Cavity ResonanceAn air cavity will exhibit a single resonant frequency. If extra air is pushed into the volume and then released, the pressure will drive it out. But, acting somewhat like a mass on a spring which is pulled down and then released, it will overshoot and produce a slight vacuum in the cavity. The air will oscillate into and out of the container for a few cycles at a natural frequency. The qualitative nature of the frequency determining factors: Actually the frequency depends upon the square root of these factors and also upon the speed of sound, as you can see in the actual calculation of the frequency. But the above illustration shows the physical factors that are involved in determining the resonant frequency.

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Cavity OscillationsThe single frequency cavity resonance suggests a parallel with the single resonant frequency of a mass on a spring. In fact, the term "acoustic mass" is sometimes used in connection with such oscillations.
Again visualizing the mass on a spring, we know that if we lift it from equilibrium and allow it to fall, it will not stop when it reaches that equilibrium point but will overshoot it and oscillate about equilibrium because the work we did to lift the mass put energy into the elastic system. Likewise, when we have done work to increase the pressure in a cavity, we have given it energy and as the air rushes out, it will overshoot the equilibrium (atmospheric pressure) and produce a slight vacuum in the cavity. This elastic system produces the cavity resonance, but it is highly damped and will not continue to oscillate like a mass on a spring. 
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Applications of Cavity Resonance

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Cavity Resonant FrequencyA quantitative analysis of the cavity resonance gives the frequency expression Note: This calculation uses the sound speed calculated from a linear approximation that only applies fairly close to room temperature. If you are interested in a resonant frequency for a really hot cavity, then you could calculate a sound speed from the more accurate gas equation and plug it in the sound speed box. The temperature it shows in the temperature box will then not be correct, but when you click on "frequency" you will get a better estimate of the resonant frequency of the hot cavity. (This note was added after someone was measuring cavity resonances at oven temperature and was not getting agreement.) 
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