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How Do Magnetic Pickups "hear" Wood?


dpm99
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My understanding is that a magnetic pickup works by sensing the velocity of the string vibration. That being said, how will wood affect tone? I realize it does. I just don't quite understand why.

Here's my best guess. The density/flexibility/whatever of the wood holds the bridge in such a way that it affects string vibration. Overtones and whatnot may well be caused by the wood resonating in such a way that the bridge somehow picks it up and passes it on to the strings. But any sound the wood itself may produce will not be picked up, and will not affect the amplified sound in any way. Wood is simply an effective conductor of resonant vibration. For this reason, contact between the pickups and the body wood serves no tonal purpose.

Is that anywhere close to correct? I hope this isn't a question that's been answered numerous times already. I tried a quick search, but didn't find anything.

Thanks,

Dave

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Picture what happens when a string is stationary, but the wood, and the pickup mounted on it, is vibrating. The pickup 'sees' the distance to the string as changing in time with it's own vibration, hence a signal is generated which corresponds to the vibration of the wood.

Now imagine you have plucked the string. The string also vibrates the wood via the bridge. Now the pickup 'sees' both the vibrating string, PLUS the vibration of the wood relative to the string. Note that, due to time delays in transmission of the string vibration to the wood, and the colouration of the vibrations due to the properties of the wood, and the whole mechanical transmission system, the two sets of vibrations are different. They add together forming a total that is richer in harmonics.

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Yep...both are pretty good descriptions. Pickups have their own physics and so characteristics. Things like impedance and magnet strength and shape of the field, the way it is wound...all make a difference to how big a voltage they put out in response to the string and how sensitive they are to different frequencies.

You could consider a pickup like a speaker...an electrical signal is sent to the speaker coil that moves the cone that moves the air in proportion to the signal. Some speakers are better at some frequencies than others. A pickup is kind of the reverse: the metal string is vibrating within the magnetic field and this generates a tiny voltage in the pickup coil.

The placement of the pickup along the string has a lot of influence...the bridge pickup is higher in harmonics behind the fundamental vibration. Being closer to the bridge (where the string is fixed and stationary) the string moves less...so the tendancy is to want a stronger pickup to balance with the neck that generally senses a bigger movement of the strings and less higher order harmonics.

Combining pickups means that some harmonics are reinforced and others are cancelled...hence the postions 2 and 4 strat hollow sounds for instance.

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The wood thing is complex. Dont forget that the neck plays and important part, as does the neck joint...more than half of the string is vibrating is suspended over the neck area potentially.

Also, the bodywood can dampen or promote resonant frequencies that affect the way a string vibrates. The pickup can only work with how the physical sting is vibrating and the harmonics in it.

...

The harmonics is where a lot of the "tone" comes from...the mix, cancellation and reinforcement through resonance can be affected a lot through the material and construction.

It is impossible to say what is "best" of an ultimate "tonewood" though...but a soft, sap ridden bit of pine is likely to have a lot of dampening aspects while a solid carbonfibre guitar might have far too much harmonic content. Still, it doesn't stop people using soft spalted tops for cosmetic reasons.

pete

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I've always envisioned it this way... the wood removes from the strings (or enforces in them) certain harmonics for every note on the fingerboard. The wood thus tells the strings how they can and can't vibrate, and the pickups reproduce this through the lens of their particular impedance, magnetic strength, position along the vibrating string, etc.

BTW, anyone ever notice how playing the highest notes on the guitar through the neck pickup sounds almost as bright as the bridge pickup? In a sense, your fingers are fretting and creating another "bridge" very near the neck pickup, thus making the harmonic balance more like it is when using the bridge pickup.

And I think the "bridgey-nes" and "necky-ness" of any pickup changes slightly for each position on the fingerboard. Say you play through the bridge pickup and fret at the octave: if the bridge pickup is positioned at 1/8 the full scale length, it's suddenly "hearing" the strings at 1/4 of the scale length (because of where you're fretting), thus picking up a different harmonic balance relative to when you play open.

Edited by Geo
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More or less what folks have said already.

Wood (and construction methods, mass and damping distribution, resonance distribution in general) acts as a filter for the string. In an acoustic setting essentially a purely substractive one (it doesn't add anything to the string signal, but only removes information/may reflect some information back, depending on resonances). Electrically it gets a touch more complex, as the amplified sound of the guitar itself also adds vibration/resonance/energy to the body(wood), which can infleunce the strings. The pickups - with their own impedance, magnetic field configuration, bla, bla bla....

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But any sound the wood itself may produce will not be picked up, and will not affect the amplified sound in any way. Wood is simply an effective conductor of resonant vibration. For this reason, contact between the pickups and the body wood serves no tonal purpose.

Is that anywhere close to correct? I hope this isn't a question that's been answered numerous times already. I tried a quick search, but didn't find anything.

Thanks,

Dave

I remember a thread about this. My answer was something like, "Remove all the strings from your guitar, plug the guitar in your amp, and knock on the guitar. You'll see that the guitar doesn't need strings on it for the pickup to pick up wood vibration."

Edited by Saber
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I remember a thread about this. My answer was something like, "Remove all the strings from your guitar, plug the guitar in your amp, and knock on the guitar. You'll see that the guitar doesn't need strings on it for the pickup to pick up wood vibration."

Guitar pickups turn the string vibration into sound thru an electromagnetic process. That's a different thing than tapping on your pickups with a screwdriver or it possibly picking up sounds from you knocking on your guitar without any strings on it.

I like the idea that certain woods & wood combos either add or subtract something from the pure quality of just the strings themselves. But you also absolutely can not ignore the guitar pickups, which really are the biggest factor in how your guitar is going to sound as they all have a sound a certain frequency response curve, resonant frequency & output. The wood type and density is what is going to determine things like sustain and what frequencies will be emphasized (or de-emphasized) the most. You even need to consider string gage, type of strings, type of bridge (fixed vs. floating) and even your fret material.

To me, a large part of this "tonewood" business is purely a lot of hype and a good reason to sell a guitar for an ungodly amount of money because it's made out of some kind of exotic wood. I will even be a total heretic and say that I have heard at least one plywood bodied Fender Strat that had a great tone to it. You really have to look at every piece of the puzzle when it comes to guitar tone.

Edited by Paul Marossy
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  • 2 weeks later...

I'm pretty much with Paul in spirit, though I impart a significant bit more importance in the importance of the wood. There's going to be a scientifically quantifiable difference in these factors, mind you. Anyone with a little bit of logic and physics will recognize that what some of the above posters have said is pretty much undisputable truth with regards to the way pickups "hear" the strings.

The real question is "but how much of a factor is it?" and that's when people like me think that direct-mounted pickups vs. pickup rings is largely a discussion about voodoo. On the other hand, to me wood resonance, transferring through the bridge to the strings, is not going to be much voodoo. That's a huge impact on your tone, which is why different tonewoods produce such different responses. But then it becomes relative again when you talk about gain/distortion in amps. Is the degree to which excited tubes impart harmonic information so much "larger" than the tonewood that the tonewood becomes less significance in the "tone ratio" as it were? Me, I tend to believe so.

And in this whole equation is the fact that different pickups sound different. The pickup will impart sonic character regardless of tonewood; regardless of direct vs. ring-mounted. The pickup itself, to me, is the most significant factor in the equation. Diminishingly so for similar pickups, but let's face it... anyone with ears knows that (comparing apples to apples in order to be fair) a single-coil tele, strat, and p-90 sound different, and that classic humbuckers sound pretty different from high-output metal-oriented humbuckers.

Not to mention the placement of the pickups themselves.

In short, all these things are factors. Even the "voodoo" discussions have a certain degree of scientific merit in many cases. Me, I just choose to let logic dictate factors are the most significant. If you are trying to engineer your guitar down to the molecule, it's all important... but if you're like me, you'll put more weight in the fact that pickups sound different from one-another than the concept (fact) that direct-mount and ring-mount will "hear" the strings infantessimally differently.

Greg

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Good points Greg. Perhaps my position is a bit skewed since I use guitars with floating bridges (Floyd Rose locking type). I suppose a fixed bridge guitar will "sense" the naunces of different tone woods a bit more. But even so, you can still mute the effect of the tone wood in any number of ways! :D

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