Asymmetric glazing pairs two panes of different thickness so their critical frequencies do not land in the same place. That frequency is roughly 12,000 divided by the thickness in millimetres, so 3,000 Hz at 4 mm and 1,200 Hz at 10 mm: a 4/16/4 unit stacks two reduction dips at one point in the spectrum, where a 10/16/4 pulls them more than an octave apart. Which index to quote then depends on the source, Rw + Ctr against road traffic, Rw + C against speech and television. And the classification of the road or railway alongside, set in France by the arrêté of 30 May 1996 as amended, says how exposed the façade actually is.
What asymmetry moves in the spectrum
The critical frequency of a pane, and why it tracks thickness
A single leaf does not reduce sound evenly. Above a certain frequency the bending wave travelling through the glass falls into step with the airborne wave, the leaf radiates, and the reduction collapses. That is coincidence, and it is inversely proportional to thickness.
| Pane thickness | Approximate critical frequency | What falls in the dip |
|---|---|---|
| 4 mm | 3,000 Hz | tyre roar, sibilants in speech |
| 6 mm | 2,000 Hz | upper mid, fast-moving traffic |
| 8 mm | 1,500 Hz | mid, raised voices |
| 10 mm | 1,200 Hz | mid, the core of urban road noise |
| 12 mm | 1,000 Hz | lower mid |
The figures are worked out for glass at a Young's modulus of 72 GPa, a density of 2,500 kg/m³ and a Poisson's ratio of 0.22, in air at 343 m/s. They do not replace a test report, they let you pick a build-up before asking for one.
Symmetric and asymmetric build-ups, dip by dip
| Build-up | Symmetry | Where the two dips sit | What to expect |
|---|---|---|---|
| 4/16/4 | symmetric | 3,000 and 3,000 Hz | both dips stack at the same point |
| 6/16/6 | symmetric | 2,000 and 2,000 Hz | same fault, shifted downward |
| 6/16/4 | asymmetric | 2,000 and 3,000 Hz | half an octave apart, modest effect |
| 10/16/4 | asymmetric | 1,200 and 3,000 Hz | over an octave, a far more even curve |
| 8/16/6.8 acoustic laminate | asymmetric and laminated | 1,500 Hz and a damped dip | the acoustic interlayer kills the second dip |
The site logic comes down to one line: you are not trying to add mass, you are trying not to stack the same fault twice. That is why a 10/16/4 often beats a heavier 6/16/6, and why moving from 4/16/4 to 6/16/4 does not go far enough when the nuisance is plainly road traffic.
Acoustic laminate and asymmetry, two distinct levers
Asymmetry moves the dips, lamination damps them. An acoustic laminate pairs two panes with plasticised butyral interlayers whose softness dissipates bending energy, attenuating coincidence instead of shifting it. The two levers add up, which is what a build-up like 8/16/6.8 acoustic does. The cavity gas, on the other hand, plays almost no acoustic role: argon serves the thermal side, and presenting it as a sound gain is a mistake that comes back at handover. The whole-façade logic is set out in our article on façade sound insulation.
Choosing the index before choosing the glazing
Rw, Rw + C, Rw + Ctr: which one goes on the quote
Rw is the single weighted sound reduction index. The adaptation terms, negative or zero on ordinary glazing, correct it for the source spectrum: C for pink noise rich in mid and high frequencies, speech, television, household activity; Ctr for a spectrum loaded with low frequencies, urban road traffic, rail at moderate speed, distant aircraft. Against a busy road it is Rw + Ctr that belongs on the quote, and nothing else. Quoting a bare Rw promises a result the client will never hear.
Qualify the source on the survey, not on the phone
The source is qualified on site and at the hour the nuisance is felt. A free-flowing dual carriageway, a railway, a school yard and a heat pump compressor on the boundary do not produce the same spectrum and do not call for the same build-up. The useful survey comes down to four points: the nature of the source, the distance and the height of the storey, the orientation of the façade concerned, and the time slots the client actually complains about. That record, not the catalogue, decides between a 10/16/4 and a laminated asymmetric unit.
The whole window, not the glazing alone
The performance the contractor stands behind is that of the complete window, glazing plus frame plus sash plus hardware, measured in a laboratory on the assembly. Glazing rated at 40 dB in a mediocre frame does not deliver 40 dB. The test report to ask the manufacturer for therefore covers the referenced window, with its test dimensions, and not the glass component alone. That sheet is filed alongside the declaration of performance.
Installation, where the decibels go
Frame, sash, seals: the contact that decides
An air leak is a sound leak, and it costs more than two decibels of glazing. The points to check are always the same: even compression of the seal all round, adjustment of hinges and keeps for a positive close, no hard spot at the cill, and the condition of the seal itself, neither pinched nor fouled. The opening type matters too, and the airtightness gap between a casement and a slider is paid for directly in decibels.
Sealing the perimeter
Performance is won at the perimeter, and continuity matters more than the amount of material. Compression tape weathers the outer face, foam fills without sealing, and tapes carry the continuity on both the inside and the outside.
- Compression tape sized to the real joint and compressed within its working range.
- Foam protected by a tape, never left exposed to air or sunlight.
- Sealing tapes laid without creases or breaks, corners and cill included.
Trickle vents and the roller-shutter box
The roller-shutter box is the acoustic weak point of most retrofitted façades, and changing the window does not deal with it. Trickle vents are not blocked either: they are replaced with acoustic vents carrying a declared attenuation, set on the least exposed face and sized to the airflow the room requires. The fresh-air rate per room is recalculated before ordering, and the other typical air leaks in a dwelling are dealt with at the same time, or the gain from the joinery stays theoretical.
Pricing and defending the offer
What you record before pricing
The number of openings involved is settled before the appointment: windows and doors are picked out on the façades and counted for pricing, which saves discovering three forgotten openings at order stage. On site, three questions settle the build-up.
- Which rooms are affected and at what hours the nuisance is reported.
- Whether the existing window closes properly, seals, sash play, trickle vent.
- What ventilation is in place, and whether the shutter box has been dealt with.
What goes on the quote
The line carries the exact glazing build-up with its thicknesses, the index used and its adaptation term, the reference of the complete window and its test report, the sealing method and what happens to the trickle vents. On the paperwork, VAT applies line by line and the mandatory wording is already drafted into the quote, which leaves room to spell out the acoustics. A quote carrying those five items no longer compares to a price per square metre.
What you do not promise
You promise neither silence nor a number of decibels heard in the room. The reduction of a window is measured in a laboratory on a test rig; on site the outcome also depends on the façade, the shutter box, the ventilation and flanking transmission. The wording that holds is the real one: the window as installed delivers that reduction in the sense of the index quoted, and the other elements are identified and priced separately if the client wants to go further.
What regulation and funding actually require of a window
Infrastructure classification, and what it is worth in a retrofit
The French arrêté of 30 May 1996 as amended sorts noisy routes into five categories and fixes the width of the noise-affected sector either side. The classification is set by the prefect and is publicly available: it is the data that turns a client's complaint into a figure.
| Category | LAeq 6am to 10pm, road or high-speed line | LAeq 10pm to 6am | Maximum sector width |
|---|---|---|---|
| 1 | over 81 dB(A) | over 76 dB(A) | 300 m |
| 2 | 76 to 81 dB(A) | 71 to 76 dB(A) | 250 m |
| 3 | 70 to 76 dB(A) | 65 to 71 dB(A) | 100 m |
| 4 | 65 to 70 dB(A) | 60 to 65 dB(A) | 30 m |
| 5 | 60 to 65 dB(A) | 55 to 60 dB(A) | 10 m |
Thresholds for conventional railway lines are raised by 3 dB(A). Mind the scope: those insulation requirements bind new buildings put up inside the sector, and the arrêté states the retained value can never fall below 30 dB. Replacing joinery on an existing dwelling attracts no requirement from that text; the classification is a prescribing benchmark and a selling argument, not an obligation to comply with.
The CEE criteria of BAR-EN-104, and the condition on what was there
Measure BAR-EN-104, version vA54-2 applying since 1 January 2024 and up to work committed before 1 July 2028, carries no acoustic criterion at all. It is about thermal performance and about what existed beforehand.
| Case | Uw | Sw |
|---|---|---|
| Window or French door, first pair | ≤ 1.3 W/(m²·K) | ≥ 0.3 |
| Window or French door, second pair | ≤ 1.7 W/(m²·K) | ≥ 0.36 |
| Roof window | ≤ 1.5 W/(m²·K) | ≤ 0.36 |
| Secondary window over an existing opening | ≤ 1.8 W/(m²·K) | ≥ 0.32 |
Two conditions rule out most of the cases that come up. The window replaced has to be single glazed beforehand, without exception, and the operation includes replacing the frame, except when fitting a secondary window. The payment is counted per square metre of window installed, 3,800 kWh cumac in zone H1, 3,100 in H2, 2,100 in H3, with Sw assessed to NF P 50-777 and Uw to NF EN 14351-1+A2. On the grant side, the conditions of the current year are set out in our article on the 2026 schedule.
Blocks, heritage constraints and the sheets to keep
In a block, joinery affects the external appearance and needs the freeholder's agreement as soon as the model, the colour or the sub-division changes. In a conservation setting the heritage officer may impose a sub-division or a material, which constrains the glazing depth available and therefore the build-up chosen: that point is raised before the quote, not after the order. The file keeps the declaration of performance of the window, the acoustic test report, the installer's certification, and photographs of the perimeter sealing taken before the reveals are closed up.
