Ah, the physics within me is being unleashed!
Unfortunately, even Wikipedia describes it in these terms, perhaps trying to be intuitive.
But I believe one can be precise even with intuitive examples, without misleading the reader.
In fact, no inert soundboard is an amplifier, but it is, more properly, a resonator.
Let me explain better.
Every signal (which for us hereafter is a body) carries with it a certain energy, which can be maintained in the signal or can be lost by ending up in other places that are not the signal; this loss is called friction.
To give an example, let's consider a stone (the stone in curling) from Curling; since the ice is very smooth, once started, the stone can travel a very long distance. If we throw it on increasingly rough ice, we will see that, for the same amount of force applied, it will cover shorter and shorter distances; friction is increasing. Conversely, if we take stones of different weights and throw them on the same ice at the same speed, we will see that the heavier stones travel a greater distance.
In physical terms, it can be demonstrated, with these and other experiments, that energy is conserved and that when things change (the stone slows down), it is only because a portion of the energy has been transformed into something else (in our case, it no longer pushes the stone, but heats and melts the ice on which the stone slides). We also see that if, while the stone is running, we sweep the ice in front of it making it smoother, the stone goes further (friction decreases). Finally, it can be seen that if, while the stone is running, we give it new energy with a push, it accelerates and goes further.
As you can see, if the energy increases, it necessarily means that we have taken it from somewhere else.
This is exactly how amplifiers work.
Consider, for example, the signal of my voice: it has a certain energy. This energy is used to operate - just like a kind of valve - a system that brings other energy (in our case, the wires that carry the current); this latter is used to move the speakers, which are capable of moving a much larger mass of air than my throat is capable of. The result is my amplified voice.
When, instead, no energy is provided, it is not possible to amplify, but only to perform different tricks; the first trick is a bit like replacing ice with dirt in a ball game (moving from bocce to Curling!, i.e., decreasing friction), the second is a bit like a 'boccia' shot (setting a stationary stone in motion using the one we throw).
If you observe closely, once the pianist has pressed the key and imparted energy to the string (or the violinist has bowed the string), no more energy is provided. Therefore, they either used the first trick or the second.
So we cannot speak of amplification, but we must speak of something else.
Very often both tricks are used. We haven't spoken about the second phenomenon yet: it is called resonance.
Resonance is the phenomenon by which the energy in one body (which vibrates) passes to another body (which also vibrates).
A classic example, which everyone knows, is the continuous transfer of energy between the tines of two tuned tuning forks (I strike the first tuning fork and the second begins to vibrate: the energy leaves the first and goes to the second, then vice versa occurs and so on; here too, the phenomenon eventually stops due to friction).
In some cases (which are the ones that interest us), we have the impression that it is an amplification because a resonator is not obliged to retransmit energy in the same way it received it.
For example, if I vibrate a volume of air, the energy distributes itself - as the wave propagates - over an ever-increasing spherical surface (this is because the wave propagates in all directions at the same speed). The result is that, as I move away, my ears receive increasingly smaller amounts of energy, because the same energy is distributed over an ever-larger surface.
Now let us imagine placing a tube close to the volume of air set into vibration.
If the tube is made of plastic, as soon as we move away, we hear almost nothing; this is because the tube is not capable of vibrating at the same frequency as the volume of air and therefore intercepts practically all the energy and uses it to deform or heat up (high friction). If the tube is instead made of brass and is capable of vibrating at the same frequency as the volume of air, what happens is that the volume stops, but the tube in turn sets another volume of air in motion, using the energy received from the first volume set in motion, and so on. Only this time, the sound wave does not spread spherically, but advances along a cylinder and, therefore, the surface over which the energy is distributed does not increase, but remains constant. Consequently, even at a great distance, I will hear the sound (impression of amplification).
The soundboard of the piano is, more or less, with various theoretical complications, a mechanism like this one that I have described to you.
Best regards
