. There is a veritable hurricane in your soundhole!

Because the guitar is only about forty inches long, so there is nothing physically large enough on it to radiate frequencies whose wave-lengths are greater than forty inches long. The fundamentals on your lower strings are far, far longer than forty inches long, so the guitar is just not up to the job of radiating them, just their accompanying higher-frequency harmonics. The fluttering plug at the sound hole, though, can generate six and ten-feet long sound waves quite handily and is responsible for the dark, hollow beauty of the acoustic soundbox. And for these, an acoustic microphone is just the thing. Recording engineers know about this, and will often attach to their board the output from the strip transducer, which supplies the "wood" and an acoustic microphone in front of the guitar to pick up the "air." A really good engineer will supply a third microphone nearby, usually to the side, off the neck, to pick up some "ambience."

But in a performance situation, a soloist, or a guitarist playing in a small ensemble in front of say, a coffee house or auditorium audience, will often require an interior microphone placed inside their guitar to complement the strip transducer. Their jealously-guarded well-loved favorite guitar will shine at its very best, then. Alas, an internal microphone will usually double the cost of the guitar's onboard amplification hardware.

There are several popular internal microphone options: Fishman has a Crown mike, reputed to be just barely adequate, but expensive, and its placement right at the soundhole makes it prone, I think, to feedback from unwanted accompanying stage sounds. Baggs has a microphone which I've gravitated to lately. I like the fact that it's imbedded in a foam block and adhered with peel-an-stick to the guitar's back plate (I've grown weary of the Crown's alligator-clip and gooseneck arrangement which chews up the x-brace and like the contact pickup, leaves you with the uneasy feeling that you can't really ever tell when it's optimally located.). I don't have first hand knowledge about it, but I've heard good things about Ken Donnell's Mini-Flex microphone which attaches through a goose neck, to the tail block.

Ah yes; a "Realistic" fourth option. A television-station techie friend of mine cued me in to this nugget: just a half hour before an on-camera TV interview, his last good Lavaliere lapel microphone failed. This was a kind of tiny thing that clipped to the speaker's lapel and picks up their voice. This was the kind that is about the size of a pea, has a German name and cost $300. And it just broke, as they are wont to do. Desperate, my friend flew to the nearest Radio Shack store and ask for a lapel mike--any lapel mike. Well, they furnished him their Radio Shack "Realistic" lapel microphone--for $29.95. On the way back to the studio, he leafed through the small-folded up scrap of technical information that accompanied the microphone. The performance specifications were identical to the $300 German model. They've been using it for several years now, and replaced all the others. Now I've furnished these to my poverty customers and they are delighted. They are a bit more feedback-prone than the other "real" guitar microphones, and they have a hearing-aid power supply/volume knob which you have to patch through a bit clumsily between the instrument and the amplifier, but hey! $29.95 and it performs like the tv-studio ones? It might work for you.


Thanks to all for your kind remarks and comments. It's quite delightful to see my list growing as fast as it is. Although I'm content to keep it free and easy (not charging makes me feel comfortable to produce it at an irregular pace), I must admit that the growing interest arouses my knee-jerk pecuniary instincts. This makes me think at times that I ought to charge something -- but then I think that would make me feel a bit more nose-to-the-grindstone about the undertaking. So I'll leave it free and easy for the time being.

Best wishes to all.

================ TAP TUNING:

Jeff Edwards wrote:
> Dear William,
> Of all the areas within guitar making, the one issue I find difficult to learn about is tuning the soundboard, both during thicknessing and during brace scalloping. I have read many books, including your excellent book, but have not be able to find anything that gives me the basics of this all important step. If this could be covered in one of your Newsletters, I am sure it would be greatly appreciated by all. As I work in complete isolation and have no guitar makers to disucess this with, my only hope is to gain some insight through the generosity of makers such as you.
> I hope you can help.
> Many thanks for the the most recent Newsletter.
> Jeff Edwards, in Perth Western Australia

You heard it here first: "Tap-tuning" has been oversold. Let's put it into perspective.

When I was learning I went through what you're going through: I had HEARD about something called tap tuning and was mystified and eager to learn about it. I presumed that it held THE secret for easily-replicable world-class results. I actually thought that the alchemy of guitarmaking was somehow locked inside this arcane act of wizardry called "tuning" the top, that it was something that only the select and most sensitive few knew about, and that they weren't going to tell me, so I would somehow have to learn it myself.

Builders, alas, often do things on guitars which they really don't know WHY their doing it, but they do it because they were taught to do what the teacher did, and they're afraid if they stopped doing it, the nice sounds they usually get will go away. So they keep doing it. Then, when you ask them why they do it, they are all too happy to MAKE UP vague, fanciful, jargon-laden accounts which will leave an impression that they know exactly why they're doing it in minute detail, and by implication demonstrate that they can somehow successfully manipulate all these invisible sonic phenomena on the guitar with ease. Luthiers usually won't stop you from believing that they are wizards. This is most painfully obvious in the realm of "tap tones."

Yes, you do "tap" a top to derive some rudimentary information about its anatomy. What you hear does give you some feedback clues which is useful and helpful. The "tuning" part is what is so misleading to beginners. At some point (most often toward the end of the process) many builders attempt to make some final changes in the anatomy of the soundbox, which the builder believes (I've selected this word carefully:) believes is exercising some control over the final results. Both, skillful experts and deluded fools, equally, scrape here and there, tap, press, reach inside, remove a little on the back braces, on the top braces, and then at some point say THERE. It's just right.

Then there are other builders, masterful experts and deluded fools alike that DON'T. They believe that they're making all the crucial decisions in the INITIAL stages of the construction: materials selection, materials dimensioning, design: that is all they need to achieve the desired results.

My good friend and colleague, Alan Chapman is confident that he has come to appreciate the difference of fine changes in bracing heights, soundboard thickness, patterns, and has come to BELIEVE that he can goose the sound quality of a certain note, or smooth out, or sweeten, or balance, or adjust the performance of distinct notes and distinct regions of the fingerboard by subtle changes in specific areas of the soundboard. With all respect to my dear friend, Alan, I believe that this is delusional. The soundboard is responsible for the production of a very limited part of the entire guitar sound spectrum. Yet his results are terrific. So I have to swallow a good portion of my skepticism when it comes to Alan's guitars. Yet, I know of other makers whos results are equally phenomenal that do NONE of that.

That is why I say now, like I said fifteen years ago in my book, that "tuning" is something that can't be taught but can be learned. This means that over time, each maker devises a series of actions which conform to a refined mental model that they've derived of how the guitar functions. This happens as a result of making, thinking, worrying, fussing, cursing, tossing and turning, eating and sleeping guitars for years and years. The mental model becomes more and more refined, and more and more PERSONAL. Jimmy D'Aquisto lectured that he made oval soundholes in his acoustic guitars, so the sound would "squirt further" like water from a similarly-constricted hose nozzle. The audience snickered when he said that. But nobody in the audience was getting $22,000 per guitar like he was. Snicker at that.

The upshot is that luthiers make excruciatingly poor sources of information on instrument acoustics. If you ask them about how they make their guitars work the way the do, and how WELL they work the way you do, I guarantee you, you will get an earful of useless mumbo jumbo. DONT ASK LUTHIERS QUESTIONS ABOUT INSTRUMENT ACOUSTICS, i.e., that if they "tune" the soundboard to G# that the "sound" improves. You'll then "tune" the top to G# and you'll never know if the sound improved for doing that, or by doing something that you didn't realize you had done, like select a stiffer top or denser neck wood, or making the bridge a little wider or fatter.


> Frederick Hoey wrote: > > I was happy to read your latest newsletter, but it brought up a question about > bridges. I am currently replacing a bridge on a war era Gibson J-45. Since this job > also involves a neck reset I can make the bridge any height. If it were a Martin I > would make it around 5/16" or 3/8". But every gibson I can remember seeing seemed to > have a bridge height of 1/4" or there about. I can only think of a few possible reason > for this. > > 1. The nominal factory bridge height is close to 1/4" > > 2. Gibson's more often suffer from poor neck angle and therefore have their > bridges shaved down to compensate > > 3. I haven't seen enough Gibson's. > > My interest is mainly curiosity, but if Gibson has a nominal factory bridge height I > would like to match it. If you have any thoughts on the subject I would be very > interested to hear them. > > Thanks William, > Rick Hoey

Gibson has a long, sorry record of very bad design decisions--and atrocious mistakes. Only recently have they been able to design their instruments sensibly. The result, however, is that many, many badly made and designed guitars are out there. The 1/4" bridge on a full-sized guitar is just one of them. Many of these 1/4" bridges were molded plastic, held on with half a pound of nuts and bolts, washers and additional hardware. And they STILL came loose. What an abomination!

I presume you're steaming off the necks. If so, I must warn you that many of those guitars didn't have a pocket behind the dovetail, which is necessary to easily steam them out. I would remove the 15th fret and make several exploratory drills to find a pocket. If you can't find the pocket, I would consider paddling off the (entire) fingerboard to reveal the joint. After removing the neck, you'd re-glue the fingerboard before resetting the neck.

The fingerboard tab may want to dive after you reset the neck. You may want to consider a wedge-shaped shim underneath it to keep it straight with the rest of the board, or you may want to attempt to loosen the upper transversal seam and slide in a spruce shim to raise the soundboard to meet the fingerboard tab.

We have improved these instruments greatly by resetting the necks to accommodate a brand new sold rosewood or ebony 5/16" to 3/8" bridge. This gives these guitars half a chance to sound real.

Dear friends, keep those suggestions and queries coming... Best wishes to all, and to all, a good night.