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Transducer magnet and making sense of a sound pickup

  1. Aug 28, 2016 #1

    I am a layperson, and have very little understanding of physics.

    I'm trying to understand the relation between a magnetic field and air and how they are similar or dissimilar in terms of disturbance in them.

    Let me lay out the context from which I came to think about the question.

    Electric guitars use magnets to detect the vibration in the guitar strings. These are called "pickups" as you probably know. From Wikipedia I learn that they are called transducer magnets. So these magnets are a variable reluctance sensor that captures or senses mechanical vibrations from stringed instruments such as the electric guitar".

    So, the magnet detects the movement of the electric guitar strings and then translate that into electric signal. This signal goes into an amplifier and converted back to sound by vibrating a speaker. Fine. I understand the mechanism in theory.

    So what is the relation between movement in the air (which cause the sound heard acoustically) and movement in the magnetic field (which is what the pickup detects)?

    To elaborate, if I play an E note on an unplugged guitar I hear the sound because the string vibrates and move the air, which is then detected by my ears. Now I am plugging in the guitar and that same sound is being projected through a speaker. The movement in the air created the same sound.

    So, when the magnet picks up the string movement what it is picking up? If it is picking up the disturbance in the magnetic field created by the strings then why exactly it is the same thing as the disturbance in the air?

    If the question is not clear I will try to further clarify. But for now I'll wait for answers. Thanks a lot in advance.
  2. jcsd
  3. Aug 28, 2016 #2


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    It's not clear what your question is actually about and some explanation may help you. You may want to follow up with a supplementary question.
    The two things are pretty well separate. Ignoring the small amount of energy that the vibrating string would lose into the air, the motion of the string over the poles of the magnetic sensor would not be different. Also, with or without the magnetic sensor, the vibrations of the string would affect the air or (more particularly) the guitar body in much the same way.
    The magnetic pickup does not respond to air movements - only to the movement of the steel string. Your ears are not aware of magnetic fields so they only 'hear' mechanical vibrations. You will be aware that the acoustic tones of guitars vary and that different pickups can also produce different sound from the same guitar. Also, of course, the more discerning players can hear the subtle differences between what comes out of the amp for the same pickup, at the same position on the string but with a different body. So the motion of the steel string is affected by the acoustics / mechanical nature of the guitar.
  4. Aug 28, 2016 #3
    Thanks for welcoming me. :) And more thanks for attempting to answer the question.

    What the question is Not about:

    1. The variations in tone created as a result of acoustic properties. I am also aware that different pickups produce different sounds.

    2. The question is also Not about the differences in the tone that we hear acoustically and that which come out of an amp. Two are created by two different vibrating objects. I understand why the sonic nature of these two sounds are different even when the sound is conveying the same musical note.


    Let me now set up the question again:

    1. A guitar string is being hit. The vibrating string causes the air to move and create sound. We hear this acoustically. Let's say it is a 'D' note.

    2. A guitar string is being hit, we still hear the acoustic sound but we will ignore it this time. A pickup transmit it to an amplifier which then amplifies the signal and then creates a noise through a speaker. The sound coming out of the speaker is the same sound: that same D note (again we understand that the sonic properties aren't identical but that is not our concern).

    So, from this information we see that the pickup is able to capture the same sound that was earlier created by a disturbance in the air. But the pickup only detects a disturbance in the magnetic field and NOT the movement of/in the air.

    How the disturbance in the magnetic field equates to the disturbance in the air created by the physical movement of the string. In other words, if pickups don't hear/detect air movement and detects only the mechanical movements how then the mechanical movements create the same thing that air movement does?

    I understand this clearly for regular microphones. It has sensors that detect air movement and captures it. But if someone said that the mic has a magnet that detects your tongue and lip movements and then translate it into speech exactly like I speak, but louder, I would be baffled. It's not a perfect analogy by any means but I hope this explains my question.
  5. Aug 28, 2016 #4


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    Right. Well the main reason that the information in the air and the information in the magnetic pickup are basically the same is that the frequencies of the vibrating string have to be in step (phase). You can't easily produce frequencies that are not there in the first place. There are processors that will do that but they are not just simple magnetic transducers.
    Your example of a microphone is an interesting one because, although most mikes sense the vibrations in the air around your face, there are throat microphones that detect the vibrations inside your throat. The two mikes are detecting different things but, again, the signals they produce will always contain the same basic frequencies of oscillation.
    If you had steel lips and mouthparts then it would be reasonable to suggest that a magnetic pickup would reproduce some of the aspects of your speech. But the resonances of your mouth cavity are what shapes the sounds you make and any sensor would have to know what to do with the signals from the magnetic sensors in order to 'know' what actual sounds you are making. It would be much more complex than just sensing the vibrations of a string - but it would be difficult (impossible?) to produce the sound of an acoustic guitar just by sensing the magnetic fields around the magnetic sensor.
  6. Aug 28, 2016 #5


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    The string vibrates.
    It moves the air causing sound waves. That works whether the string is steel or nylon.
    If the string is steel, its movement varies the magnetic field in the pickup, just like moving a piece of iron near a coil containing a magnet.
    If it were nylon, there would be no signal in the pickup.
    Here is a Youtube video (just the first I found, not necessarily the best.)
  7. Aug 29, 2016 #6
    Hey Merlin,

    I understand how guitar pickups work. What I don't understand is how the disturbance in the air can be understood also from a disturbance in the magnetic field. I get that the pickups DETECTS the movement. But why the disturbance in the magnetic field is the same information as the disturbance in the gas around us?

    I'm probably not understanding something that is obvious to you but I just can't relate the two.
  8. Aug 29, 2016 #7

    jim hardy

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    Why do you assume they are the same ?
    They must have the same periodicity because they have the same cause which is displacement of the string
    but when expressed as % of background versus displacement of string
    they'll have different amplitudes ,
    different linearity ,
    therefore different harmonic content,
    and different "sound" to the ear that's paying attention.

    So, the only sense that they're "the same" is base frequency .

    I submit that, for a given acoustic guitar, a spectral analysis of its air signal from a microphone will differ in harmonic content from a spectral analysis of the electric signal from its magnetic pickup. You know it's true for a solid body guitar.

    That's why live music from real instruments "sounds" different than electronic. Harmonics are 'overtones' that give color to the instrument.
    Do you not hear a difference in tone between a pickup-equipped acoustic guitar when played by itself versus amplified ?

    that's my thought.

    old jim
  9. Aug 29, 2016 #8
    The pickup has nothing to do with the air... i.e. an electric guitar would still "work" in a vacuum.

    In both cases the source of the signal ( waveform) is the string, which when plucked would vibrate with or without air.. Case 1 ) Energy from the vibrating string is transferred to the air and creates a sound wave that your ear can detect. Case 2 ) Energy from the string disturbs the magnetic field of the pickup, which creates a detectable electrical signal in a winding in the pickup.

    The main reason you do not her a difference is the predominant influence is the vibrating string and it's natural harmonics ( many factors here due to overall instrument) - the signal you ear received directly through the air is nearly identical to the signal the pickup receives, and then the whole concept of Fidelity comes in to the electronics and speaker; basically the system should not be affecting the signal, unless some desired distortion is added.
  10. Aug 29, 2016 #9


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    The string vibrates - moves from side to side perpendicular to its length.
    Viewed from a bit of air nearby, when the string moves nearer, it compresses the air and when it moves away, it rarefies it. The alternating compression and rarefaction of the airthus matches the movement of the string, both in time and in amplitude.
    Viewed from a magnet near the string (wire string) when the wire moves closer, it intensifies the magnetic field in the coil and when the wire moves away, it weakens the magnetic field in the coil. Changes in the magnetic field induce voltages in the coil, which will be amplified by the amplifier (naturally!) and used to cause currents in the speaker, which make the cone move forwards and backwards, compressing and rarefying the air, again copying the movement of the string*. The speakers can move a lot more air and make much more sound.

    I more or less agree with Jim's comments. But the reason I see for the difference in the two sounds (acoustic direct and electronic via magnetic pickup, etc), is that they represent different bits of string! The magnetic sensor is mainly affected only by the movement of the bit of steel near to it, I'd guess maybe a cm or so, but mainly the bit right next to the core of the pickup. The air will be moved by all parts of the string, though I guess more by the central region where the amplitude of vibration is greatest. Also the vibrations will be transmitted through the bridge, tailpiece and even to some extent the head and neck to the body and the vibrations of all parts will move air and add to the total sound produced in the air. (Not nearly so much as in an acoustic guitar, where this is intended to happen, and the body is made of thin boards which vibrate more than the solid electric guitar body.)
    The way different parts of the guitar vibrate will "colour" the sound by accentuating or attenuating some harmonics more than others. Similarly, the position of the pickup will mean it is more or less sensitive to different harmonics of a string's vibration. If it is positioned near the nodes of a particular harmonic, the amplitude of vibration at that frequency is small and will have less effect on the pickup. Harmonics which have antinodes near the pickup will conversely have greater effect. I see some electric guitars have more than one pickup and I wonder if this is to allow the artist to select a balance between the signals from the pickups to achieve a colouration that (s)he likes.

    * My account of the pickup leads to the electronic signal being the differential of the movement of the string. But thanks to the magic of Mr Fourier's maths and the fact that our ears are indifferent to phase, it all sounds the same in the end.
  11. Aug 29, 2016 #10
    At the expense of sounding extremely dumb, Its amazing how you could figure that out by using science. Yes, the pickup nearer to the guitar bridge is very treble heavy and the overtones are totally different through a neck pickup.

    I'm starting to understand more I think. I'm going to post a final supplement the original question.
  12. Aug 29, 2016 #11
    Hy everyone,

    Merlin, Windact, Jim, and Sophie

    Thanks so much for taking the time to write these answers. I'm certainly understanding more than I did before.

    But the problem is that I think you are not understanding what I am not understanding.

    Let me try to walk you through my thinking that led to my question.

    Pickups collect information and then through the magic of electricity and engineering it can be translated back into sound. I have no idea how that happens. The circuit boards do their magic. But I can kind of make sense of it when the receiver, in our case pickups, receives "something" > converts it into electricity > that gets interpreted correctly > converted back into the original "thing". In case of microphones, in my mind, it makes sense. Because even if i do not understand the mechanisms that is used in those circuit boards I can see that it received a vibration in the air then converted into electricity and then converted it back into what it received. So the original "thing" was vibration in the air, which we recognize as sound. But guitar pickups never received the vibration in the air. How then it receives a different thing, converts it back into what it did not receive?

    Am I making any sense?
  13. Aug 29, 2016 #12
    Well yes and no... - Have you ever seen a waveform? We have great an much loved tool called an oscilloscope that lets us "see" the waveform.

    What we hear, is a waveform of varying air pressure, or sound pressure, with the pick up we also start with a wave form... what is interesting if we compare a good microphone waveform recording of a guitar on our fancy oscilloscope to a pick up wave form - they are very similar. Basically any way we try to visualize a sound waveform, it will look nearly identical to the electrical waveform we see with the Oscilloscope. I am thinking that you may be making this a little more complex than it is. -- Another example is the old analog records -- the pattern on the grove in the record also will look almost identical to our electrical signal - before record players were electrical with small magnetic pick ups, you just put a needle in the grove attached to a converter and a horn - the system was all mechanical. Gramaphone

    The sound pressure - is small variations in air pressure occurring at different frequencies, our electrical signal is small variations in voltage over the exact same frequencies.
  14. Aug 29, 2016 #13

    jim hardy

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    Have you ever played with magnets ? Or a compass ? They're silent and do not care whether there's a breeze blowing or they're in a vacuum on the surface of the moon.

    Get out your compass and bring a nail near it. The needle will move. The compass "senses" the position of the iron nail from a small distance.
    Similarly the magnet in the pickup is affected by the proximity of the string. It senses changes in position of the string. That change occurs at the frequency of the string and whether you sense that by ear(air pressure) or by light(photodiode) or by magnetism(guitar pickup), it's still the frequency of the string.
    And your ear responds to air pressure, and your cochlea discerns the frequency.

    Do not ask me what is magnetism, it's just something that occupies or distorts space but unlike air it has no mass or atoms.
  15. Aug 29, 2016 #14


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    I've drawn something to try to illustrate what I said before.
    1 - Movement of the string creates movement of air, either;
    a) directly (acoustic guitar), or
    b) by changing the magnetic field by the PU, which causes a voltage, which is amplified, which moves the speaker cone, which moves the air.

    2 - By either method the way the air moves is effectively the same - fast vibrations produce fast changes in voltage, fast movements of the speaker and fast vibrations in the air (sound waves of high pitch.) Slow vibrations likewise produce slow vibrations in the air (sound waves of low pitch.) Big movements of the string cause big movements of the air (loud sounds) - in the case of the electric guitar, via big changes in magnetic field, big voltages and big movements of the speaker.

    3 - The little "graphs" on the right are supposed to show the movement of the string relating to all the other things which are changing. It shows a "sinewave" which represents a single tone - for a higher pitch, the wiggles are closer, for a lower pitch, further apart. For a louder sound the wiggles are bigger, for a quieter sound, smaller. But however you vary it, the other elements in the chain vary in the same way -
    more movement of string = more change in magnetic field,
    more change in magnetic field = more voltage,
    more voltage = more movement of the speaker cone
    more movement of the speaker cone = more movement of air.
    So whichever route you take, acoustic or electric, the movement of the string ends up as a matching movement of air. And vibrating movement of air is sound.

    4 - As I put as a trivial footnote before, I've shown the wiggles may get a little out of step via the electric route. This is phase shift and and at this level of discussion makes no difference to what you hear.

    5 - Tone differences are not shown here. The way the string vibrates is not the same at all points, so what the magnetic PU sees depends on its position along the string. It will generally be different from the vibrations picked up by the bridge in an acoustic guitar.
  16. Aug 30, 2016 #15
    Windadct, Jim, Merlin

    Thanks so much. This has to be one of the most friendly and helpful forums.

    I think I understand now. Wind's answer got through to me and the others confirmed it. So it appears that the originator of the whole thing is the movement of the string and the relation between air and magnetic field is that they both get impacted by the same movements, so, it matters not that pickups did not "hear" air movement. The waves in the magnetic field carry the same thing/signature/whatever as the air waves. I think I can live with that. :)

    Thanks again.
  17. Aug 30, 2016 #16


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    not quite .... the steel string moving in the magnetic field of the pickup causes variations in the field
    those variations generate a current in the wire coil(s) surrounding the pickup magnets

    An electric guitar will work just the same in a vacuum, where there is no air

  18. Aug 30, 2016 #17
    Hey Dave, I probably articulated that incorrectly. But I do realize that an electric guitar will work in a vacuum. In fact, that's exactly what baffled me so much.
  19. Aug 30, 2016 #18


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    ok No probs, just checking, as you constantly speak of air and magnetic fields in the same sentence as tho they are somehow related

    If you understand that they are not, then my comment in my first sentence ....
    really answers ALL your questions
    one of the others should have put it like that many posts ago as it would have probably saved much confusion :wink:

  20. Aug 30, 2016 #19


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    I think in threads like this, the problem is working out what it is that someone needs to understand. We each look at the questions and make our own interpretations of what is needed. Is it, "how can the pickup detect the vibration at all?", "why is the sound produced by the electric amplifier the same as the sound produced by the string?", "why is the sound from the electric amp similar but different?", etc. And then one is wondering just where the gap in understanding is that needs to be plugged.

    Anyhow, it's fun for us. Makes a change from crosswords and sudoku.
    And as usual it reveals the gaps in my own understanding. I confidently explain the electric pathway (brushing over the bits that are a bit difficult) only to realise that I don't understand how the sound is produced in an acoustic guitar!
    I know the string moves very little air and would be hardly audible on its own. But how does the body of the guitar get the vibrations from the string? Obviously the bridge is the key, but isn't that, almost by definition, at a node for all transverse vibrations? Perhaps, since the bridge isn't totally non-compliant, it is not a node and the virtual end of the string is just past the bridge. Then there would be an end correction to the length of the string (cf antennae) and the frets would need to be spaced from this virtual end.. Whatever, I find as usual that even an apparently simple mundane thing, I only partially understand.
  21. Aug 30, 2016 #20


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    Have we dealt satisfactorily with your query yet? You originally asked why the two methods seems to produce very nearly the 'same' effect on the ear. Well that's just due to the fact that the same frequencies in roughly the same proportions are heard by the ear in both cases. No new frequencies can be produced if the system is 'linear' (good fidelity). Guitar amps are often coupled with distortion boxes which have strong non-linearities (limiting amplifiers) and weird phase effects. These will produce new frequencies that are not present in the original string vibrations. (That's a matter of taste - and age)
    Something else that I don't think has been dealt here with is that, despite what people say about our hearing not being sensitive to the phases of the frequency components, we actually have some time discrimination in our hearing of sounds so differences in the timbre of notes and the attack and decay can be detected. This is an evolutionary thing because it gives us the ability to estimate the size of space (rooms / caves ) we are in, from the effect of delayed sound reflections on the wave shape of what we hear. (More subtle than just hearing well defined ECHOS . . echoes ...echos)
    Music is often processed to maximise the 'loudness' of sound by changing the phases of frequency components to suppress peaks (compressing) so that the overall gain can be increased without 'clipping'. That can be heard and the amount of compression is regulated to keep the impairments acceptable.
  22. Aug 30, 2016 #21


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    If there were no energy lost then the note would sustain for ever. Energy is, in fact, transferred into vibrations of the bridge so it isn't a perfect node. As you suggest, you can regard the bridge as a node with different positions depending on the frequency involved. However, with the sort of Q factor that you get with a vibrating string, the proportion of energy lost at each cycle, the "end effect" won't be very different. The same effect will occur at the frets but the lost energy will then go into the fingers. I imagine that a fretless instrument like a violin (which has a much shorter sustain time) must be affected much more by this - but the player would compensate by placing the finger appropriately. All these are second order effects, I guess, and allow us to differentiate between good and bad players. :smile:
  23. Aug 30, 2016 #22
    Yes. Thanks to all of you.
  24. Aug 30, 2016 #23
    I think that is exactly right ("...in threads like this, the problem is working out what it is that someone needs to understand").

    It seems that most of the responders did not understand what I wanted to know (I think you did though). This is not because of their lack of knowledge, but because, I think, they were presuming I possess certain knowledge/understanding that I did not have and which to them was obvious knowledge.
  25. Aug 30, 2016 #24


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    Absolutely. It's a great temptation for some OPs to say "What, exactly is blah blah blah?" when they actually couldn't deal with what exactly blah blah blah is. They often need some hooks to allow them to connect a bit of Science with their everyday knowledge. There is a great temptation to be over smart with answers and for established PF posters to take over with big boys' talk and to frighten the poor OP away.
    Most 'visitors' are usually too polite and they really should be prepared to give a WTF reply and bring the conversation down to a decent level. The P I A filters on the General Physics Forum can be useful - when we bother to read them, that is.
  26. Aug 30, 2016 #25

    jim hardy

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    If you ever watched guitar strings vibrate it's not only in the horizontal plane
    i kept an old piano action for a while just out of curiosity. The strings go left-right, up-down, precess , and make interesting patterns.
    Up-down component of motion will be transmitted via bridge into the thin wood top which acts as a diaphragm.
    While helping my friend Harry work on old violins i saw they're more than just a resonant sound chamber underneath the strings
    the bridge support is asymmetric and cantilevers motion into the top plate, and into the bottom one via a soundpost. Top also has a "bass bar" under the low frequency string (G?) i suppose to encourage it to vibrate as a large plate, and to make the bridge rock which flexes the top.

    violn cross section.jpg

    Next guitar i see open i'll look for its subtleties .


    Yes, we all want and try to "answer the question" . We generally answer the one we heard. The wide difference in answers reflects how our perception is colored by our background .
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