Guitar Pickup Analysis Web Link

[bbsailor]

Below is a web link for a good technical analysis of guitar pickups.
http://www.moore.org.au/pick001.htm

Joseph Rogowski

[David Schwab]

Great info there!

[guitician]

Pickup too close to the strings???

Yeah, I was looking at the page titled "Unified Pickup Measurements" and noticed how he states that.......

"If you put a relatively powerful magnet near the strings, they may be very marginally pulled towards the magnet. The pitch is effectively not changed and the string still sustains as before."

I've always noticed a "Warble" when the pickup is too close to the strings. Am I just amagining this?

[bbsailor]

Yeah, I was looking at the page titled "Unified Pickup Measurements" and noticed how he states that.......

"If you put a relatively powerful magnet near the strings, they may be very marginally pulled towards the magnet. The pitch is effectively not changed and the string still sustains as before."

I've always noticed a "Warble" when the pickup is too close to the strings. Am I just amagining this?

guitician,

The warble is the caused by the magnet being too close to the strings. The up and down string motion is the only part of the string motion that generates a pickup voltage. Sideways string motion does not contribute to output, according to induction theory.

Typically, magnets that have more mass (diameter, length), magnet type, string diameter, string-to-pickup distance and string tension contribute to the "warble effect". Thicker strings are affected more than thinner strings. Loose strings are affected more than tighter strings.

The recommended maximum pickup height adjustment procedure is to move the pickup close enough to the strings to hear the warble than lower it until the warble stops. Then, adjust the low-E to high-E string balance or individual string balance to suite your ear.

Joseph Rogowski

[guitician]

Yeah, that's what I thought. I think the statement I quoted was refering to sustain and pitch, which is not the same as an attack distortion or warble. Thanks

[Mike Sulzer]

Yeah, that's what I thought. I think the statement I quoted was refering to sustain and pitch, which is not the same as an attack distortion or warble. Thanks

I think the quote is wrong. Think of the total motion of the string as divided into two modes, one in which it moves vertically, the other horizontally. A strong vertical B field affects the frequency of the vertical mode. But the modes are not completely independent; energy from one moves into the other. So the "beat" that you hear is from two nearly equal frequencies present at once.

If you use really strong magnets and light up the string just right, you can see the normally elliptical motion of the string become sort of wild. I am tempted so say "chaoitic", but I doubt that this really qualifies under the formal definition of chaos.

[guitician]

I guess that it pulls a discordant harmonic into the vibrating string.

[Mike Sulzer]

I guess that it pulls a discordant harmonic into the vibrating string.

I think the when the string moves vertically in the vertically-directed B field, the average tension on the string is altered as the force due to the field varies, thus changing the frequency. Horizontal motion, that is parallel to the field, does not result in a change in the force from the B field. With both motions present at once, the situation is nasty, resulting in a coupling of the two modes. I do not know the details, but this seems reasonable.

[Mal88]

Hi Mike

I used to think that the pickup picked up on vertical and horizontal motion relative to the strings but my experiments showed that relative horizontal motion gave very little output compared to the vertical relative motion - and that is why I wrote what vertical relative motion gets translated into electrical signal.

You are right that the string appears to move elliptically (if not round), but when you start to analyse the magnetic field from the magnets' pole pieces - you see that the magnetic field very quickly ends up with a uniform gradient up into / through the strings - so if the string moves on a constant magnetic potential line, vertical (normal) to the magnetic lines of force, then you will see that the strings do no 'work' - hence they will not produce an electrical signal via this lateral motion.

Be aware that the magnetic field stength / potential is not linear with distance from the magnet's centre and so the linear motion of the string will produce a non-linear voltage, and this accounts for the strong even harmonics (or that 'warm' sound), that can be accentuated by single-ended amplification without feedback - but that is another story!

Cheers, Malcolm

[Mike Sulzer]

Hi Mike

I used to think that the pickup picked up on vertical and horizontal motion relative to the strings but my experiments showed that relative horizontal motion gave very little output compared to the vertical relative motion - and that is why I wrote what vertical relative motion gets translated into electrical signal.

I agree that the horizontal motion of the string results in very little output. That is OK, but let us look at what happens when a string vibrates. Ideally, in the simple situation a physicist would analyze as a first attempt, if you started a string vibrating in the horizontal plane, it would continue to do so. That is, there would be no vertical motion. But you are not really going to achieve this simple situation in practice. There are asymmetries, both mechanical (how the string is supported), and magnetic. So one expects that the vertical and horizontal get coupled. So if the h and v vibrations are different in some way, as caused here by the B field, then this difference is audible when the h affects the v (which the pickup senses). String pull (the warbling sound) is something that occurs at a slow rate, much slower than the frequency of the string. So to explain it, we have to look for a frequency difference. This is provided by the differences of the vibration properties in h and v when B is present. This is a "seat of the pants" explanation; the details of how it works are probably quite complicated, and not so easy to analyze.

[Joe Gwinn]

I think the when the string moves vertically in the vertically-directed B field, the average tension on the string is altered as the force due to the field varies, thus changing the frequency. Horizontal motion, that is parallel to the field, does not result in a change in the force from the B field.

With a blade pickup this would be strictly true. With magnet and especially slug poles, there is also a variation in magnetic field with horizontal motion.

With both motions present at once, the situation is nasty, resulting in a coupling of the two modes. I do not know the details, but this seems reasonable.

The classic extreme example and demonstration is the Wilburforce Pendulum.

http://www.ap.stmarys.ca/demos/conte..._pendulum.html

[David Schwab]

Hey Malcolm, nice for you to join us here!

Bill Bartolini had an interesting take on the up/down vs. side-to-side string motion. He showed that if you have a tall vertical pattern over your pole (such as from round rod magnets), the string's side-to-side motion will be picked up because the lines of flux are more vertical, so the sideways motion cuts through them.

He then designed the wide planar pole tips to produce a low field with more horizontal lines of flux to capture the up-and-down motion of the string, as he felt that was more akin to an acoustic guitar's soundboard motion, as they produce little sound from the sideways motion.

From his two patents:

Basically, the tone of a plucked or a struck string instrument is judged by the richness and complexity of the acoustic output in the "attack" or beginning portion of a note. In acoustic string instruments, the bridge structure constrains the motion of the soundboard such that those components of string motion which are perpendicular to the, plane of the soundboard are well amplified, while those components ofthe string motion which are parallel to the plane of the soundboard are not. The path described by any arbitrarily small segment of a smoothly released, plucked string is a precessing elliptical orbit of decreasing radius which rotates about the quiescent position ofthestring. Accordingly, the asymmetrical amplification of string motion provided by the bridge of an acoustic instrument yields a rich, full and complex tone of continuously varying, harmonic content.

The prior art variable reluctance pickup systems are characterized by separate pole tip and/or pole pieces for each string. Each pole tip and/or pole piece provides a distinct magnetic field region around the quiescent position of each string. The distinct magnetic field regions of prior pickup systems render them relatively insensitive to the plane of vibration of the particular string.

For example, pickup systems with circular pole pieces provide a magnetic field having the form of a symmetrical sinusoidal shell and a string vibrating within such a magnetic field will generate approximately equal magnitude electrical signals for string vibrations both parallel and perpendicular to the string plane.

The pole pieces and the poletip faces provide a magnetic field region proximate each string which has a large magnitude magnetic flux gradient in a direction perpendicular to the string plane and a small magnitude magnetic flux gradient in a direction parallel to the string plane (parallel to the sound board).

[Mike Sulzer]

With a blade pickup this would be strictly true. With magnet and especially slug poles, there is also a variation in magnetic field with horizontal motion.

The classic extreme example and demonstration is the Wilburforce Pendulum.

http://www.ap.stmarys.ca/demos/conte..._pendulum.html

Nice link! I had not seen that one.

Yes, there is some horizontal variation in B. It is smaller than the vertical, though; so I hope one can ignore it when looking for reasonable, simple, explanations!

[Mal88]

I used to think that the magnetic rods in pickups like the Strat and Tele would have created a series of ridges that would give signals a sense of 'ownership' between pole and string - but if you pull a string over a couple of the magnetic poles - do you hear a distinct drop and increase in level as the string transverses? I don't!

It therefore follows that the magnets in a single file form 'sheets of equivalent magnetic field strength' that sits like blankets on a matress - so you don't really feel the springs in the bed as you run your hand over the sheets - and that is equivalent to the strings moving in a plane a few mm over the magnetic poles, and the further you move away the more consistent these sheets become.

With Antennae, Hygens theory matches these comments! I did some finite element magnetic modelling and it shows that after about 2 mm there really is an almost flat magnetic field strength above the line of magnetic poles.

Interesting to note that the magnets lose a lot of their fields between each other in the strip, and it would also follow that because the magnets are separated, then the eddy current losses (in these highly conductive magnets) are isolated into the separate magnets and consequently these frequency dependent losses are somewhat limited - and that is good!!

[David Schwab]

I used to think that the magnetic rods in pickups like the Strat and Tele would have created a series of ridges that would give signals a sense of 'ownership' between pole and string - but if you pull a string over a couple of the magnetic poles - do you hear a distinct drop and increase in level as the string transverses? I don't!

You do to an extent. That's why a lot of people like blade poles, so they don't have drop outs when bending. You can see the profile of the rod magnets in the Bartolini patent picture above marked as "II".

It all depends on the size of the poles. You especially hear it with bass pickups.