An Advanced software plugin or software called a spectrum analyzer.
You can use this plugin in conjunction with allmost all audio applications that support it.
You can use it to analyze harmonic structure of instruments and compare them simply by looking at the visual graph and taken note of harmonics compared to first harmonic (which is the initial sound itself) and wether the harmonic are mostly even or odd. i.e. A pleasant sounding instrument will have mostly even harmonics. Noise will have mostly odd ordered harmonic
e.g An Acoustic guitar pluck :
1st Harmonic =800hz (initial sound)
2nd Harmonic =1000hz
3rd Harmonic = 1200hz
and so on......
Hope this was helpful not confusing.
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