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:

  1. 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.

  2. 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.

  • If you bolted the violin strings to the wall and drew them over a bridge that was mounted to the wall, then the wall would probably do a fine job of amplifying the sound. If you put a wall "in front of" the violin, then very little energy is coupled into the wall; most is reflected off. – hobbs Nov 7 '14 at 4:16
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    A good answer to this question would explain how the sound chamber in a musical instrument is an impedance matcher between the string and the air. – DanielSank Nov 7 '14 at 6:43

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).

  • Yes I understand this, but I don't understand how what you described can make a sound source more efficient. Frequency response of a wall or any passive object can be spiky or in other words "resonant", but it will never make a sound louder. So what is the mechanism which makes the sound production process more efficient? – Tony Nov 7 '14 at 16:54
  • Double post. But to explain more, I am stuck thinking that if you put objects near a sound source it should dampen the source, since pretty much everything is going to absorb more power than air. This is obviously wrong, but I don't understand where I am making the error. – Tony Nov 7 '14 at 16:54
  • Most hard surface reflect sound very well. You just get a buildup of sound at the resonant frequency of the cavity, until the rate of input equals the rate at which the sound being absorbed. – user56903 Nov 7 '14 at 20:02

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