In order to gain a basic understanding of ‘Acoustical Damping’, there are a number of equally basic principles that one should first understand.
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).
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).
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.
There are two typical types of applied ‘Acoustical Damping’ materials;
Extensional Damping; Complete surface of a relatively lightweight material is fully covered with the ‘Damping’ medium. This is typically applied as a liquid or semi-liquid and when dry is flexible to the point of having a Natural Frequency below the frequency range of human ‘hearing’ and typically has a dry weight equal to 1 ½ times the mass of the treated material. Alternatively, a fully cured ‘sheet’ with a Pressure Sensitive Adhesive (PSA) backing can be applied over the entire surface of the radiating surface.
Product: Antivibe (Aquaplas) – Blachford
Constrained Layer Damping; As with the effect of placing one’s finger on a resonating cymbal, only a relatively small area needs to be addressed to reduce predominant frequency radiation or completely eliminate element resonant energy radiation from the lightweight material surface. These materials simply consist of a thin gauge metal or inert semi-rigid material having a different Natural Frequency than that of the radiating surface, attached to a ‘mastic’ (a thin layer of soft semi-solid material similar to ‘Plasticine®’). In effect, the vibrating surface and the thin gauge material vibrate at different Natural Frequencies and the driving energy of the vibrating surface is dissipated in the intimately attached mastic interlayer as it attempts to ‘telegraph’ its predominant frequency(ies) to a material, also intimately attached to the mastic, that has inherently different Natural Frequency(ies). Roughly, the amount of extensional damping material required need only cover 15% of the radiating surface area but should be cut into variously sized pieces, randomly spaced in and around the center, rather than the perimeter, areas of the radiating surface
Product: VSC Constrained Layer Damping Pads
Rule of thumb;
Large surface area vibrating/resonating surfaces are better treated with extensional damping materials rather than with constrained layer materials as they are more effective on the lower frequencies typically generated by larger surface areas.