How Do You Choose Acoustic Glass for a Curtain Wall?
Two panes of 6 mm glass with a 12 mm cavity. Now change one pane to 10 mm and leave everything else alone.
Sound reduction improves measurably, and the reason has nothing to do with the extra thickness itself. Equal panes resonate together at the same frequency, creating a window in the spectrum where the unit barely performs. Making them different thicknesses moves those resonances apart so one pane is still working where the other is weak.
That single principle, asymmetry, explains most of what separates an acoustic glazing specification from a standard one. The rest is interlayers, cavity width, and sealing the frame properly.
What it is not is cavity gas. Argon helps thermally and does essentially nothing for sound, which surprises people who assume the two performances travel together. PRANCE supplies glass curtain wall systems with acoustic glazing specifications.
Curtain Wall Acoustic Performance STC and OITC
Two rating systems in North American practice, measuring different things, and specifying the wrong one is a common error.
- STC, Sound Transmission Class, tested to ASTM E90 and rated to ASTM E413. It was developed around speech frequencies, so it weights the mid and high range heavily and tells you relatively little about low-frequency noise.
- OITC, Outdoor-Indoor Transmission Class, tested to ASTM E90 and rated to ASTM E1332. It weights low frequencies much more heavily, reflecting traffic, aircraft, and industrial noise, which is what actually reaches a facade.
For a curtain wall, OITC is the more honest number. A unit can rate STC 38 and OITC 29, and the occupants will experience the lower figure because road and aircraft noise sits where OITC measures.
European practice uses Rw with spectrum adaptation terms, where Rw + Ctr serves a similar role to OITC by penalizing poor low-frequency performance.
What the Numbers Mean in Practice?
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Standard double glazing, 6-12-6: approximately STC 35, OITC 29
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Asymmetric, 6-12-10: approximately STC 38, OITC 31
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Laminated with standard PVB, 6-12-10.8 lam: approximately STC 40, OITC 33
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Acoustic PVB interlayer: approximately STC 42 to 45, OITC 35 to 38
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Wide cavity with acoustic lamination: STC 45 and above, OITC 38 and above
Indicative values for comparison. Laboratory ratings for the glazing alone, which an installed facade rarely achieves.
A 5-point STC improvement is clearly audible. A 10-point improvement reads as roughly half the perceived loudness.
The Four Variables That Matter
Ranked by how much they move the result per unit of cost.
1. Asymmetric Pane Thickness
The cheapest meaningful upgrade available, and frequently omitted.
Equal panes share a coincidence frequency, a point where the glass becomes nearly transparent to sound. Different thicknesses place those frequencies apart, so the weakness of one is covered by the other.
A 6 mm and 10 mm combination performs better than 8 mm and 8 mm, despite similar total mass and similar cost. Specifying symmetric panes in an acoustic facade wastes an upgrade that costs almost nothing.
2. Acoustic Laminated Glass
The largest single improvement available in a glazing unit.
Laminated glass bonds two panes with a polymer interlayer. Standard PVB at 0.76 mm provides safety performance and a modest acoustic benefit. Acoustic PVB uses a softer, more damped formulation specifically designed to absorb vibrational energy rather than transmit it.
The difference between standard and acoustic PVB is typically 2 to 4 STC points at the same thickness, which is substantial for a change invisible in the finished facade.
Interlayer choices that carry both duties covers how lamination serves safety and acoustics simultaneously, which matters because most curtain walls need both.
3. Cavity Width
Wider cavities improve acoustic performance, and the optimum differs from the thermal optimum.
Thermal performance peaks around 12 to 16 mm with argon, beyond which convection currents reduce the benefit. Acoustic performance keeps improving well past that, with 20 mm and wider cavities performing measurably better.
Where both requirements apply, there is a genuine trade. A 16 mm cavity serves both reasonably. A 12 mm cavity optimised for thermal performance gives up acoustic performance that a wider unit would have delivered.
4. Mass
More glass blocks more sound, following the mass law, roughly 6 dB improvement per doubling of mass.
It is the most expensive route to a given result, since doubling glass weight has consequences for frame design, structural load, handling, and cost. Asymmetry and acoustic lamination deliver more per unit of spend, which is why they come first.
Why Installed Performance Falls Short of Laboratory Ratings?
This is where acoustic specifications most often disappoint, and the glass is rarely the reason.
A glazing unit rated STC 45 in a laboratory sits in a frame, in a wall, with gaskets, joints, and penetrations around it. Every one of those is a potential flanking path, and sound finds the weakest one.
- Frame construction. The mullion and transom profiles transmit sound. A thermally broken frame performs better acoustically as well, since the break interrupts the structural path.
- Gaskets and seals. Compression gaskets at glazing pockets, and continuous seals at perimeter joints. A gap of a few millimetres over a long joint can cost several points.
- Perimeter junctions. The head, sill, and jamb details where the curtain wall meets structure. Acoustic sealant and backing rod, properly installed, rather than foam alone.
- Spandrel zones. Often weaker acoustically than vision areas, with back pans and insulation that were specified for thermal rather than acoustic performance.
- Opening vents. Any operable unit, including aluminum door systems within a facade, is a weak point unless specifically detailed with acoustic seals.
A facade is only as good as its worst path. Specifying STC 45 glass behind an unsealed perimeter joint produces an expensive facade performing at STC 35.
Setting the Target
Acoustic requirements come from an environmental noise assessment, not from a product catalogue.
The process measures existing external noise levels, applies the internal criteria for the building type, and derives the required facade reduction. Internal targets are commonly expressed as dB LAeq for a given period.
| Building type | Typical internal target | Indicative facade requirement |
|---|---|---|
| Commercial office, urban | 40 to 45 dB LAeq | STC 35 to 40 |
| Office near major road | 40 to 45 dB LAeq | STC 40 to 45 |
| Hotel bedroom, urban | 30 to 35 dB LAeq | STC 42 to 48 |
| Hotel near airport | 30 to 35 dB LAeq | STC 48 and above |
| Residential, urban | 30 to 35 dB LAeq | STC 40 to 48 |
| Hospital ward | 30 to 35 dB LAeq | STC 42 to 48 |
Indicative only. An acoustic consultant derives the actual requirement from measured site noise and the applicable standard.
Thermal breaks and frame performance interact here, since the frame contributes to both results and a system chosen for one affects the other.
Checking a Supplier Can Deliver It
Acoustic glazing requires specific fabrication capability, and not every curtain wall supplier has it.
Acoustic PVB interlayers need lamination facilities and correct autoclave processing. Asymmetric units need careful handling to avoid panes being assembled the wrong way round, which happens more often than anyone admits and matters because the coatings that sit inside the cavity are surface-specific, so a reversed unit loses its thermal performance as well as its acoustic intent.
Test evidence should cover the actual build-up, tested to ASTM E90 or ISO 10140, rather than a similar configuration. Checking a fabricator can actually deliver it is worth doing before award rather than at submittal.
Frequently Asked Questions
1. Does argon gas improve acoustic performance?
No, essentially not. Argon improves thermal performance by reducing convection in the cavity and has negligible effect on sound transmission. Acoustic performance comes from asymmetric pane thickness, acoustic interlayers, cavity width, and mass.
2. What is the difference between STC and OITC?
STC weights speech frequencies heavily and was developed for interior partitions. OITC weights low frequencies much more, reflecting traffic, aircraft, and industrial noise. For a facade, OITC is the more representative figure, and a unit can rate STC 38 while rating OITC 29.
3. Is laminated glass better than thicker glass for acoustics?
Usually yes, per unit of cost. Laminated glass with an acoustic PVB interlayer damps vibration rather than simply adding mass, typically outperforming monolithic glass of similar weight while also providing safety performance.
4. Why does installed acoustic performance fall below the laboratory rating?
Because sound finds the weakest path. Frame construction, gasket compression, perimeter joint sealing, spandrel zones, and any operable vent all transmit sound around the glazing. A facade performs at the level of its worst flanking path rather than at the rating of its glass.
5. Can a curtain wall meet both acoustic and thermal targets?
Yes, though the optimum cavity width differs for each. Thermal performance peaks around 12 to 16 mm while acoustic performance continues improving beyond 20 mm. A 16 mm cavity with asymmetric laminated panes serves both requirements reasonably in most commercial applications.
Specifying Acoustic Glazing
Start from a measured requirement rather than a product. Use asymmetric panes, since the upgrade is nearly free. Specify acoustic PVB where the target is demanding. Widen the cavity if thermal requirements allow. Then spend as much attention on the frame, the gaskets, and the perimeter seals as on the glass, because that is where the performance is usually lost.
PRANCE supplies curtain wall systems with acoustic glazing, asymmetric laminated build-ups, and tested assembly data for the configuration quoted. Send your acoustic report and facade elevations and request an acoustic glazing specification.