Glass for Sound Control
Glass can be used as an excellent insulator against sound transmission. Starting from a single 6mm glass lite with an STC of 31, we can achieve STC ratings of as high as 50 with different combinations of laminated and insulated glasses. Although the increase in absolute numbers seems pretty small, it results in big difference in performance. An increase from 28 to 38 means 90% of the noise is reduced. A change from 28 to 43 represents a noise reduction of over 95%. The following useful tips will help you derive better sound performance from glass glazing and windows.
Use thicker glasses: Increasing the thickness of a single-pane glass enhances the glazing’s sound insulation, for e.g., a 4mm thick glass provides an R of w 29 dB, which can increase to 35 dB for a thickness of 12mm. However, increasing glass thickness is generally a poor choice for applications such as city structures which are primarily subjected to lower pitched sounds. This is because increasing glass thickness shifts the critical-frequency trough towards lower frequencies which results in weakened protection against low-pitched sounds.
An important observation here is that compared to a monolithic 4mm or 8mm glass, the performance of 4- 12-4 insulated glass deteriorates at lower frequencies. This is due to the mass-spring-mass (m-r-m) system in double-glazed units where the resonant frequency is located in the low-frequency zone of approximately 200 to 300 Hz depending on the glass thickness. Although the m-r-m effect can be reduced using thicker air-spaces (to make the spring created by the air-space more flexible), its has a serious limitation of making the glazing too wide and impractical.
To enhance the level of sound insulation provided by double-glazing, glasses with sufficiently different thicknesses should be used so that they can hide each others’ weaknesses when the overall unit reaches its critical frequency. This therefore produces a coincidence trough in a broader frequency zone but compared to symmetrical glazing the trough is less intense (as seen around 3,200 Hz). In this case, the increase in mass in relation to 4-12-4 glazing also helps to reduce the trough at low frequencies.
Use laminated glasses: Use of laminated glasses is one of the most popular methods in sound-reduction applications. The PVB film used in laminated glasses have a shear damping effect that has substantial sound-attenuation characteristics. The main contribution of acoustically effective interlayers to reducing sound transmission resides in their capability to drastically reduce the amplitude of vibration the partition (in this case the laminated glass lite) is subjected to under resonance. This effect roots in the microscopic shear motion the interlayer is subjected each time a fraction of the glass laminate is displaced from its equilibrium position as it vibrates. While the whole laminate starts vibrating under the influence of the impinging sound waves, bending waves build up, consequently introducing a slight curvature in the whole structure as shown.
As the shear modulus of the interlayer comprises a significant non-elastic component, any shear deformation will lead to a dissipation of mechanical energy into heat. This process accounts for the damping action of the interlayer inside the laminate. Depending on the magnitude of the loss modulus at service temperature, large differences can exist from one interlayer type to another. Acoustic PVB’s are designed around this principles and are formulated to exhibit a high loss modulus at service temperature.
The sound-attenuation characteristics of PVB and acoustical PVB films can be understood by the following comparative graph. Considered here is the performance of a monolithic 4mm glass with a 4.76mm (2-0.76-2) regular PVB and 4.76mm (2-0.76- 2) acoustic PVB.
Although the transmission curve for 4mm monolithic glass is shifted to lower values owing to its slightly smaller mass if compared to the laminated glasses, the superior performance of the PVB glass (and more so in the acoustic PVB laminate) is clearly evident in the coincidence region. The reduced plate vibrations below 800 Hz also help enhance the sound-reduction properties of the laminated glass assembly.
Use combination of insulated and laminated glasses: Further increases in sound-reduction performance can be achieved by using combinations of insulated and laminated glasses. These units offer the dual benefit of greater mass and different frequency resonance of insulated glasses coupled with the damping effects of PVB laminated glasses.
The graph and table below comparatively illustrate all the discussed glass options.