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How do scientists know how acoustics work? (piefed.social)

submitted 5 days ago by breb@piefed.social to c/askphysics@lemmy.world

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I've seen lot of theory about how it works.
But how do they get to that conclusion?
As far a i know, you can see that it's air vibrating bc when there's a loud noise you can feel the floor vibrating or if i drop something in a table and i place my hand on it i can feel the table vibrating as well. But how do they know it in more detail. How do they know about the pith and that it's a wave?

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[–] sga 2 points 5 days ago

We record them. as some others have pointed, we measure all kinds of sources, be it a sound wave which is recorded by a mic, where we have some diaphragm, which physically moves, and depending on how the mic works (for example, a mic with a working principle of capacitence would use this motion of diaphragm as one of the capacitors plate, and measure change in capacitence by measuring voltage across the plates)(also, i am not saying this is how all mics work, just something i came up with for a example, but it works in theory).

At a much better level, for complex sources, for example, you gently resting your hand on a table, the sound currents produced are not strong enough, so one could potentially amplify, but doing that also amplifies noise (which means erroneous data here, and not like bad sounds), so we go to a better measurement technique, for example seismography (very complex stuff, which i am not going to go into).

in a lab setup, we may actuallu pick a material, and excite it. Think of like plucking a guitar string, but instead, we use for example a piezoelectric material, and give a input source voltage (which will vary periodically) and use it to excite a wave, and we can, for example optically measure the motion of material (if for example, we start with a block, we can measure it's compression and expansion). though this requires quite high quality cameras, this can be done.

acoustics as a phenomenon originates as large scale effect of molecules vibrating (i will not go into detail about vibrations here, because that is way out of scope), but think of it like this - you may know that electricity, or to be precise current, is just flow of electrons, so if electrons move slightly on application of a electrical voltage, and large scale effect is current, then sounds or all mechanical motions can be roughly considered as large scale motion of these vibrations. at this level, there are not really any optical cameras that can measure, so what we do spectroscopy (fancy words warning - we shine different wavelengths of light, and material absorbs wavelengths corresponding to particular vibration's resonant frequencies)

tl:dr - we record the motion