by sussexpob » Tue Aug 13, 2013 12:00 pm
This is where sound engineering turns into full on mechanics, so my terminology and explanation my not be technically brilliant.
Every material has an inclination to respond at greater amplitudes when the frequency of its oscillations match its natural frequency of vibration. This is known as the resonance frequency, and a material can have more than one, the lowest frequency being known as the fundamental frequency. Because of this increased amplitude, even in the presence of other frequencies being created, the sound you hear is dominated by the fundamental frequency's pitch, and any subsequent integer multiple of this frequency also creates a resonant frequency, known as a harmonic.
The reason that the resonance frequency produces greater amplitudes is because of increased energy absorption at these frequencies producing greater vibrations in the material, while less natural frequencies do not absorb as much energy.Because the system responds to small amounts of energy and amplifies certain frequencies, momentum can build up in the vibrating substance producing resonance, which keeps the vibration going.
This is counteracted by friction and damping though, so without reapplication of force that maintains the absorption of energy in the system, eventually the point of equilibrium is reached and the vibration stops. When you think of catastrophic instances of resonance, mainly in the form of the famous bridges that wobble and collapse, there is always a continual application of force/energy, usually in the form of something like the wind.
To put it into context, when the marble strikes the table sound will be produced, amplified at all the levels of frequency that are harmonics of the Fundamental, but also the unamplified overtones that are not part of the fundamental will mix, giving the contact a unique sound.
The second sound cannot be resonance because there is no second source of energy after the striking of the table that replenishes the energy absorbed in that collision that is eventually dampened or lost to friction.
In the soundwave the initial high amplitudes are a mix of your harmonics and overtones, which produces the big crash on contact. After this you can see that the sound sustains at around the same level, and this where your marble is resonating back to the point of equilibrium.
My guess is that it is an interference.... it might have been created as simply as you reaching to turn off the camera, and you pushing air into the microphone with that motion.
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