I am a student in the IB program and am required to write an Internal Assessment [IA]. I have decided to focus on soundwaves and would like to know how I could most accurately measure how long it takes for a soundwave (created by playing a key/note on a piano) to decay/how long the wave lasts using Audacity.
OS: Windows 10
Audacity 2.3.2
If it is a real (acoustic) piano, then the strings will ring forever (due to air currents and molecular vibration, the strings will never be totally silent).
What actually happens is that the sound will decay to a point where it becomes substantially quieter than the noise floor of the recording set-up - the sound of the strings will be drowned out by background noise.
To work around this problem, the decay of sound is usually measured as the length of time for the sound to drop by a specified amount. The method is commonly used for measuring the decay of room echoes (“reverberation decay”). A standard way of measuring this is “RT60” (or “T60”), which is the time it takes for the sound to decrease (“decay”) by 60 dB.
To measure the T60 decay, you will need a very “clean” recording, in which the noise floor is more than 60 dB below the peak level of the piano sound. You then measure the time from the peak of the sound, to the point where the sound is 60 dB lower.
how long the wave lasts using Audacity.
I can tell you some of that. It will last forever and your experiment is going to be limited by your ability to measure it. A real world microphone, preamplifier and digitizer all have internal noise and when that noise volume and the tone volume pass each other (the tone is decaying and the noise isn’t) that’s the end of the experiment. The tone is still going.
You might get rid of some of these problems with an electric guitar. No microphone, you can measure the pickup.
On a guess, drag-select a bit of the sound at a predictable time and measure it with Effect > Amplify. Don’t actually apply anything. Just read the top value. That’s the difference between where you are and 0dB, the maximum. Measure periodically down the tone.
You’ll want to be in dB rather than percent. dB measures really quiet things well. 60dB is 1000/1.
You can cheat. Derive the decay rate from the pieces you can measure and predict where it’s going to be next week. I suppose it will eventually flatten out at the molecular movement of the string which is a constant with temperature.
Koz
it will eventually flatten out at the molecular movement of the string which is a constant with temperature.
You will, of course, want to freeze the piano with liquid nitrogen…
Koz
I expect that in reality, vibration from passing cars, people walking in the room upstairs, air currents, and other sources will outweigh the effect of molecular vibration. Beside which, the piano strings will probably become brittle at -200C.
the piano strings will probably become brittle at -200C.
Not to mention the wood. Given the tension in a piano you might have a thermo-mechanical event. Even if you fail the actual experiment, the YouTube posting of the explosion will go viral instantly. Video everything.
Koz
Wow, I definitely didn’t expect such a quick reply! Thank you so much, this is very helpful advice (except the liquid nitrogen plan, maybe)! I do actually have a MIDI controller and might just play piano piano notes via Ableton, because it seems like getting a clean recording using an iPhone is pretty hard (actually, one of my classmates weirdly owns a studio microphone which he can lend me, so if the controller idea is rejected I have a backup plan). I’ll consult wth my Physics teacher and hopefully get the experiment done soon! 
P.S.: Happy to know that we are using °C here 