Two people describe a problem with a room. The first says it does not sound right in there. The second says you can hear it through the whole house. Both are acoustic problems. They need almost entirely different work, and the money spent on one buys very little of the other.
That distinction is the single most useful thing to understand before planning a cinema, a listening room, or any room that has to behave. It is also the thing most often got wrong — usually late, when the walls are up and the options have narrowed.
The two jobs
Acoustic treatment is work done inside the room. It manages reflections and the way low frequencies behave in that particular volume, so that what you hear is the system rather than the room’s opinion of it. It is why speech is easy to follow, why bass is even from seat to seat, and why a good system can sound disappointing in a bad room.
Sound isolation is work done to the boundaries of the room. It stops sound getting out to the rest of the house, and stops the house getting in. It is a construction problem: mass, separation, and a genuinely continuous sealed line.
Treatment does not stop sound leaving. Isolation does not make a room sound good. A perfectly treated room with lightweight walls will still empty the bedroom above during a film. A properly isolated concrete box with bare surfaces will be quiet from outside and unpleasant to sit in.
Most real projects need some of both, in different proportions. Knowing which one a given complaint calls for is where the money starts being well spent.
Why the confusion is expensive
The costly version of this mistake runs one way far more often than the other: someone spends on treatment hoping it will fix a noise problem.
It is an understandable hope. Absorption is visible, purchasable and satisfying. It arrives in boxes. Isolation is invisible, has to be built, and mostly consists of things not being there — no rigid connection, no unsealed gap, no ventilation path acting as a duct for sound. So the room gets panelled, sounds distinctly better inside, and carries on being audible upstairs, because nothing that was done addressed that at all.
The reverse mistake happens too, though it is rarer and cheaper to correct. A heavily isolated room with no internal treatment is quiet from outside and hard work inside.
Treatment: what it actually involves
The low end is where rooms are won and lost
Most rooms succeed or fail at the low end. Treble and midrange are comparatively easy to manage. Low frequencies are long, they interact with the dimensions of the room itself, and they are the reason a room can look right on paper and still sound heavy in one seat and thin in the next.
They are also the reason acoustics cannot be bought by the square metre. Room dimensions and materials influence which frequencies need attention, so two rooms of the same size and different proportions need different work.
Depth has to come from somewhere
Controlling low frequencies takes depth, and depth comes out of the room.
On a garden-room cinema near Gloucester, that meant more than 30cm of broadband acoustic treatment in the front wall, with the treatment designed to remain effective down to approximately 60Hz. The ceiling could not give the same: it was held between 50mm and 68mm, the maximum that the acoustic fabric run could conceal. After treatment, the room’s reverberation profile measured textbook from around 60Hz upwards.
In a Victorian living room in Frome, modelling the room in acoustics software showed severe cancellations in the bass response before anything was built. Because it was known early, the answer could be structural: a 50cm-deep wall of absorption built into the design and concealed behind a stretch-fabric system, with 15cm absorbers on the front wall and a final bass trap above the vinyl storage.
Those are two real rooms, not rules of thumb. The general point is the one they share: meaningful low-frequency control is measured in tens of centimetres, and that space is taken from somewhere — floor area, head height, or a wall build-up that has to be agreed before it is built.
It does not have to look like a studio
The first objection is almost never technical. It is that nobody wants to live inside a recording studio.
That is a fair objection to grey foam wedges, and it is not an objection to absorption, which is indifferent to what is stretched over it. On one cinema the panels carry a cherry-blossom print and read as a decorative wall. On another, treatment became a run of illuminated vertical columns. In the Frome listening room, half a metre of absorption disappeared behind fabric and joinery.
All three do the same job. The constraint worth defending is depth, not appearance. Agree the depth early and the finish can be almost anything.
The aim is not a studio. A room with everything absorbed is technically dead and unpleasant to sit in; a hard, bright room throws everything back at you and is tiring. Good acoustic design sits between those extremes, with enough control that speech is clear and bass is even, and enough life that the room still feels like part of the house.
Isolation: mass, separation, and the details that undo them
The principles are not exotic. Mass resists the energy. Separation stops the structure carrying it. A continuous sealed line stops it simply walking around the mass. Any competent build can achieve all three.
What undoes them is ordinary. A well-built separated wall with a standard internal door in it. A ceiling sealed everywhere except the ventilation route. Service penetrations cut after the acoustic line was signed off. Isolation is only ever as good as its weakest path, and the weakest path is usually the one drawn last or drilled later.
This is why isolation belongs in the coordination conversation rather than in a product schedule. The door, the threshold, the ventilation strategy and the service routes are all somebody’s drawing already. They simply need to be the same drawing.
It is also why there are almost no good photographs of isolation. By the time a room is worth photographing, every part of the isolation is behind plasterboard.
Measure, treat, measure again
Acoustics is the part of a system where taste stops and measurement starts.
On a basement room in Bath, the constraints were a low ceiling and untreated surfaces that would otherwise have produced uncontrolled reflections and modal problems. Treatment went on three of the four perimeter walls: timber-framed, filled with insulation, with an engineered air gap behind. The air gap is the detail worth knowing about — it increases the effective acoustic thickness of the treatment and helps trap lower-frequency energy, which is dissipated as heat through friction within the insulation fibres. An irregular adjacent space was deliberately left as it was, because its shape helps disrupt strong modal resonances.
The room was then measured with REW, and room correction applied with ARC Genesis.
That order matters. Correction applied to an untreated room is making the best of a problem rather than removing it, and there are things — the cancellations that modelling found in Frome — that no amount of equalisation resolves, because they are a property of the room and not of the signal.
Neither belongs at the decorating stage
Both jobs change the building.
Treatment changes wall build-ups and ceiling depths, and takes usable floor area. Isolation changes the structure, the doors and the ventilation strategy. Once the plasterboard is on, the wall thicknesses are fixed, the ceiling void is committed, and the honest options have narrowed to whatever fits in the space nobody planned to lose.
Brought in while the room is still a drawing, depth is something you allow for. Brought in afterwards, it is something you sacrifice.
What is worth knowing before the first conversation
Very little, and none of it is equipment.
What the walls, floor and ceiling are made of — solid masonry and timber stud behave differently, and the answer changes both what treatment is needed and what isolation is possible. What is above, below and next door, which is the isolation question in its entirety: a cinema under a bedroom is a different project from the same cinema over a garage. The room’s dimensions. And what the room is for — films, music, both, or a family room that has to do all of it and still look like a living room.
A floor plan and those four answers are enough to start.
A note on numbers
Every figure in this article is from a Hidden Home Technology project: the depths, the frequencies, the software. There are no general performance figures here — no decibel reductions, no absorption coefficients, no typical improvements — because they depend entirely on the room and the construction, and a number quoted without a room attached is worth nothing.
If a room has to sound right, or has to stay quiet on the other side of its walls, the useful first step is a plan and a conversation. We work with homeowners, architects and interior designers across Bath, Bristol and the Cotswolds, and we would rather be in the discussion while it is still a drawing.
