IB Physics IA idea · Music and sound
IB Physics IA idea: how does the pluck position change the harmonics?
Pluck a string in the middle and some overtones vanish. Measure how the spectrum depends on pluck position.
These ideas are starting points, not an IA to copy. Never copy an idea blindly: make the question, method and analysis your own, and check your plan with your teacher.
Example research question
How do the relative amplitudes of the 2nd, 3rd and 4th harmonics of a plucked string depend on pluck position, from 0.1 to 0.5 of its length in eight steps, read from phone-recorded spectra and compared with the ideal sin(nπx)/n² model?
Change the system, the range or the method so the question is yours. Check it against the research question checklist in our Physics IA guide.
Variables
- You change
- pluck position as a fraction of the string length, 0.1 to 0.5 in 8 steps
- You measure
- relative amplitude of the 2nd, 3rd and 4th harmonics (as a fraction of the fundamental)
- You keep the same
- string and tension
- pluck force
- microphone distance
- recording and FFT settings
Method
Pluck a monochord or guitar string with a plectrum at marked positions and record 2 seconds with a phone. Use a spectrum or FFT tool to read the harmonic amplitudes.
Data analysis
Theory: harmonic n has amplitude proportional to sin(n pi x)/n^2 for an ideal pluck. Plot the amplitude ratio against x and overlay the model, discussing where the damping and finite-width pluck cause deviations.
How it works
You change Pluck position (fraction of length) and measure Harmonic amplitude ratio in Plucked string, keeping String and tension, Plectrum and force, Recording distance the same. Expect a graph of Amplitude / fundamental against Pluck position x / L: Some harmonics vanish at set pluck positions.
Make it yours
Use nylon and steel strings, or a thicker pick, to test the finite-width pluck.
Extended Essay angle
Build a physical model of a plucked string with damping and stiffness and compare with the measured spectra.
Safety
Wear eye protection when tuning.
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