Acoustic sound absorbing materials are used to reduce the acoustic vibration within a structure or system. In order to gain an understanding of how acoustical damping works, following are a number of helpful principles.
We know that all airborne ‘sound’ (airborne energy to which the human ear is responsive) is created by vibration. Acoustic energy is simply energy converted from a state of vibration that is in a physical, relatively dense medium to a relatively lower density medium such as air. This usually results in a significant increase in ‘reaction’ to this incoming energy by the lighter mass medium.
Example
A good example of this ‘reaction’ is a common audio speaker. When an electrical current, with varying frequencies, excites a relatively high mass electromagnet, the lightweight speaker ‘cone’ magnifies the energy. This magnification increases the physical amplitude of the incoming vibration. It does this 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. This increase in amplitude is a function of the relative energy to mass ratio of the cone versus the energy to mass ratio of the medium.
To understand the effects of damping on acoustic energy, it’s important to know that all physical media has a natural frequency. This natural frequency is the vibrational frequency at which physical media can vibrate most efficiently. Efficiently, in this case, means the least amount of lost energy.
The natural frequency of a string on a guitar can be changed by ‘fretting’. This, in effect, changes the excited length of the guitar string. Hence, the most efficient frequency at which it can vibrate. However, most media are of a set size and shape and, as such, have a set natural frequency.
Example
A cymbal, as an example, when struck vibrates at its natural frequency (ies). We say ‘natural frequency(ies)’ because sound does not naturally exist as a single, discrete frequency. It exists as an envelope of acoustical energy at different frequencies largely centered around one predominant frequency. Secondarily sound exits around its ‘Harmonics’ (frequencies twice, three times (and so on) the predominant natural frequency).
This envelope of sound is referred to as ‘tone’. Where multiple, very unique specific tone ‘envelopes’ can be created it is referred to as voice, in the music world.
To quickly stop a cymbal from radiating sound, it needs to be fully covered with a cloth or simply touched. Effectively, this action of sound absorption 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 waves. Of this, the most predominant is the natural frequency of the driving component or the component vibrating with the highest amplitude.
However, the combined acoustical vibration energy, from the various components, will transmit, to anything to which the main component is attached. The exception being if the main component has been vibrationally isolated. In this case, the acoustical vibration energy, would not transmit.
This combined energy will become airborne as it ‘drives’ lighter mass components along its path of travel (see ‘speaker’ analogy above). Generally, the energy imparted to the air will be at the predominant natural frequencies and harmonics of the various components. However, every lightweight surface has its own natural frequency and can be excited into ‘resonance’.
This amplifies the lower level vibration energy. The result is that what it radiates/sheds to the surrounding air is significantly higher than the incoming predominant frequencies. The effects of ‘resonance’ are 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). These products practically eliminate ‘resonant’ frequencies, being ‘telegraphed’ to the surrounding air.
There are two typical types of applied acoustical damping materials. At Vibra-Sonics, we are proud to carry both materials in the form of excellent quality products.
Extensional Damping
The complete surface of a relatively lightweight material is fully covered with the ‘damping’ medium. This medium is typically applied as a liquid or semi-liquid. When dry, it is flexible to the point of having a natural frequency below the frequency range of human ‘hearing’.
Typically it 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 radiating surface.
The extensional vibration damping product that we sell is Antivibe (Aquaplas) – Blachford. This damping compound is an off‐white, paste‐like liquid that is easy to apply by spray, trowel or brush. Antivibe is highly resistant to water, oils, solvents, acids and alkali. The material is non‐toxic, non‐flammable (wet or dry), almost odourless and is non‐aging.
Antivibe is highly effective in reducing vibrational resonance and unwanted noise which is often described as humming or tininess. Applying this product to affected surfaces will dissipate structure‐borne vibrational energy. It will also dissipate the directly related and radiated air‐borne noise energy. This damping liquid or putty is a water based compound that dries to be flexible and solid.
The visco‐elastic properties, of this product, convert vibrating surfaces into poor transmitters of acoustical energy. This damping material is unable to transfer the energy of the vibrating source fast enough to effectively transmit much noise or vibration. Applying this product to affected surfaces will dissipate structure‐borne vibrational energy and the directly related and radiated air‐borne noise energy.
Antivibe, extensional vibration damping compound can be used in a wide range of applications:
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.
Constrained layer damping materials simply consist of a thin gauge metal or inert semi-rigid material having a different natural frequency than that of the radiating surface. This material is 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. The driving energy of the vibrating surface is dissipated in the intimately attached mastic interlayer. Here, it attempts to ‘telegraph’ its predominant frequencies to a material, also intimately attached to the mastic, that has inherently different natural frequencies.
Roughly, the amount of extensional damping material required need only cover 15% of the radiating surface area. To install constrained layer damping, the material should be cut into variously sized pieces. Then, the pieces should be randomly spaced in and around the center, rather than the perimeter, areas of the radiating surface.
The product is very easy to apply being peel and stick with a built-in pressure sensitive adhesive. This adhesive will stick to most constructed surfaces. It can be painted over immediately without bleeding or other adverse effects.
At Vibra-Sonic, the constrained layer damping product we carry is VSC Constrained Layer Damping Pads. VSC Damping Pads are a specially plasticized, high tensile, high elongation, constrained viscoelastic damping pad. They consist of an energy-absorbing mastic integrated with a constraining layer of .010″ flexible aluminium. The combination of mastic material with the aluminium top layer provides the maximum capacity to block sound.
Extension and compression of the mastic sound damping material converts vibratory energy into heat. This works to reduce noise levels of the damped substrate. The aluminium top layer maximizes this effect by re-directing any of the escaping vibratory energy back into the mastic layer. Generally only 10 – 20% of a noise generating surface needs to be treated.
VSC Damping pads can be used on most constructed surfaces such as:
Selecting the best type of acoustic damping material for your project.
The goal of all our sound proofing materials and products is to control unwanted noise and vibration. To do this, it is important to ensure you select the product that best fits your particular environment.
Large surface area vibrating and resonating surfaces are better treated with extensional damping materials rather than with constrained layer materials. This is because extensional damping materials are more effective on the lower frequencies typically generated by larger surface areas. When installing constrained layer damping pads, typically, only 10 – 20% of the noise generating surface needs to be treated.
If you would like some help with your project to find the right product and solution, give us a call, email or fill out the contact us form. We’d be happy to help!