3

A standard electric guitar utilizes passive induction pick-ups, which generate fluctuating voltage as the vibrating string moves in a magnetic field. This is high-school physics. Because it's passive, the signal is likely to be unbalanced - the signal coming down the guitar cable is a literal reflection of how each string interacts with the magnetic field[s], not how much noise each string is making.

But, how is it unbalanced? It will vary slightly between strings and pickups but your lower strings are heavier than your higher strings, and your higher strings vibrate faster than your lowers strings - both of these factors should have an effect.

What are the characteristics of a typical signal? Is a signal typically weak in the bottom end, weak in the mid-range, or weak in high-end?

  • Well, it will be unbalanced mostly because you send it over an unbalanced line, i.e., you basically ground one of the ends of the coil that is in the sensor. – yo' Dec 8 '14 at 16:41
  • Google search of "guitar pickup physics" yields several articles like this google.com/… – Dave Dec 8 '14 at 16:42
  • Interesting - I'll try to dig through it - but I'm really after skipping to the end result, i.e. by asking the question hoping someone who knows that stuff well can explain what the implications are. – Mr. Boy Dec 8 '14 at 16:51
  • "This is high school physics." It's not nice to assume that we all took physics, ever. – Chris Cirefice Dec 9 '14 at 20:31
  • Why do you think I bothered explaining things in my first paragraph, except for those who don't know the physics? – Mr. Boy Dec 11 '14 at 21:45
2

The similarity between "how much noise each string is making" and "how each string interacts with the magnetic field" is greater than your question implies.

In an acoustic guitar

  • The vibration of the string transfers through the bridge of the guitar to the sounding board. This saps energy from the string causing its vibration to decay.
  • The sounding board transfers its vibration to the air, by virtue of being a big flat surface that pushes and pulls air as it vibrates. This saps energy from the sound board causing its vibration to decay. The sound board has resonances of its own that affect the tone of the sound.
  • Other components resonate or absorb vibrations at certain frequencies - the neck via the nut, etc. This subtly affects the overall tone and sustain of the instrument.

In an electric guitar

  • The vibration of the string transfers through magnetic induction to the pickup. This saps energy from the string causing its vibration to decay.
  • The resulting electric current is carried to effects, amp, speaker. These components have properties of their own that affect the sound.
  • Other components resonate or absorb vibrations at certain frequencies - the body of the guitar via the bridge, the neck via the nut, etc. This subtly affects the overall tone and sustain of the instrument.

The actual vibration of an acoustic string versus an electric string isn't much different. A mathematically "ideal" electric guitar would be completely rigid, so that no energy is lost other than through the pickup - and solid-body guitars come closest to that. Because the loudness of the amplified sound comes from another source of energy, a pickup takes very little energy from the string, so electric guitars can have long sustain.

Other than that, the string behaves the same on both acoustics and electrics. Thick heavy strings vibrate slowly for a bass sound, light thin strings vibrate quickly for a treble sound. If you pluck near the middle of the string you get close to a pure sine wave. If you pluck away from the centre you get more layers of vibration: harmonics.

You use the word "unbalanced", but I'm not sure what you mean by it.

Typically when talking about electrical sound signals, "unbalanced" refers to a signal path that doesn't compensate for interference. If I ran 100 metres of guitar cable -- which contains a signal wire and a ground wire -- and plugged it into a guitar and an amp, I would hear a load of crackle and hum, just from stray magnetic interference in the environment.

If I ran 100 metres of balanced microphone cable in the same way, to suitable equipment, I wouldn't hear this interference. That's because balanced cable has one ground and two signal wires. Each signal wire has the same signal, but one is inverted. They both pick up almost the same interference. At the receiving end, one is inverted again and the signals are combined: the interference cancels out, and the signal is retained.

The same effect happens with pickups. A standard pickup is just a single coil around a magnet. It picks up interference, and that's audible as a hum. That hum is always there under the signal from the vibration of the guitar strings.

A "humbucker" pickup counters this by using two coils next to each other, and wired again such that the interference cancels out. However, because the coils aren't exactly in the same place, and because of complex magnetic fields, the combined signals interfere with each other a little, and it has a little less treble than a single coil pickup -- a sound that's often described as "fatter" or "rounder".

  • When I say "unbalanced" I mean that my understanding is an acoustic amp is pretty "flat" - with all the EQ knobs at 0 it is ideally not modifying your sound - but an electric amp with all the EQ knobs at 0 will be doing some baseline adjustments (such as pumping the mid) as a kind of 'correction' before anything interesting happens. – Mr. Boy Dec 11 '14 at 21:47
  • 1
    Mr.Boy - a) that is not what unbalanced means, and b) a truly flat amp does no boosting – Doktor Mayhem Nov 20 '15 at 9:08
2

This is a really old thread, but it is still missing some of the basics. An electric guitar output can be quantified by the voltage amplitude of the string being plucked or strummed. In the magnetic coil pickups that dominate the market, the peak voltage will usually correspond to the peak velocity of the string because this gives the peak rate of change in magnetic flux in most configurations. This, in turn, through Faraday's law, gives the peak voltage in the coil.

The string peak velocity depends on both the amplitude of the initial pluck, and the frequency, with higher frequency strings (or frettings) having a higher velocity than lower frequency strings (or frettings).

But that's not the whole story. The reason the coil generates a voltage is that the presence of the string modifies the net magnetic field going through the coil as stated above. Some people like to view this as the permanent magnet field inducing a magnetic dipole in the ferromagnetic string, which in turn generates it's field which causes the change when it moves during vibration. But, when calculated this way, the field of the string dipole needs to be calculated correctly by noting any magnetically permeable materials in the vicinity (like magnet or steel poles, etc.) - i.e., it is not the field of a magnetic dipole in free space. Others prefer to look at the problem in its entirety and solve for the proper solution for the magnetic field of the permanent magnet in the pickup which will depend on the proximity of any magnetically permeable materials in the vicinity - most notably the string and its changing location during vibration. When done correctly and self-consistently, these two pictures are identical.

The relevance of this is that the thickness of the string - the amount of ferromagnetic material - also has a great influence on the voltage produced.

So for a properly "balanced" set of strings, for a comparable pluck or strum, they are chosen with thickness, tension, and a net amount of ferromagnetic material such that the desired composite of all these effects give a balanced sound. It's actually kind of nice that nature made it not too different from the strings you'd use on a steel string acoustic.

1

You have nearly answered your own question. Except for one thing.

the signal coming down the guitar cable is a literal reflection of how each string interacts with the magnetic field[s], not how much noise each string is making.

Frequency and volume are the same, but you can't forget velocity. If two strings are tuned to the same frequency but one is vibrating at a velocity higher than the other, the one with a greater velocity/volume will be sensed by the pickup more.

But, how is it unbalanced? It will vary slightly between strings and pickups but your lower strings are heavier than your higher strings, and your higher strings vibrate faster than your lowers strings - these should both have an effect.

So, are the characteristics of a typical signal that it is weak in the bottom end, or the mid-range, or what?

Another thing to consider the sensitivity of the pickups. Here's an example. You pluck the high E and low E with the same velocity. The high E will vibrate at a frequency of 329.63 Hz. The low E will vibrate at a frequency of 82.41 Hz. Knowing what portion of the EQ will dominate the signal is up to the characteristics of the pickups. Some pickups are more sensitive to bass, so the bass notes are more prominent. In this instance, the lower E will dominate the signal.

It is logical to assume that most pick ups are weak in the mid-range as the low-end will likely overpower the rest of the signal.

0

The term for the characteristic you're looking for is "frequency response". There's a thorough exploration at buildyourguitar.com for example, but basically any pickup is more efficient at conveying some frequencies (pitches) than others, and the frequency that is most efficient is the resonant frequency. The guitar's frequency response curve shows the relative strength of the output voltage across a large frequency range.

Frequency response is affected by the type of pickup (humbucker, single coil) the number of windings, the circuitry in the guitar, the materials and physical construction of the guitar, and even the cable.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.