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Alembic Club » Alembic Basses & Guitars » Archive through October 26, 2012 » Still trying to understand acoustic cavities in solid body instruments... :-/ « Previous Next »

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Senior Member
Username: jazzyvee

Post Number: 3140
Registered: 6-2002
Posted on Sunday, September 09, 2012 - 4:44 am:   Edit Post

This could be a long thread as I've been randomly thinking about this for a while and trying to think through the answers. In case I digress, all I'm trying to understand is what benefit or detriment having a cavity in a "solid bodied" neck through instrument has on the signal induced in the magnetic pickup and hence the tone and why my alembicised strat now has more of an amplified acoustic guitar sound with a maple scratch plate than the plastic one it had in the past.

Now I understand how a magnetic pickup works. simplistically string moves through a magnetic field and induces a current into the coils on the pickup which then goes out to something else in the signal chain then to a speaker then sound.

Now I understand somewhat about how an acoustic guitar works, basically vibrating string transfers energy into the top of the guitar, that vibrates giving sound.

So on an Alembic neck thru instrument, the pickups are fixed to the body of the instrument either side of the neck. Start the string vibrating and the coil picks up the signal, and the vibes from the string flow through the body and neck and certain frequencies are emphasised or attenuated accordingly and the body also vibrates thus losing some energy as audio sound into the air from surface of the instrument. Once the body is vibrating then everything attached to the body will also vibrate including the pickups. Now with the pickups moving, it's magnetic field will be moving relative to the strings so other signals will be induced in the pickup coils resulting in additional signals being present in the pickup output and possibly some cancelled out. How much I don't know.

Now we put a cavity in the body, in reality all of the instruments have a cavity for the wiring. The cavity in an alembic from what I've seen on pictures is contained within pretty thick pieces of wood so the wood on the top and bottom of the cavity is not going to flex as freely as say an acoustic guitar but more than the solid body version so I presume there will be a amount of energy loss from the body due to the vibration of this section of the top and bottom into the closed cavity this vibrating the air inside the cavity. This i again presume would result in a reduction in amplitude of certain frequencies in the body and resulting in an apparent emphasising of others.

Now back to the reason for my initial point. I have alembic activator pickups in a strat. It used to have these pickups mounted on a plastic pickguard but when I had it refurbished, I had a quilted maple pick guard made which the pickups mount directly to now that change has suddenly made the guitar sound close to that of an amplified acoustic guitar more than it's previous sound. The only reason for that I can deduce is that the maple pick guard must be now vibrating more freely than the plastic one and causing additional vibrations from the body to be transferred into vibrating the pickups and hence inducing additional signals into the output.

I contrast also have a solid body strat style guitar with no scratch guard and pickups mounted directly into the mahogany body and this one has has none of the acoustic guitar properties in the sound so I'm almost convinced the vibrating platform the guitar pickups are attached to is the cause of the sound quality changing.

Anyone got any views/facts on this or maybe can point me to some useful web sites.

Senior Member
Username: gtrguy

Post Number: 496
Registered: 9-2004
Posted on Sunday, September 09, 2012 - 10:38 am:   Edit Post

I do know I once tried taping over the sound holes in a Telecaster Deluxe and Charvel Surfcaster and it did not make any real difference to the sound that I could tell, which surprised me.

I think for a solid body guitar: pickups and electronics first, then neck (neck-through vs bolt on, fret board wood, neck wood, scale length), then body, then accessories like bridge and nut and strings. Of course number one is your fingers!

I may be wrong though; there must be experts out there?

Username: nnek

Post Number: 77
Registered: 8-2009
Posted on Sunday, September 09, 2012 - 12:19 pm:   Edit Post

In a strat the pickups are suspended from the pick guard so i would imagine any motion no mater how minute might cause an effect in relation to the vibrating string's magnetic attunuation. the normal pick guard materials are pretty dead... Can you get more pronounced string generated feedback at high volume now?
Username: adriaan

Post Number: 2967
Registered: 6-2002
Posted on Sunday, September 09, 2012 - 1:51 pm:   Edit Post

So why not find a Strat with similar specs but standard pickups and pickguard, and make recordings of both guitars. Then swap the pickguards with the pickups and electronics, and record again. Finally, swap the pickups and electronics only, amd record. By then, you should have a pretty good idea of what is causing the "acoustic" quality in the sound. (Plus it will make you appreciate the added value of having threaded inserts.)

(Message edited by adriaan on September 09, 2012)
Senior Member
Username: jazzyvee

Post Number: 3142
Registered: 6-2002
Posted on Sunday, September 09, 2012 - 4:18 pm:   Edit Post

I can see where you are going Adriaan, however with due respect, that's easier said than done and since I intend on keeping the alembic pickups in this guitar I'm really more interested in the reason for the sound change with the alembic pickups in the plastic vs alembic pickups in the maple pickguard and really the general theory as to why this is the case.

Senior Member
Username: lbpesq

Post Number: 5223
Registered: 7-2004
Posted on Sunday, September 09, 2012 - 10:33 pm:   Edit Post

I suspect it has something to do with the different manner in which plastic and wood respond to vibrations. Of course the Alembic pickups have something to do with it too. They tend to be cleaner and clearer than the usual DiMarzios, etc. Such a clean clear tone is more easily associated with an acoustic tone.

Bill, tgo
Senior Member
Username: elwoodblue

Post Number: 1451
Registered: 6-2002
Posted on Sunday, September 09, 2012 - 11:16 pm:   Edit Post

just a guess...
I would say the resonant freq. of the wood pickguard would be the most noticeable sonic attribute.
I'd bet it has lower resonant peaks and is more responsive than 3 ply plastic .

Does the tone change if you dampen the guard?
(put some pressure on the guard right next to whichever pickup is active while playing).

On a Banjo you can really manipulate overtones by where you dampen the head.

Whether these resonances add or subtract to the final overtone distribution would depend on the nuances of the individual instrument .

A strong pickup/pickguard connection might create a sort of notch filter effect ,
(and maybe it affects the sustain/decay more than the plastic would),
where a weak pup/guard coupling might add some phase play).
Senior Member
Username: edwin

Post Number: 1283
Registered: 5-2002
Posted on Monday, September 10, 2012 - 3:00 pm:   Edit Post

It seems to be that plastic is more flexible in some ways than wood. The plastic pickguard might dampen a wider spectrum of frequencies than a wood one, which probably only resonates at a few frequencies. This change might be perceived as more acoustic sounding. I'd be curious to hear how it would sound with the pickups screwed right into the body. I bet it would sound more like the wooden pickguard.
Senior Member
Username: gtrguy

Post Number: 497
Registered: 9-2004
Posted on Monday, September 10, 2012 - 8:58 pm:   Edit Post

I do know that Strats are very sensitive to pickup height.
Senior Member
Username: jazzyvee

Post Number: 3144
Registered: 6-2002
Posted on Monday, September 10, 2012 - 11:05 pm:   Edit Post

The pickups are much lower than usual due to them having a slice of maple fixed to the top to match the scratchplate although the output is pretty hot.

here is a pic you can see how low they are from the black casing.

Senior Member
Username: dfung60

Post Number: 564
Registered: 5-2002
Posted on Tuesday, September 11, 2012 - 10:19 am:   Edit Post

I think the change in sound you're hearing is because the pickups are dropped so low, per the picture. The pickups already look pretty low and you have the additional spacing from the wood cover on the pickup.

As the pickups get farther from the strings, there will be less bass and less attack, which may account for the acoustic-like tone. If you have another Strat, try cranking the pickups all the way down and you'll probably hear some similar stuff going on.

With all other things equal, you probably won't hear the difference between pickguard materials as they're relatively tightly attached to the body. There will be an effect from the mass of the pickguard (a 70's brass pickguard will be a lot heavier). A steel pickguard would change the tone because is distorts the magnetic field. The big bridge plate on a Tele is doing this and is very obvious if you switch to a brass Tele bridge (the way it sounds explains why you almost never see brass Tele bridges, and makes me wonder if there should be a steel plate for Strats).

The reason normal Strats are so sensitive to pickup height is that stock pickups use magnetic polepieces which are so powerful that they damp the vibration of the string if they're too close or can even cause false harmonics.

Although the pickups being mounted to the body cause them to be moving when sensing the strings, I think that effect is very minimal as well. When you're playing a note on a guitar, the motion of the body vs. strings should be proportional to the ratio of the string mass vs. guitar mass. That's a big ratio, so it's probably a pretty small effect relative to the circus of other stuff that's going on when you hit a note.

This ratio can change with the weight of the instrument, but there's a much bigger effect there, as the string motion is affected as the body accelerates up to the pitch of the string. You'll hear this as a big difference in attack between a heavy guitar and a light one, but this is somewhat separate from pickup motion.

Mica often points out that neck woods dominate the sound of Alembics to a greater extent than on other instruments because of the neck-thru construction. Both ends of the vibrating string are sitting on the same piece of wood and the laminated neck is engineered to give better stiffness and consistency than a neck made of a single piece of wood, which may or may not be good.

As you mention (and I think I did in the past), the guitar is a big mechanical filter with many variables, which include mass, stiffness, resonance, etc. If you want to have pure string vibration, then having a stiff, uniform neck is the best way to achieve that. If you make the neck of one wood and the body of a different wood, the pure vibration of the string is disturbed where the materials interface. This is a physical impedance mismatch. This is often illustrated in an introductory Physics class when they ask you to look at the wave motion of a rope tied to a doorknob and then change from a uniform rope to a thick rope attached to a thin one. The non-uniform rope will try to set up different resonant frequencies in the different parts and, depending on how well those frequencies "get along", you may not be able to set up a steady motion. That's the difference between a "good" Strat and it's "bad" brother hanging right next to it at your local guitar store - even if they were build one after the other in the factory, each piece of wood is different and affects the attack, sustain, and tone of the guitar. Having a through-body design gives you the best chance of preserving the natural string motion, particularly the fundamental.

The cavities in the body will have the effect of changing the body mass, but in an Alembic the effect is reduced because of the thru-neck. Even though the caps are thick, I do think a Series bass is much louder acoustically than a solid body bass, but don't really hear that in the amplified tone.

David Fung
Senior Member
Username: funkyjazzjunky

Post Number: 827
Registered: 5-2007
Posted on Wednesday, September 12, 2012 - 8:48 am:   Edit Post


Senior Member
Username: bigredbass

Post Number: 1926
Registered: 9-2002
Posted on Wednesday, September 12, 2012 - 10:52 pm:   Edit Post

And, as well, the magnetic field the pickups are projecting is spherical, though I'm sure with some 'dents' in that 3-D shape. We always tend to think that the pickups field is pointing up like a flashlight, when it's really more like a bare bulb. This is rarely brought up (probably since pickup design is a black-enough art as it is). So what's this got to do with it? Beats the hell outa me, I just know it, although I suppose I know it only on this arcane level.

I remember reading a quote from Ron where the idea was for the neckthru, the bridge (on its sustain block), the brass nut, and the low impedance pickups was to interfere with the vibrating string as little as possible. Yet the woods unequivocally 'voice' the tone. . . .

I definitely agree that single-coil Strat pickups, being high impedance, can definitely 'run away' when cranked too close (and it's a slim, slim difference in height between warmed up and wolf tone city). This is why for some guys, EMG's are such an answer to these problems: Blade poles, low impedance, no noise, pretty much fool proof, and will drive thru a long effects chain. Over time, Fender (like Porsche with the 911) has slowly civilised these things, but most garden variety Strats are a long way from a current American Deluxe, or any of the 'aftermarket' good Strats.

j o e y

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