A window does not have one acoustic number that remains unchanged in every room and every test. At low frequencies, the receiving space and measurement procedure can influence the reported result enough to change how two products appear to compare.
What the 2026 study examined
Researchers tested two window types—one with an integrated shutter box and one without—across five glazing configurations. Measurements covered 50 to 5000 Hz, with several standard-based approaches compared specifically in the 50 to 100 Hz range.
All of the work took place under controlled laboratory conditions. The comparison included laboratory, in-situ-style and hybrid procedures rather than pretending that one method could be exchanged for another without consequence.
Below 100 Hz, the room becomes part of the result
At low frequencies, sound fields in a small receiving room are not uniform. Room modes create peaks and dips, so microphone position and the way the receiving-room level is averaged become particularly important.
The study found that the calculated insulation and the apparent difference between the two window types varied significantly with the method below 100 Hz. Its hybrid approach produced the lowest insulation values, while another in-situ-style treatment reduced the apparent distinction between the windows.
A single-number rating cannot show every weakness
Manufacturer ratings are useful for comparison when the specimen, standard and test conditions are understood. But a headline value compresses a frequency-dependent result. A window can behave differently at the low frequencies associated with some transport or amplified-music sources even when its overall rating looks strong.
The study also found that the window without the integrated shutter box generally performed better above 100 Hz, yet showed a more pronounced low-frequency reduction under laboratory conditions. That is a finding about these specimens—not a rule that every shutter box or every unboxed window will behave in the same way.
The specimen details still matter
A window assembly is more than its glazing. Frames, opening arrangements, seals, vents, boxes and junctions can create different paths. In this study, the integrated shutter box reduced performance particularly at mid and high frequencies.
The products and construction context studied should not be transferred uncritically to a British home. The wider lesson is that a component attached to a window may change the result, and that the effect can be frequency-dependent.
Ask what the number is meant to predict
For an ordinary specification, standard laboratory data may be the best consistent evidence available. For a difficult low-frequency complaint, the question may be whether that data represents the room, source and complete façade closely enough.
- Check the test standard, specimen size and frequency range behind the rating.
- Compare spectra as well as single-number quantities when the source justifies it.
- Separate glazing performance from whole-window and whole-façade performance.
- Consider vents, shutter boxes, frames, seals and installation junctions.
- Treat an in-room measurement as evidence about that room and installation—not a new universal product rating.