In previous articles, we discussed Acoustic PVB (sound insulating) glass, the importance of asymmetric thicknesses, and other sound insulation technologies. In this article, we take a step further and enter the world of technical sound insulation ratings - the numbers an engineer (or an informed homeowner) needs to know in order to correctly select the glazing unit that matches their street, neighbourhood, and the noise source that bothers them.
Simply saying "I want sound insulation" is not enough. You need to learn to read the technical datasheets of glazing manufacturers so you can objectively compare your options. Let's see how.
Before diving into the ratings, let's remember the fundamental rule we saw earlier: Decibels (dB) are NOT a linear scale (like kilograms or metres). Instead, they are a logarithmic scale.
In practical terms, this means that an increase of +10 dB = Doubling of perceived loudness. Conversely, a 10 dB reduction = Halving of perceived loudness. A glazing unit that reduces noise by 40 dB doesn't seem "slightly better" than one that reduces by 30 dB - it sounds almost twice as effective to your ears!
Rw (pronounced "R-W") is the primary, internationally established sound insulation rating printed on every glazing datasheet (according to standard EN ISO 717-1).
Rw expresses the weighted (average) ability of a glazing unit to block sound across a broad frequency range (100 Hz – 3150 Hz), i.e. in an "average acoustic environment". It is a single number, in dB, giving you a quick, general picture.
A glazing unit with Rw = 38 dB means that if road noise is measured at 75 dB, theoretically only about 75 − 38 = 37 dB reaches the interior of the room (a very quiet space!).
On its own, Rw does NOT tell the full story. Next to the Rw value on serious technical datasheets, you will always see 2 additional numbers in brackets: C and Ctr.
The C value (always a negative number, e.g. -1) is used as a correction for medium-high frequency sounds: human voices, music, television, phone calls, nearby playgrounds, close-range aircraft noise, and high-speed trains. If your problem is a neighbouring playground, the real sound reduction of a glazing unit with Rw 40 (-1; -4) is: Rw + C = 40 + (-1) = 39 dB.
The "tr" comes from the word Traffic. The Ctr value (usually a larger negative number, e.g. -4 or -6) is used as a correction for low-frequency sounds - i.e. the DEEP, bass-heavy rumble: urban road traffic (cars, buses, trucks), bass-heavy music from clubs and bars, trains, and distant aircraft. If the building is on a boulevard, the real sound reduction is: Rw + Ctr = 40 + (-4) = 36 dB. Notice how a "good" 40 dB glazing suddenly loses 4 dB of performance at low frequencies!
💡 Example: A glazing unit rated Rw = 38 (-1; -4) has: Sound reduction for voices/aircraft = 38 + (-1) = 37 dB. Sound reduction for road traffic = 38 + (-4) = 34 dB. If you live on a boulevard, the number that matters is NOT Rw (38), but Rw + Ctr = 34 dB!
Rw 30–32 dB - a typical energy-efficient double glazed unit (e.g. 4-16-4) will suffice.
Rw 35–38 dB - asymmetric thicknesses (e.g. 4-16-6) or acoustic laminated glass.
Rw 40–45 dB - Triplex Acoustic PVB + asymmetry + wide Argon-filled cavity.
Rw 45+ dB - Triple glazing or specialised heavy-duty acoustic systems.
Use panes of different thickness (e.g. 6 mm outer, 4 mm inner) to "break" the resonance and improve performance at low frequencies.
Specify acoustic damping interlayers (Acoustic Laminated Glass) designed to dramatically improve bass-frequency behaviour, minimising the Ctr "penalty".
Increase the gap between panes (above 16 mm if the profile allows) and fill with Argon gas. The wider the cavity, the better low frequencies are blocked.
The Rw rating (and the C, Ctr corrections) is your "compass" in the sound-insulating glazing market. When specifying windows for a demanding urban project, simply requesting "glass with Rw = 40 dB" leaves dangerous legal and technical gaps. You must identify the dominant noise source and demand certified sound reduction test results with the Rw (C; Ctr) values. Only on the basis of these numbers can you make a rational decision, rather than a guess.
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