In order to gain a basic understanding of ‘Acoustical Damping’, there are a number of equally basic principles that one should first understand.
From Vibration comes Airborn Energy; Sound
We know that all airborne ‘Sound’ (airborne energy to which the human ear is responsive) is created by vibration. Acoustic energy is simply converted from one state (vibration) in a physical, relatively dense medium, to a relatively lower density medium such as air, usually resulting in a significant increase in ‘reaction’ to this incoming energy by the lighter mass medium. A good example is a common audio speaker; An electrical current, with varying frequencies, excites a relatively high mass electromagnet and the lightweight ‘cone’ of the speaker magnifies the energy, increasing the physical amplitude of the incoming vibration, as a function of the relative energy to mass ratio, incoming (ie: from the electromagnet), to the relative energy to mass ratio of a lighter substrate (ie: the very lightweight diaphragm of the speaker cone). However, the speaker cone is also intimately surrounded by air, which is of even lower mass than the speaker cone, so it, in turn, is excited … and again the acoustic energy increases in amplitude in the surrounding air as a function of the relative energy to mass ratio of the cone versus the energy to mass ratio in this medium (air).
Effect of Damping on Sound
In order to understand the effects of ‘damping’ on acoustic energy, one should also understand that all physical media have a Natural Frequency; the vibration frequency at which they can vibrate most efficiently (least amount of lost energy). The Natural Frequency of a ‘string‘ on a guitar can be changed by ‘fretting’ which, in effect, changes the excited length of the string, hence the most efficient frequency at it which it can vibrate. However, most media are of a set size and shape and, as such, have a set Natural Frequency. A cymbal, as an example, when struck vibrates at its Natural Frequency(ies). I say ‘Natural Frequency(ies)’ because sound does not naturally exist as a single, discrete frequency, but rather, as an envelope of acoustical energy at different frequencies, largely centered around one predominant frequency and secondarily around its ‘Harmonics’ (frequencies twice, three times (and so on) the predominant Natural Frequency). This envelope of sound is referred to as ‘Tone’ or, where multiple very unique specific tone ‘envelopes’ can be created, as ‘Voice’, in the ‘music’ world.
If one wants to quickly stop a cymbal radiating ‘sound’, one need only fully cover it with a cloth or touch it with one’s finger. Effectively, this action is ‘Acoustical Damping’. Every piece of equipment, whether rotating and/or responding to electro-magnetic flux, vibrates. Further, where gears, fan blades and other physical components are involved, other related vibration frequencies will be generated. Some airborne acoustic energy will be radiated off the high mass housing/casing of the driving component directly as airborne sound, the most predominant of which being at the natural frequency of the driving component or the component vibrating with the highest amplitude (ie: read … ‘shedding the highest amount of energy’). However, the combined acoustical vibration energy from the various components will transmit (unless vibration isolated), to anything to which the main component is attached. This combined energy will become airborne as it ‘drives’ lighter mass components along its path of travel (see ‘speaker’ analogy above). Yes, generally, the energy imparted to the air will be at the predominant Natural Frequencies and Harmonics of the various components, BUT, every lightweight surface has its own Natural Frequency and can be excited into ‘Resonance’, amplifying that lower level vibration energy such that what it radiates/sheds to the surrounding air is significantly higher than the incoming predominant frequencies. (Note: The effects of ‘Resonance’ is clearly evident when a professional singer holds a high tone note and shatters wine glasses that are filled to a level that makes the glass resonate to the point of destruction).
Using Acoustical Damping Products
Acoustical Damping Products, when applied to a light weight surface, reduce the amount of predominant vibration frequency(ies), and practically eliminate ‘resonant’ frequencies, being ‘telegraphed’ to the surrounding air.
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