The common belief that “any internal window will stop noise” is only half true. Many people install secondary glazing only to find that the roaring of trucks or the hum of the city remains audible.
The reason isn’t poor product quality, but a lack of consideration for two physical phenomena: “Air Gaps” and “Glass Resonance.”
In this article, we’ll explain the soundproofing equations for a successful window transformation.
1. The Magic of the “Air Gap”#
In window soundproofing, the distance between the existing window and the internal window is more important than the thickness of the glass itself.
- 10mm Gap: Standard for thermal double-glazing. While great for insulation, it is nearly useless against low-frequency noise.
- 70mm to 100mm Gap: The standard installation distance for secondary glazing (like internal windows). This distance allows the air “cushion” to effectively damp sound waves.
Why is distance needed?#
When sound enters a narrow, sealed space, the air itself acts like a “spring,” causing both windows to vibrate together (Low-frequency resonance transmission). To avoid this and achieve significant attenuation, an “Air Gap of 100mm or more” is ideal.
2. Asymmetric Glazing: The Trap of Matching Thickness#
A common mistake is choosing “double glazing” for your internal window that matches the thickness of your existing glass.
The Coincidence Effect Chain#
If you layer glass of the same thickness (e.g., 3mm and 3mm), they will resonate at the same frequency (the “Coincidence Effect”), allowing specific sounds to pass through easily.
- Fail Example: 3mm (Existing) + 3mm (Internal) = Specific frequencies leak through.
- Success Example: 3mm (Existing) + 5mm or 6mm (Internal) = Shifting the resonance frequencies ensures stable insulation across the entire spectrum.
this is called “Asymmetric Glazing,” and it is a fundamental rule in professional soundproofing.
3. Laminated Glass: Turning Sound into Heat#
For ultimate performance, consider “Acoustic Laminated Glass.” These consist of two layers of glass with a specialized interlayer (PVB resin) that converts acoustic vibrations into “Heat Energy,” absorbing the sound rather than just blocking it.
In severe environments, such as apartments along major highways or late-night piano practice, a combination of a high-end internal frame and 12mm acoustic laminated glass can achieve over Dr-40 (blocking 40dB of sound)—a level of silence that makes you forget the outside world.
Conclusion: Choose “Configuration” Over Specs#
Three rules to avoid regret in window renovation:
- Maximize the “Air Gap” even by 1mm.
- Choose a “Different Thickness” from your existing windows.
- Never Compromise on Airtightness (especially at the seals and handles).
Are you looking for a room that is “somewhat quiet,” or the silence of a “private sanctuary”? If you choose the right configuration based on physics, your windows will become your strongest sound barrier.