How does a pickup work?

[curtisa]

Higher frequencies have less amplitude than lower frequencies, all along the string. While the lower frequencies decrease in amplitude closer to the bridge

So a bridge pickup sounds brighter because the higher frequencies still achieve full amplitude, while the lower frequencies have less amplitude

And a neck pickup sounds warmer (and louder) because the amplitude of higher frequencies remains the same, while the amplitude of lower frequencies is greater

Very nearly. The string's fundamental tone (the one we hear as the note) has maximum displacement at the mid point of the string's length. Along the length of the string are a number of harmonically-related higher pitched overtones having much shorter wavelengths with corresponding maximum swings at various points. The bridge pickup being near the end of the string cannot hear much of the fundamental, but can hear a lot of those overtones. The higher ratio of overtones to fundamental is what makes the pickup sound brighter. Conversely the neck pickup being closer to the mid point of the string will hear a different balance of fundamental and overtones and sound warmer.

The part of the pickup that "listens" to the string is probably only 1-2cm wide, so when it's placed within a few millimeters of the string it's not surprising that moving the pickup a little bit makes a big difference to the tonality of the signal that comes out the other end. Anyone who's experimented with jamming a mike in front of a screaming 4x12 cab will know how much difference an inch of movement will make to the reproduced sound given that the part of the mike that hears what's going on is about the size of your pinky fingernail.

Edited February 13, 2014 by curtisa

[Prostheta]

Thanks Curtisa! I've had such difficulty relating concepts like this into words this week. Physically and mentally drained.

[Crusader]

Thanks Curtisa! I've had such difficulty relating concepts like this into words this week. Physically and mentally drained.

Same here, I can see now how what I said can be taken the wrong way

but I think we're all saying the same thing, just choosing different words

A picture paints a thousand words so I put this together

Its not exactly to scale but portrays that higher frequencies have less amplitude/displacement than lower order harmonics. Some of the diagrams I've seen on the net show them all having the same amplitude, which is a bit misleading/confusing

This diagram represents the Fundamental, 2nd Harmonic and 8th Hamonic

The bridge pickup is located where the 8th Harmonic has its maximum Displacement (1/16th the scale length) Many bridge pickups are placed around this area

At this point the first Harmonic displacement is greatly reduced, resulting in less volume from that mode of vibration but the 8th Harmonic is still in full swing, resulting in a brighter sound

Thats my theory (now) Later I might show some ideas I had a few years ago

[Crusader]

Crusader as usual its a little more complicated than that but yes for all intents and porpoises you got it

Yes I realise I'm only looking at basics and I might not be quite right about the 440Hz etc. But at times in the past I've been told (quite emphatically) that these points of maximum and no displacement in harmonics have nothing to do with the tone you get from pickup position, its just "distance from the bridge" So I looked for other reasons why pickup position changes tone

This picture shows a vibrating string and as you can see it forms a "V" shape closer to the bridge, and I thought the bridge pickup might be brighter because of this. But now I realise these pictures only show the Fundamental mode of vibration

Neck pickup. String passes more evenly

Bridge pickup. String passes unevenly

I want to say more but its getting late and I have some heavy work tomorrow

[westhemann]

All of that is not true at all...it's tiny little gremlins in the pickup that start squawking every time they hear the vibrations...they match those vibrations...just like when the little one was humming in the movie.

It's an oversimplification,but for all intensive porpoises...

[Prostheta]

You know what? Between our respective butchering of the physics, I am sure we've gotten 50% of it right. No ideas as to which 50% though. We've quite possibly gotten our Macs fixed up.

[Crusader]

Just want to explain those last two diagrams again. I pondered if the Bridge pickup was brighter because of the string cutting across on an angle. That was just one of the ideas I came up with a couple of years ago.....hang on, I just checked when I made those diagrams. It was 5 years ago! But my main point, it was just a theory

I think Curtisa explained it best and its perfectly logical, its exactly the same as for accoustics

...The speed of the "Alternating" in AC is what we hear as pitch. The magnitude of the AC swing is "volume" to us.

I think it gets confusing when you start to think about higher frequencies (upper harmonics) How does the pickup read all the different harmonics at the same time?

I was just thinking its like when you're on the telephone, you can hear people at the other end and they can hear you at the same time, all on the same wire

[curtisa]

I think it gets confusing when you start to think about higher frequencies (upper harmonics) How does the pickup read all the different harmonics at the same time?

The simple explaination is probably "it just does". In the same way a mike can reproduce the complex sound of a human voice, or a camera can "see" a near-infinite pallette of different colours.

Probably what's more interesting/amazing is that a moving string has all that going on in the first place. The pickup is the boring bit. It's the source material that makes it rock.

[Tim37]

It is actually pretty simple to understand. When you look at those charts you see the whole string and get confused. The pickup sees a very short section of string it just reproduces that movement.

[pan_kara]

its just a superposition of movements (at different frequencies) - if you swing your arm back and forth and at the same time make some fast wiggling motions .. you're superimposing two frequencies on top of each other. If you attach a smaller pendulum to a bigger pendulum.. the motion of the weight at the end of the small one will be the sum of the two oscillations. And so on.

The string does the same, just superimposing many more types of motion at once.

And then the motion of a "piece" of strings that is sensed by the pickup is just a sum of these motions. And precisely this motion is what the pickup reads. It just reads whatever movement the string is making. So to join in the game: to me the magic is not in the string vibrating with all those frequencies at once, its not in the pickup picking up the vibration - its in our ear (brain) that takes the complex movement and actually decomposes it so that we hear individual pitches in this complex waveform.

Re why the bridge pickup has more treble.

As Tim37 is saying - the pickup just senses a part of the string and looks at the vibrations in that one part. The signal strength (amplitude) is proportional to the amplitude of the vibration.

At this point.

The first harmonic is at the 12th fret. Moving a pickup from the 12th fret towards the bridge decreases the volume of the root harmonic. The 2nd harmonic has its maximum above the 24th fret and then again decreases towards the bridge. For the higher ones it gets of course more messy, the 4rd one disappears above the 24th fret etc.

So the more you move towards the bridge, the less of the (at least) first 2 harmonics are present in the sound. The higher ones are more or less constant, so the balance shifts --> more treble in the sound.

[Crusader]

you're superimposing two frequencies on top of each other...

...It just reads whatever movement the string is making

Superimposition, that's the word

Apart from Googling 'superimposition' and so forth, I've just been playing around with harmonics. And when you play the 8th harmonic (or any harmonic for that matter) the string barely moves at all, they're just ripples. This helps me understand how all the harmonics can be on the string at the same time

What I find most amazing is the pickup can read these microscopic movements, all thanks to electromagnetism!

Well I think my question has been answered and I thank you all for your contributions. The other night when I posted this question I didn't think it would go for so long and the more I think about it, the more questions I find. Now its giving me a bit of a headache

Lets us conclude, "IT JUST DOES"

Edited February 18, 2014 by Crusader