Decibel Addition and Subtraction :

Sound level decibels are logarithmic and so cannot be manipulated without being converted back to a linear scale. You must first antilog each number, add or subtract and then log them again in the following way:

Measuring Sound Reduction :

There are several ways by which sound reduction is measured. The Sound Transmission Class (STC), measured in dB, is the common measure by which acoustical performance is rated. It is the weighted average over the frequency range 100 to 5000 Hz of the STL (Sound Transmission Loss ) and measures cthe decibel reduction by a partition. The higher the STC rating, the more able the material is to resist the transmission of sound. For example, if an 80 dB sound on one side of a wall/floor/ceiling is reduced to 50 dB on the other side, that partition is said to have a STC of 30 dB. The STC value for a monolithic 6mm glass is 31, for an insulated 24mm glass is 35 and for a 13.52mm laminated glass is 39.

STC ratings are used for windows doors, walls & most building materials. It is a logarithmic scale and ranges from 18 to 50 for windows. Below table list some examples of STC levels.

In addition to STC, another popular method of measuring sound reduction is the weighted sound reduction index R The European Standard EN ISO 717-1 describes a method to express the airborne w. Sound insulation by the single-number quantity. This index is a common method of rating sound insulation of buildings materials for two noise spectra’s described below. It is applicable to walls, ceilings, floors, roofs, doors or windows. The index allows sound absorbent properties of materials to be calculated and is measured in a laboratory.

Optimum performance of a unit is achieved when it provides insulation at frequencies where noise is greatest. Performance of a window cannot be determined based on glass alone. Therefore, to ensure optimal acoustic insulation the correct type and composition of glazing should be selected.

Rw (C; Ctr )
index provides weighted correction for the human ear based on type of surrounding noise.
R (C; C ) in which,
Rw (Weighted sound reduction index in dB) is sound insulation of a building element over a range of frequencies,

C is the adaptation term when noise in question is outside background noise.

Ctr is the adaptation term when noise in question is road traffic (mostly low frequency noises)

According to the nature of the sound source to be insulated the right adaptation term can be chosen.

Following are common examples of noise source for each type of adaptation term.

Both C and Ctr are generally negative and deducted from Rw to calculate noise reduction property of the material. For example, R (C; Ctr ) = 40 (-2 ; -6) means that sound insulation for a window is 40 dB and is reduced by 6dB for traffic noise i.e. 40 – 6 = 34dB. Similarly, sound insulation against radio or TV noise is reduced by 2dB i.e. 40 – 2 = 38dB. This methodology allows designers to select optimum window specification based on the required application.

Glass Performance In Acoustic Insulation :

Unwanted sound is considered noise when it intrudes on our daily lives. To minimize this intrusion, all aspects of the building construction need to be evaluated. However in this instance we will only analyze the acoustic qualities of glass. The first step in this analysis is to determine the source of the unwanted noise. This is a critical step, as the noise source can vary from low frequency traffic noise to high frequency aircraft noise. 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 small, it results in a 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%.

Use Thicker Glasses :

Monolithic glass has specific critical or coincident frequency at which the speed of incident sound in air matches that of bending wave of glass. At this critical frequency glass will vibrate allowing sound waves to penetrate without significant attenuation. the thickness of a single-pane glass enhances the glazing’s sound insulation, for e.g., a 4mm thick glass provides an R of 29 dB, which can increase to 35 dB for a w 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 sound.

Use Glass Configurations with Different Thicknesses :
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 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 (preferably with Acoustic Interlayer) :

The poly vinyl butyral inter-layer (0.38m m to 1.52mm) used in laminated glass provides a dampening effect that reduces vibration by absorbing the sound waves hence reducing sound transmission.Laminated glass also has  superior sound insulation qualities in the higher frequency range where the noise from sources such as aircraft is a problem.

The PVB film used in laminated glasses have a shear damping effect that has substantial sound attenuation characteristics. When the outer glass layer is exposed to bending waves, the PVB layer creates a shear strain within itself and the bending of wave energy of glass is transformed to non-directional heatenergy, Which is barely noticable. During this phenomena the sound waves are absorbed by the PVB layer and not transmitted to the second glass layer. This results in reduction of the amplitude of vibration and sound transmission as shown below.

Increasing the inter-layer thickness has marginal effect on the performance of laminated glass. Acoustically enhanced PVB’s are designed to have higher damping characteristics that further reduce the amplitude of the sound waves. The graph below shows comparitive decay in vibration observed in laminated glass with Standard PVB and Accoustic PVB.

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 following chart demonstrates the STC and Rw performance of some common glass types. Double glazed unit with certain gases also provide sound insulation characteristics.

Areas around Windows :
It is important to note that no matter how good the noise insulation qualities of the windows are, there should be no gaps or cracks around the window frame. As long as the Rw of a window remains under c35 dB and the frame area doesn’t exceed 30% of the window area, the influence of the frame on the total acoustic performance can be neglected. However as soon as Rw lies between 35 and 40 dB, it is advised to reinforce each frame element. Windows with Rw larger than 40 dB are specific for the window concept itself which makes special advice necessary.

Factors not Affecting Insulation :

The following factors have no effect on the sound insulation properties of glass assemblies:

• Tint/color of glass
• Coatings (reflective/low-e) on glass
• Position of glass
• Tempering
• Annealed glass of a particular company