Help me understand resonance I don't fully understand how resonant chambers work. My confusion stems from the fact that the chambers are passive elements or filters, yet somehow are able to amplify the sound. 
Questions:


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*How can I differentiate between passive objects that can amplify a source and ones that can't? Where is the connection? Why can I add a chamber to a violin and it gets louder, but building a wall in front of it diminishes the sound.

*Do the resonant chambers vibrate / produce sound themselves or merely aid a string or other tone producing objects in vibration IE. Does a string make a louder sound because of the chamber, or are we hearing a combination of faint string plus a much louder chamber? Put in yet a other way, does the resonator move the primary point source of the sound?
My current understanding on the matter is that the chamber actually does not amplify the tone as function of it's resonant nature. Rather, what is tied to each end of the string affects the efficiency of the string. The chamber then just alters the tonality and directionality of the tone. This means an electric guitar is not as loud as an acoustic not because of the chamber, but the material used for the strings and body.
If you can recommend a technical and precise book on acoustics that deals with the matter, that would be very helpful.
 A: Resonance occurs when small amounts of energy accumulate because the rate at which they are being added matches the the frequency of the system itself and the phase. The classic example is pushing a child on a swing. If you add energy, ie push, at a frequency that matches the fundamental frequency of the pendulum motion of the swing the amplitude slowly increases. The phase also matters because for optimum energy transfer from you to the swing you need to add it at the correct phase ie at the end points of the cycle.
So if you want to (apparently) amplify a particular frequency of sound you need to put it in a chamber where that frequency is also the resonant frequency of the chamber (or a multiple thereof - think pushing the swing 1 in every every 2 or 3 swings etc).
A: Try to get your hands on the book "The Physics of Musical Instruments"  By Fletcher and Rossing. All those issues are well discussed there; many physicists have digged on those subjects.
Short summary for your questions:
Violins and guitar bodies use a vast array of tricks to amplify the sound.
A vibrating string, by itself, radiates as sound only a small part of the energy; the most part is wasted, so a string alone has a feeble sound.
The violin body couples to string vibration, extract energy from it and efficiently transform it in vibration of the air (sound). We all call it "amplification" altough is not strictly correct, since no extra energy is injected.
The top plate (soundboard) vibrates directly under the string pressure; being it large and flat, it moves a lot of air and produces a lot of sound.
The sound board must be flat, light and flexible to do that.
The resonance chamber does several jobs, one of wich is to transfrom the small vibrations of the violin body into in-out motion of the air through the sound holes, that again increase sound radiation efficiency. It also suppress destructive self-interference of body surfaces. 
It must be "of the right size & shape" for the frequencies involved, and have some coupling to the sound source.
Playing with those effects you have all resonancies (vibration modes) of the body and cavity, that affect the response (amplification) at each string frequency.
