Acoustic laminated glass

Acoustic laminated glass is the most misunderstood product we sell. It is a real, measurable improvement over ordinary glass, and it is also the thing most often sold to people whose noise problem was never in the glass. This page explains what it does, how sound actually gets into a London room, and how to tell whether a glazing change will fix your problem or just empty your wallet. We make acoustic laminated glass to size in our own Wembley workshop and fit it across the capital, so we have no reason to sell you a pane that will not work.

The construction is simple enough. Two sheets of glass are bonded to a thin acoustic interlayer, a softer and more flexible grade of PVB than the standard kind. Ordinary glass is stiff and transmits vibration efficiently. The acoustic interlayer lets the two sheets move very slightly against each other, and that shearing movement turns sound energy into a tiny amount of heat instead of passing it into the room. The glass is also laminated safety glass by definition, so you get fragment retention and ultraviolet filtering as part of the deal.

How sound actually gets into a room

Picture a bucket with several holes in it. Plugging the biggest hole helps. Plugging it while leaving the others open does far less than you would hope. Airborne sound behaves the same way: the result you get is set by the weakest path, not the strongest element. Replace the glass and leave three leaks open and you will hear almost no change, which is exactly why so many people feel cheated by an acoustic upgrade that was technically delivered as specified.

In a typical London flat or terraced house, the competing paths are usually these. Trickle vents, which are deliberate holes in the frame and often the single loudest route. Letter plates and cat flaps in a front door. Extract fans and cooker hood ducts. Worn sash meeting rails and shrunken staff beads, where you can often feel the draught with the back of your hand. The joint between the frame and the brickwork behind the plaster reveal, which in older houses is frequently nothing but crumbled mortar and foam. Service penetrations for boiler flues, waste pipes and gas supplies. Suspended timber floors with airbricks beneath them, which carry street noise straight into a ground-floor front room. And the box sash void itself in a Victorian window, which is a hollow chamber connected to the outside world.

Any of those can dominate. We have surveyed bedrooms in Islington where sealing the reveal and fitting an acoustic trickle vent would have done more than the glass, and we said so. We have also surveyed first-floor flats on the A40 where the glass genuinely was the main path and the upgrade was worth every penny. The only way to know which you have is to look.

Why 3dB is barely noticeable and 10dB sounds like half

Decibels are not a normal scale and reading them as one leads people badly astray. The scale is logarithmic, which means it compresses very large differences in sound energy into small-looking numbers. Two practical rules are worth carrying around. A 3dB change is the smallest difference most people can detect at all in a real room, and many cannot. A reduction of around 10dB is generally perceived as roughly halving the loudness, even though the actual energy reduction is far larger than half.

This has two consequences for buying glass. First, a quote promising a handful of decibels is promising something you may struggle to notice, and it is worth asking whether the money is better spent elsewhere in the window. Second, decibel figures quoted for a pane come from laboratory tests on that pane in a test rig, not from your window in your wall. We will talk to you in terms of what you are likely to perceive and where the remaining noise will come from. We will not put a decibel figure for your room in writing, because nobody honest can predict one from a survey, and any company that does is quoting you a laboratory number and letting you assume it is a promise.

The coincidence dip, and why two different thicknesses beat two the same

Every sheet of glass has a frequency range where it stops resisting sound and starts cooperating with it. Sound waves hitting the pane at an angle line up with the pane’s own natural bending wave, the sheet flexes in sympathy, and sound passes through far more easily than the thickness would suggest. This is the coincidence dip, and it is why glass performs unevenly across the frequency range rather than blocking everything equally.

Where that dip sits depends on the thickness of the sheet. Two identical panes dip at the same frequency, so they fail together and leave a hole in the performance at exactly that pitch. Make the two sheets different thicknesses, say a 4mm bonded to a 6mm rather than two 4mm, and the dips sit at different frequencies, so each sheet is still working where the other one is weak. Add an acoustic interlayer and the damping flattens the dip rather than just moving it. This is the whole reason good acoustic laminates are asymmetric, and it is the single easiest thing to check on a quote. If a supplier has offered you a symmetrical make-up for an acoustic job, they have offered you laminated glass and called it acoustic.

What an acoustic interlayer can and cannot do

Be clear on the physics and you will not be disappointed. Acoustic laminate is good at mid and high frequency airborne noise: voices in the street, car doors, dogs, brakes, aircraft passing overhead, rail squeal. It is much weaker against low frequency noise, because low frequencies are controlled mainly by mass and cavity depth rather than damping, and a pane of glass simply does not weigh enough to stop them. Heavy lorry rumble on the North Circular and the deep part of a train passing on the Overground will be reduced, not removed.

It does nothing at all about structure-borne noise. Vibration from a train in a tunnel beneath you, footfall from the flat above, a lift motor, or a plant room on the roof travels through the building rather than through the air, and the window is not on that path. If what you feel is the floor rather than hear through the glass, a glazing change is the wrong purchase and we will tell you that at the survey rather than after the invoice.

Acoustic laminated glass or secondary glazing?

For airborne noise, the biggest single lever is the depth of the air gap between two layers of glass, and a sealed unit does not have much. A standard double glazed cavity is optimised for heat, not sound: the gap that performs best thermally is relatively narrow, and for acoustics you want it as deep as you can reasonably get. Secondary glazing sits a hundred millimetres or more inside the original window, which is why a well-sealed secondary system will usually beat any replacement pane of the same cost.

That matters enormously in London because so much of the housing stock is in conservation areas where the original sash has to stay. In Islington, Camden, Hackney, Richmond, Dulwich Village and Bedford Park we have told plenty of people that their money is better spent on secondary glazing with an acoustic laminated inner pane than on trying to shoehorn a thick unit into a 19th century sash box. You get the deep air gap, the damping interlayer and a window the council will not object to. Where secondary glazing is genuinely unacceptable to the owner, slim acoustic laminated units in refurbished sashes are the next best thing, and we are straight about the fact that it is second best.

In a sealed unit or as a single pane?

Both are normal. In a double glazed unit, the standard approach is an acoustic laminate on the inner or outer leaf and a plain pane on the other, with the two deliberately different in thickness for the reasons above. That gives you an asymmetric unit that handles sound and still meets the thermal requirements of Approved Document L when the window is being replaced. In single-glazed positions, including many secondary systems, internal screens and conservation work, acoustic laminated glass goes in as a single pane and does its work on damping alone.

Where a window is being replaced outright, there is a trade-off to have out loud. Pushing the cavity wider helps sound and eventually hurts heat loss through convection. Filling the cavity with a denser gas helps heat and does very little for sound. Thickening the glass helps sound and adds weight the hinges and the frame have to carry. We work out where the compromise sits for your actual priority rather than specifying one house-standard unit for everybody.

How we survey before promising anything

A noise survey for us is a physical inspection, not a phone call. We look at the window from both sides, check the frame for movement and worn weatherseals, feel for air leakage at meeting rails and beads, check whether there are trickle vents and what condition they are in, pull back a corner of the reveal finish where we can to see what is actually behind it, and look at the floor construction and any airbricks. We ask what the noise is, when it happens and which room you notice it in, because a lorry at 6am and a pub at 11pm are different problems.

Then we tell you the order to spend your money in. Sometimes that is acoustic laminated glass. Sometimes it is sealing and draught-proofing first and glass later. Sometimes it is acoustic secondary glazing. Occasionally it is nothing we sell, and we say so and leave.

We will not put a decibel improvement for your room in writing. Nobody can calculate one honestly from a survey, and the figures on a glass data sheet describe a laboratory, not your bedroom.

Sizes, cutting and edgework

Because we laminate and process in Wembley, laminated glass cut to size is ordinary daily work rather than a special order. Laminated can be cut after manufacture, which is a genuine practical advantage over toughened. Both sheets are scored and broken and the interlayer is parted with heat, so a pane can be trimmed to a site measurement, a template can be matched to an out-of-square Victorian opening, and a late change on the day does not automatically mean a reorder. Toughened glass is heat treated at the end of the process and cannot be cut, drilled or notched afterwards, so every toughened mistake is scrap.

Edges are finished to suit the position: arrissed and hidden where the pane sits in a rebate, flat ground or polished where an edge will be seen or touched. Holes and notches in laminated glass are possible but need care and sensible margins from the edge, and we will tell you when a cut-out you have drawn is too close to the perimeter to survive handling.

Standards and paperwork for acoustic laminated glass

Laminated glass is manufactured to BS EN 14449 and comes with a declaration of performance that identifies the make-up. In critical locations under Approved Document K, including doors, side panels and low-level glazing, the pane must be safety glass satisfying BS 6206 Class A or the equivalent classification under BS EN 12600, and a laminated pane of adequate thickness achieves that by retaining its fragments. Where the acoustic glass is also doing a security job, the relevant classification is BS EN 356, and where it forms part of a barrier the loading comes from BS 6180. We leave the paperwork with you rather than keeping it on file, because the person who needs it next is your surveyor or your building control officer, not us.

On price, the things that move the number are the make-up, the overall area, the number of panes, whether it is a single pane or a sealed unit, the edgework, and the access. Weight is not a footnote: acoustic laminate in a large opening can need extra labour, and an upper-floor flat with a narrow staircase changes the job more than the glass specification does. We price all of that into a fixed written quotation after the survey, so the figure you accept is the figure you pay.