How does an electric guitar waveform differ from a pure tone?

Question

I've been writing a program that functions similar to a MIDI interpreter, but the pure tones I'm currently using very poorly resemble the actual tones produced by guitars, pianos, and other instruments.

I know that instruments exhibit an overtone series, but I can't find any information about the specific overtone profiles of guitars. How exactly would I mathematically simulate the tone of an electric guitar? How exactly does its tone differ from a pure tone?

Answer 1

I know that instruments exhibit an overtone series

Yes - but even talking about 'the overtone series' of an instrument is a big simplification. In reality, each of the overtones might be considered to change amplitude and pitch over the duration of the note, according to a large number of different variables.

https://www.soundonsound.com/techniques/synthesizing-plucked-strings is looking at an acoustic guitar, but most of these aspects are still relevant:

• Each string produces a different waveform depending upon the plucking position;
• The shape and hardness of your fingers or the plectrum influences the high-frequency content of the initial waveform;
• The amplitude envelope of the oscillators depends upon the direction in which you pluck the string(s);
• The strings' harmonics are 'stretched' as the pitch increases and/or the excitation increases in amplitude;
• The six strings interact with each other in different ways, depending upon their pitches and the number of them which are free to vibrate at any given time;
• Each string interacts with a system of complex resonators (the guitar body) that absorbs energy and then directs it back to all the strings, exciting harmonics that may not be present in the initial waveform;
• The body has many densely packed resonances and anti-resonances that can not be imitated using conventional equalisers or filters;
• The nature of the resultant sound is extremely dependent on the position of the listener and the angle between the listener and the instrument.

It's probably no exaggeration to say that no two guitar plucks are exactly the same! It may be unintuitive, but a simple bit of wire tied to a bit of wood has incredibly complex behaviour...

If you want to dig into all this, the synth secrets series is an excellent primer - https://www.soundonsound.com/series/synth-secrets-sound-sound.

Answer 2

I'd like to start with a note that if I knew a good answer to this question, I would be spending my time earning money on selling guitar emulation virtual instruments, rather than sharing the information for free.

Note also, that as far as I'm aware, presently the best guitar virtual instruments are based on sampling, rather than modeling. This may give a hint about the difficulty of the task.

• The first part of the sound is attack. It's percussive, i.e. continuous spectrum and broad band, depending on whether the string is plucked by a finger, finger nail, or plectrum, its shape, texture, and also angle and time duration of the contact with the string. Guitar body resonance (at least with classical and acoustic instruments) may contribute significantly to the attack sound.

• Plucking the string starts with displacing it from the equilibrium into a triangular shape, with the tip rounded (due to finite size of the finger, nail or plectrum, and the string rigidity). Fourier transform of this shapes gives the initial ratio of amplitudes of the harmonic components.

• As the string vibrates, the amplitude of various frequencies decays in various ways, as the energy is transferred to the instrument body, the air, or dissipated internally in the string, which depending on the string parameters.

• Various characteristics of guitar body and construction affect how quickly energy various harmonic components is dissipated, for all types of guitars, both classical/acoustic, and for solid body electric as well.

• There are (at least!) two effects causing inharmonicity:

  1. frequency of harmonic components is slightly increased due to string rigidity
  2. When a string vibrates with large amplitude, it's effective tension increases, resulting in pitch changing as a function of the decaying amplitude (time)

The inharmonicities depend on string material (Young's modulus), string gauge, length and tension. They also may result in transferring energy between harmonic components.

• The string transfers energy to the guitar body via the bridge. For nylon strings this is primarily via transverse vibrations, while for steel strings both transverse and longitudinal vibrations contribute to a similar degree.

• Finally, how the actual sound is produced? In classical/acoustic instruments the sound is primarily emitted by the top plate, with some additional contributions of the sound hole (resonance of the air cavity), and the back, altogether captured by a microphone, placed at some location. Then you have instruments with piezo pickups, and finally electric guitars with electromagnetic pickups. There are also some more exotic systems like optical transducers or pickups capturing vibrations of the body. Any of these systems, and its parameters affect the final sound significantly.

• Last but not least are various additional sounds, characteristic for guitar. What comes to my mind is the noise when sliding the finger along the string (in particular the wound strings) and fret buzz. Also, guitarists use multiple articulation techniques which extend the spectrum of available sounds.