Acoustic glazing and rain noise

Two noise problems that share a piece of glass

They are different and they are solved differently, which is why generic acoustic advice so often disappoints.

External noise, from a road, a railway or aircraft, arrives as airborne sound that has to pass through the glazing to reach you. It is dominated by low and mid frequencies, particularly the 50 to 300Hz band that carries traffic rumble and aircraft. Mass and asymmetry deal with it.

Rain noise is generated at the glass. A drop strikes the outer pane, deforms it briefly, and the pane radiates that impulse into the cavity and out of the inner pane as sound. The energy spectrum is broadband but weighted high, mostly between 1kHz and 5kHz, which is exactly the band the ear is most sensitive to and exactly the band that wakes people.

A specification optimised for traffic is not optimised for rain, and a unit that performs well on a road-noise test can still be loud in a downpour.

How rain noise is actually produced

The outer pane is a diaphragm. A raindrop delivers a short impulse to it, and the pane responds by vibrating at whatever frequencies its size, thickness and edge restraint favour.

Three things then govern how much of that reaches the room. How much the pane moves for a given impulse, which is a function of its mass and stiffness. How efficiently the vibration couples across the cavity to the inner pane, which is a function of the gas and the edge connection. And how efficiently the inner pane radiates sound into the room, which depends on its own area and stiffness.

Note what is not on that list: the number of panes as such. A triple unit is quieter than a double mostly because it has more total mass and usually mismatched thicknesses, not because there are three of them.

Mass, and the first rule of sound reduction

The mass law states that sound reduction through a single partition rises by about 6dB for each doubling of surface mass. It is the most reliable relationship in the subject and it sets the floor for everything else.

In practice that means going from a 4mm outer pane to a 6mm one buys roughly 3dB, and 4mm to 8mm buys around 6dB. A 3dB reduction halves the sound energy. A 10dB reduction is heard as roughly half as loud.

The limit is weight. Every millimetre of glass adds 2.5kg per square metre, so specifying an 8mm outer pane on a large flat-roof unit adds real load to the structure and the hardware. Mass is the blunt instrument. The clever answers cost less weight.

Asymmetric panes and the coincidence dip

Every sheet of glass has a critical frequency at which bending waves in the glass travel at the same speed as sound waves in air. At that frequency the glass stops resisting sound and effectively becomes transparent to it. This is the coincidence dip, and it is the reason a heavy symmetric unit can perform worse than a lighter asymmetric one.

For 4mm glass the dip sits near 3000Hz. For 6mm it drops to about 2000Hz, and for 10mm to around 1200Hz. Two panes of identical thickness share the same dip, so both fail at the same frequency and the unit has a pronounced weak point.

Make the panes different thicknesses and the dips separate. A 4mm outer and a 6mm inner have their weaknesses in different places, and each pane covers for the other. On rain noise this is particularly effective, because 3000Hz is right in the middle of the impact spectrum.

Asymmetry usually costs nothing. A 4mm and 6mm build-up is standard stock at most fabricators and it outperforms a 4mm and 4mm unit by 2 to 3dB across the range that matters.

Laminated glass and what the interlayer does

Laminated glass is two sheets bonded by a plastic interlayer, typically 0.38 or 0.76mm of polyvinyl butyral. It is required for the inner pane of overhead glazing anyway, on safety grounds, which makes it the most useful acoustic upgrade available because you are partly paying for it regardless.

The interlayer damps the glass. Instead of ringing freely at its critical frequency, the sheet loses energy to shear within the plastic. That fills in the coincidence dip and adds around 2 to 3dB over monolithic glass of the same weight.

Acoustic laminated interlayers go further. They use a softer, more dissipative resin, sometimes in a three-layer construction, and they add 4 to 6dB over standard laminate at the same thickness. The premium over standard laminate is modest and the effect is real, particularly on rain, where the damping directly attacks the impulse response of the pane.

What the ratings mean

Three numbers appear on acoustic datasheets and they answer different questions.

A unit quoted at R_w 38 (-2; -6) reduces general noise by 38dB, speech-range noise by 36, and traffic noise by 32. The last figure is the one to compare if you are under a flight path.

The rain figure is the one most manufacturers do not publish and can usually supply on request. Anything at or below 46dB under the standard heavy rainfall test is quiet. Above 55dB is loud enough to interrupt sleep.

Cavity width, and where it helps

Acoustically, wider is better, and this puts acoustic requirements in tension with thermal ones.

The cavity acts as a spring between two masses, and the system has a resonant frequency at which it transmits sound freely rather than blocking it. A 16mm argon cavity resonates somewhere near 200 to 250Hz, which is inconveniently close to traffic noise. Widening the gap lowers that resonance out of the way, which is why secondary glazing with a 100mm gap outperforms any sealed unit for road noise.

Within the 12 to 20mm range available in a rooflight, the effect is small. You are not going to solve a low-frequency problem with cavity width in a sealed unit, and widening it beyond 20mm damages the thermal performance for reasons set out on low-emissivity coatings and argon fills.

Argon has a marginal negative acoustic effect against air because it is denser and couples the panes slightly better. It is far too small to trade against the thermal benefit.

Polycarbonate and plastic domes

The loudest option available, by a wide margin, and worth stating plainly because they are frequently sold as an economy choice.

A multiwall polycarbonate dome weighs a fraction of a glass unit, so it has almost no mass to resist impact. It is also stiff and thin, which makes it an efficient radiator. Under heavy rain, a polycarbonate dome over a bedroom is loud enough to make conversation difficult, and the sound has a hard drumming quality that people find harder to ignore than a broadband hiss.

If a plastic dome is being replaced with glass, expect a difference of 12 to 18dB under rain. It is the single largest acoustic improvement available in this category of work and it is one of the more common reasons people call us about a unit that is otherwise still watertight.

Flat-roof units and the deck around them

On a flat roof, the rooflight is not the only thing making noise. The deck itself is a large, thin, stiff membrane over an air void, and rain falling on a single-ply or felt covering over plywood produces its own broadband noise.

People often attribute the whole of it to the rooflight because the rooflight is the visible thing. Standing under the same roof away from the unit usually settles the question in a few seconds.

Where the deck is the problem, the answers are in the roof build-up rather than the glazing: a heavier covering, a mineral-surfaced cap sheet rather than a smooth one, or insulation with more mass. Where the rooflight is the problem, the glazing specification is where it gets solved.

The frame, the seals and the path around the glass

Sound takes the easiest route, and a well-specified pane in a poorly sealed frame gives it one.

An opening rooflight has a compression seal between sash and frame, and its condition governs how much sound flanks the glass entirely. A perished or badly adjusted seal can lose 5dB of the unit’s rated performance. Adjusting the hinge compression on an opening rooflight is a five-minute job and it is worth doing before anyone concludes the glass is at fault.

The frame material matters too. A thin aluminium frame section radiates sound more readily than a timber one of the same size, and rain striking the exposed aluminium flashing around a roof window is itself a noise source separate from the glass. Timber-cored units with an aluminium outer skin sit between the two.

The reveal, the ceiling and the room below

A rooflight opens into a splayed shaft, and that shaft behaves like a horn coupling the glass to the room.

A deep, narrow, hard-plastered reveal reflects sound down efficiently and adds a slight amplification. A shallow, wide, splayed one disperses it. Nobody should design a reveal around acoustics, because daylight matters more, but it explains why two identical units in different rooms are reported so differently.

The room itself matters as much. A loft bedroom with a carpet, curtains and a bed absorbs the impulse. A tiled kitchen extension with a polished concrete floor and glazing on two sides reflects everything, and the rain noise there will be reported as roughly twice what it is in the bedroom.

Vents, flaps and the hole you left deliberately

Any deliberate opening in a glazed assembly is an acoustic hole, and sound goes through holes far more readily than through glass.

A trickle vent in a rooflight frame or a night-vent position on a roof window can cost 8 to 10dB of the unit’s rated performance while it is open. That is not a fault. It is the trade between ventilation and quiet, and it is worth knowing about before it surprises you.

Acoustic trickle vents exist, with a lined baffled path rather than a straight duct, and they recover most of the loss for a modest cost. If you are specifying vents on a rooflight over a bedroom near a road, specify the acoustic version. The ventilation requirement itself is covered on whether rooflights need trickle vents.

Bedrooms, and where the specification changes

The single most useful rule we apply is that any rooflight above a bed gets an acoustic specification whether the client asked for one or not, and we price it as a visible option rather than burying it.

The reason is that the complaint threshold in a bedroom is far lower than anywhere else. A level that goes unnoticed in a kitchen at four in the afternoon will wake somebody at three in the morning, and the moment it does the rooflight becomes the thing that ruined the room.

The specification that answers it is not expensive: asymmetric panes, an acoustic laminated inner, and an opening seal that is properly adjusted. That combination typically takes rain noise down 8 to 10dB against a standard symmetric unit, which is heard as roughly half as loud.

Aircraft and road noise, a different problem

Oxfordshire has plenty of properties under approach paths and beside busy roads, and those need a specification aimed at low frequencies rather than at rain.

Here the numbers to compare are R_w with the C_tr correction applied, and the levers are mass and asymmetry rather than damping. A 6mm outer pane against a 6.8mm acoustic laminated inner, in the widest cavity the frame will take, is a sensible domestic answer.

Be realistic about what glazing can achieve. If the walls and the roof covering are the weak elements, upgrading only the rooflight moves the total very little. Sound finds the weakest element and the overall result is governed by it, so an excellent rooflight in a poorly insulated roof is money spent on the wrong component.

What cannot be added afterwards

The list is short and it is absolute.

• Pane thicknesses and asymmetry: fixed at manufacture.
• Laminated construction and the interlayer type: fixed at manufacture.
• Cavity width: fixed by the frame rebate.
• Frame material and section: fixed by the product chosen.

What can be improved later is limited to seal adjustment, adding an acoustic baffle to a vent, and soft furnishings in the room. Everything with a meaningful decibel figure attached is decided when the order is placed, which is why we raise it at survey rather than at handover.

Specifying quiet without overspending

Take the reductions in order of value for money and stop when the room is answered.

Asymmetric pane thicknesses cost nothing and buy 2 to 3dB. A laminated inner pane is required for safety overhead in any case, and buys a further 2 to 3dB. Upgrading that laminate to an acoustic interlayer costs a modest premium and buys 4 to 6dB more. Increasing the outer pane from 4mm to 6mm costs weight and a little money and buys around 3dB. Beyond that, you are into heavy glass, structural consequences and diminishing returns.

For most bedrooms, the first three items are the whole answer. Tell us which rooms have beds in them and which elevation faces the road, and we will specify accordingly. Call 01865 704245 or email info@heritageskylightsoxford.co.uk with the room uses and we will put the glass make-up in writing, pane by pane, before anything is ordered.