Acoustic underlay is one of the few building products that is bought almost entirely on faith. It goes in, it disappears, and nobody sees it again. Most people never find out whether the one they chose was appropriate, or whether it was installed in a way that let it do its job, until the flat below tells them.
That is a shame, because the principles are not difficult. What makes this confusing in Central London is not the physics. It is that the requirement itself is different in every building, and people go looking for a single right answer that does not exist.
There is no standard requirement — there is only your lease
Let us deal with this first, because it is the question everyone actually arrives with. There is no single acoustic specification that applies to flats in Central London. What you must achieve, if anything, is set principally by your own lease and by any block regulations made under it.
Those provisions vary enormously. Some leases say nothing specific about acoustic performance and instead restrict hard finishes or require carpet. Some require the landlord's consent for any change of floor finish, leaving the specification to be agreed case by case. Some name a standard or a level of performance that any hard finish must achieve. Some blocks have their own house rules, which can be more detailed than the lease and can be updated over time.
Because of that, we are not going to quote you a decibel figure or name a required standard on this page, and you should treat anyone who does so casually with suspicion. A number that is correct for one block can be irrelevant or insufficient in the next. The only figures that matter are the ones in your paperwork, confirmed by your managing agent in writing. The broader lease picture is covered in our guide to floor noise rules in mansion blocks.
The two kinds of noise, and which one this is about
Understanding this makes every conversation with a supplier easier.
Airborne noise is sound travelling through the air — speech, television, music — which then sets the structure vibrating. Impact noise is energy delivered straight into the floor: footsteps, heels, a dropped phone, chair legs, a dog. It bypasses the air entirely and travels through the building as vibration, radiating out of the ceiling below.
Acoustic underlay under a hard floor is aimed overwhelmingly at impact noise. This is the distinction that trips people up. A householder who fits underlay and still hears their neighbour's television has not necessarily been sold a bad product; they have solved a different problem from the one that was bothering them. Improving airborne insulation through a floor is a much bigger intervention involving mass and separation, not a resilient layer under a board. Be clear which problem you are actually trying to solve before you spend anything.
How it works, in one paragraph
A resilient layer between the finish and the structure absorbs and damps the energy of an impact so that less of it reaches the deck. Different products do this differently — rubber crumb, foams, fibre mats, cork, composites with a dense layer bonded to a soft one — and they trade performance against thickness, compressive strength and cost. The important consequence is that the layer only works if it is genuinely continuous and genuinely resilient in use. Compress it too hard, bridge it, or puncture it, and the energy simply goes round.
Flanking: the reason good products fail
This is the part that separates a floor that works from one that does not, and it gets almost no attention in showrooms.
Acoustic underlay only helps if the impact energy has nowhere else to go. If the floor finish touches something rigid that is connected to the structure, the energy takes that path instead, and the expensive layer underneath is simply decoration. In a real flat there are a lot of these paths, and every one of them has to be thought about.
The floor must not be hard against the skirting or the wall — it needs a perimeter gap, treated appropriately and covered rather than filled with something rigid. Pipes rising through the floor, radiator tails in particular, must not be grouted tight to a floating floor. Door thresholds, hearths, fitted furniture bases and heavy kitchen units can all bridge the layer. A single screw or nail through the resilient layer into the joists, which is easy to do without thinking on a floating floor, connects the finish straight to the structure at that point.
Older mansion-block floors add their own routes. Gaps around joist ends, service penetrations cut through decades of alterations, and voids that run between rooms all give sound alternative paths. This is why the same underlay can perform well in one flat and disappoint in another, and why installation detail deserves as much scrutiny as the product name.
Height: the constraint that decides the product
In a Central London flat this is frequently what determines what you can actually use.
Your build-up is the acoustic layer plus any levelling or overlay plus the board itself plus adhesive. In a period flat, the doors, thresholds, skirting details and lining bottoms were all set out for the floor that is already there. Add height and you may be easing every door, remaking thresholds, and looking at a skirting proportion that no longer reads correctly. Where a floor meets a bathroom or a kitchen, or where there is a level-access requirement, the tolerance can be almost nil.
The tension is obvious: better acoustic performance often means more thickness, and thickness is exactly what you do not have. This is why the choice of finish and the choice of underlay have to be made together rather than sequentially. Thinner engineered boards frequently win here for no other reason than that they leave room for the layer beneath, which we cover in our guide to engineered versus solid wood flooring. Measure the whole stack before you order anything.
Underlay is not the only approach
Where a lease demands more than a resilient mat can reasonably deliver, or where the existing floor is in poor condition anyway, the answer may sit further down. Options that get used in this stock include building a floating deck on resilient battens or cradles, adding mass within the floor construction, treating the void, or working on the ceiling below — though that last one involves the flat below and their agreement, which is a different conversation entirely.
These are more substantial than laying an underlay. They cost more, they take height or headroom, and they are much more likely to need consent, potentially as part of a licence to alter. They are worth knowing about because if your managing agent sets a demanding requirement, they may be the only route to it, and finding that out before you buy boards is considerably better than after.
Product claims, and reading them properly
Manufacturers publish test data for their products, and that data is useful. It is also frequently misread, so a few cautions are worth stating.
Laboratory test results are obtained on a specific floor construction with a specific finish. Your Edwardian timber floor with a century of alterations is not that construction. A figure achieved on a concrete slab tells you very little about the same product on joists. Performance figures are properties of a whole assembly, not of a mat in isolation.
So read what construction a claim was tested on, and check whether it resembles yours. Ask the supplier directly for data on a floor like yours rather than accepting the headline. And where a managing agent has set a requirement, ask them whether they will accept the manufacturer's data as evidence or whether they want something else. That is a question with a real answer, and it is far better asked in advance.
Getting the specification approved
Here is the sequence that avoids trouble. Read your lease and the current block regulations. Ask the managing agent or freeholder in writing what is required and what consent process applies. Get your contractor to put the proposed build-up in writing — the layer, the board, the total height, the method, and how the perimeter and any penetrations are detailed. Send that to the managing agent and get their response in writing. Only then order.
Ask the agent one more question while you are there: is anything required after the work? Some will want confirmation of what was installed. It is much easier to keep the delivery notes and take photographs of the layer going down than to prove after the fact what is under a finished floor. Photograph the underlay in place, before the boards cover it, in every room. It costs nothing and it is the single most useful piece of evidence you can hold.
The practical side in an occupied block
Acoustic layers arrive as bulky rolls or heavy boards, and they have to come in past neighbours. Lift booking, protection of communal parts and delivery timing all apply, as set out in our guide to porter access for mansion-block works. Preparation of the subfloor before the layer goes down is often the noisiest part of the whole job, and noisy work is commonly restricted to set hours, particularly in Westminster — see our guide to working hours restrictions. Tell the flat below what is happening and when. They are the person this entire specification is about, and a conversation before the work costs nothing.
How we specify it
We start with your lease and what your managing agent requires, not with a product. We will set out a proposed build-up in writing that you can put forward for approval, we will tell you what it does to your doors and thresholds, and we will detail the perimeter and penetrations properly rather than laying a good mat and then bridging it. Our wood flooring work starts from £30/m², with a fixed quote confirmed before anything begins, and where the floor is part of a wider refurbishment we get the consent sequence right first. If you would rather just ask a question before committing, get in touch.