Sound Barriers - What Are They?
Mass Loaded Vinyl is the material of choice for use as a sound barrier. There are numerous “brand names” of MLV being sold by numerous companies, who each have various claims, only some of which are true. This application note provides some background on MLV, describes how it works from a technical standpoint, examines some of the claims, and recommends best practices for using MLV for soundproofing.
What is MLV?
Mass Loaded Vinyl has been used for soundproofing since at least the 1960s. It is essentially a viscoelastic material, such as polyvinylchloride (PVC, or vinyl), that is loaded with an inert material, such calcium carbonate or barium sulfate. Viscous materials, such as honey, when subjected to a force, will flow and take on a different shape. On the other hand, elastic materials, such as rubber, when subjected to a force will deform and snap back to their original shape. Viscoelastic materials exhibit both behaviors – they will flow, but when the force is removed or reversed, they will go back to their original shape. This property, known as hysteresis (or damping), contributes to the ability of MLV to attenuate sound. It is the mechanism by which many companies claim that MLV “converts sound to heat”, although it is not the fundamental means by which MLV attenuates sound.
How does MLV work?
In order to understand how MLV attenuates sound, we need to first make an analogy with the field of mechanical vibrations. MLV essentially acts like a tuned mass damper, that is capable of reducing the magnitude of the single peak into two peaks (shown in magenta), where the lower peak is the vibration mode of the coupled system, and the higher peak is the vibration mode of the MLV sheet. MLV with a greater degree of viscoelasticity will be seen as providing greater damping, as shown in the green peaks. In actuality, a sheet of MLV will have several vibration modes that occur over a wide frequency range.
Is one brand of MLV better than another?
MLV is manufactured by only a handful of companies in the US, however, these same products are sold by numerous soundproofing companies. Because of this, the soundproofing companies try very hard to differentiate their brand from others. In terms of the inert materials (calcium carbonate or barium sulfate), there is very little that differentiates one brand from another, although some companies claim an advantage for being “barium free”. While exposure to high levels of elemental barium can be toxic, barium sulfate is inert and insoluble in water, and does not pose a significant health hazard.
Differences in the vinyl sheet, have a significant impact on soundproofing behavior. As most of us are familiar, when vinyl is exposed to low temperatures, it becomes very stiff, and when it is exposed to high temperatures it becomes very compliant. Going back to our vibration analogy above, when the elastic substrate becomes very stiff, the vibration modes of the sheet are tuned to a higher frequency, compromising its ability to attenuate noise at low frequencies. Conversely, as the temperature increases, the MLV becomes very limp, to the point where it does not have enough compliance to vibrate. MLV manufacturers use plasticizers to ensure the proper compliance of the MLV over the operating temperature range. The user should therefore examine the flexibility of the MLV to determine if it has adequate compliance.
Other factors that differentiate one brand of MLV with another concerns uniformity. MLV is produced in a sheet extrusion process, that if not properly controlled, will lead to inconsistent thickness, voids, clumping, and delamination, resulting in diminished soundproofing capability. The user should examine the MLV to look for bubbling (or orange peel) on the surface, which may be indicative of voids and lumping, and examine cuts through the thickness of the MLV to look for variances in thickness and the presence of stratification or layering of the sheet.
Is a thicker MLV sheet better than a thinner sheet?
A one-eighth inch thickness of MLV will have a weight density of one pound per square foot, and a one-quarter inch thickness will have a density of two pounds per square foot. Generally, the greater the mass and thickness, the better the low frequency attenuation capability of the MLV sheet. This is due to the fact that the thicker material will provide a greater impedance mismatch than a thinner material, which will tend to extinguish an acoustical wave that tries to pass through the material. However, going back to our vibration analogy, it is important to note that it is the ratio of stiffness to mass that determines the effectiveness of the MLV to attenuate noise. Therefore, a stiff MLV sheet with higher density may not be as effective as a compliant MLV sheet with a lower density. The user must therefore examine the compliance of a thicker sheet to ensure that it will work better than a thinner sheet at the frequency range of interest.
Can I bond MLV directly to a wall or partition?
There are many manufacturers of “peel-and-stick” MLV, which has a layer of adhesive on one side to facilitate attachment of the sheet to a structure. Some users may want to reduce noise attenuation through a wall by bonding MLV directly to a wall using construction or carpet adhesive. Bonding the MLV to a wall or partition defeats the purpose of MLV, and will result in only a modest amount of sound attenuation.
Going back once again to our vibration analogy, it is our desire to allow the MLV to vibrate independently as it will attenuate acoustical energy in two ways: (1) the acoustic waves encounter different impedance interfaces by having to propagate from the air, through the MLV, through the air, then through the wall or partition, and (2) the acoustical energy is dissipated by the tuned mass damper effect of the MLV sheet. By bonding the MLV directly to the structure, the MLV sheet cannot act independently of the structure and therefore cannot attenuate acoustical modes. We therefore do not advise that MLV be stuck directly to a wall or partition
Should I mount the MLV vertically or horizontally within a wall or partition?
Generally, the orientation of the MLV when hung on a wall or partition should not matter. However, the MLV should be hung in a manner that will minimize the amount or number of seams. Seams in the MLV will comprise sound transmission flanking paths that need to be blocked using acoustical caulking or tape. Therefore, when soundproofing a wall in a house (for example), it will be more effective hanging the MLV sheet horizontally, leaving a single continuous horizontal seam that can be more readily treated with caulking or tape.
Sonic-Shield has an engineering staff with advanced technical capabilities who can assist our residential, commercial and industrial customers in the selection and installation of soundproofing materials and products to solve their noise and vibration problems.