Alongside significantly improved thermal performance, Double glazed windows provide substantial acoustic benefits to minimise sound intrusion into your home. While windows are not the only design element to consider in overall acoustic performance, they are likely to be the single biggest area of improvement, with an enormous difference between a poorly designed basic window and the high-end Miglas composite suite.
Acoustic performance can be measured through a Sound Transmission Class Rating (STC Rating). This is a measure of the amount of sound a product will prevent, or stop (Transmission Loss). As a generic rating, it can have some limitations – however in general the higher the sound transmission class rating, the better the product. An STC rating measures the Transmission Loss in decibels (dB) across 16 different frequencies in accordance with ASTM E413 to give an overall understanding as to how well a product performs in a broad spectrum of acoustic applications.
In General, lower frequencies (bass tones, drum sounds, truck engines etc.) are harder to minimise than that of higher frequencies (General Conversation, Whistling etc.) An STC Rating will give an indicative figure as to how many dB of sound a product prevents. It is also important to note, that dB is a non-linear scale so whilst a change of 1dB is almost imperceptible, a change of 10dB is twice, or half, as loud. From a volume perspective, 50dB is quiet, while 140 dB is so loud that it can immediately cause hearing damage.
In general, the thicker the glass the better the acoustic performance. However, this effect can be restricted at certain frequencies where the “coincidence dip” occurs. The coincidence dip is the frequency at which the glass panel vibrates in unison with the frequency of the source sound and as a result, the acoustic insulation properties of the glass are strongly reduced at this point. The coincidence dip for 4mm float glass is approximately 3125Hz, whilst that of 12mm float glass is approximately 1040Hz. An effective design principle in double glazing that is not possible in single glazed applications is to specify varying thicknesses of glass to create an ‘overlap’ in where the coincidence dip occurs and maximises the acoustic performance.
Below are some 'centre of glass' STC ratings of common acoustic glazing combinations to provide an understanding as to the varying levels of performance that can be achieved. Effectively the combination possibilities are limitless, and it is always best to discuss your objectives with the Miglas team to ensure the most cost-efficient and effective solution is obtained for your project.
The benefits of acoustic performance from a double glazed window do not only come from that of the glass. However, the frame design, seal engagement and compression from locking systems play a vital role in overall performance.
Another important figure to consider when assessing acoustic performance is the Air Infiltration (air-leakage) of a window product. A product with high Air infiltration is similar to leaving the window slightly ajar, it doesn’t matter how thick or much insulation it has– the sound finds a way around.
Miglas Windows use bespoke seal designs and high-performance multipoint locking systems that ensure the sash is pulled in tightly against the seals to provide minimal air-leakage. For example, the Miglas product has an Air Infiltration window of 0.07 under AS2047 testing conditions, whilst a generic window on the market is 5.0 (over 70 times more air leakage!)
Similarly to this, the frame design and material are also important factors to consider. If you visualise a hollow aluminium window section versus that of a solid hardwood timber profile, which product attenuates the most noise?