The distinction that saves people the most money
Absorption changes how sound behaves inside a room. Isolation stops sound getting from one room to another. They are different physics, different materials and different budgets, and the products for one are useless for the other.
Porous absorbers — fibrous panels, mineral wool, thick fabric — work by making air move through a resistive material, converting the motion into a tiny amount of heat. That kills reflected energy in the room. It does nothing meaningful about transmission, because transmission is governed by the mass of the barrier, how well it is sealed, and whether the two sides are mechanically decoupled. A panel hung on a wall adds no useful mass, seals nothing, and decouples nothing.
So if the complaint is that the room sounds echoey, boomy, or smeared, treatment is the answer. If the complaint is that someone else can hear it, treatment is not the answer and no amount of it will become the answer. That job is construction: more mass in the wall, resilient channel or a decoupled second leaf, and obsessive attention to every gap, because sound leaks through openings the way water does.
Finding the first reflection points
The reflections that damage stereo imaging most are the earliest ones, arriving within a few milliseconds of the direct sound. Your ear cannot separate them from the original, so instead of hearing a reflection you hear a blurred, wandering version of the source. The geometry of where they land is simple: a reflection behaves like a straight line from a mirror image of the speaker, so the reflection point on a side wall lies on the line between the speaker and the mirrored position of your head.
The calculator does that arithmetic for a rectangular room and gives the distance from the front wall for four points: the near-speaker side wall reflection, the cross reflection from the opposite speaker, the ceiling, and the floor. Verify them with a mirror before you commit. Sit in the listening seat, have someone slide a hand mirror flat along the wall, and mark every position where you can see a speaker driver reflected. Those marks are the truth for your actual room, including the parts that are not a neat rectangle.
The floor reflection is on the list because it is real and strong, and it is also the one you almost never treat with a panel. A rug with underlay in that position does the job and nobody has to look at it.
How much is enough
Coverage is usually discussed as a percentage of total room surface area — walls plus floor plus ceiling. Somewhere in the region of 15 to 25 percent is a working band for a listening or media room, and the calculator defaults to 20. Below about 10 percent the change is audible but partial. Above about 30 percent, small rooms start to sound dead in a way people find unpleasant rather than clean: voices go flat and close, and the space feels like a padded box.
| Coverage | What it usually delivers | Typical use |
|---|---|---|
| 5-10% | First reflection points only, imaging tightens | Living room where panels must stay discreet |
| 15-20% | Noticeably shorter decay, dialogue clarity | Media room, home theatre |
| 20-30% | Controlled, quite dry room | Critical listening, mixing |
| Over 30% | Often too dead in a domestic room | Rarely wanted outside a vocal booth |
Position matters far more than quantity at the low end of that range. Six panels at the reflection points and the front wall do more than twenty panels distributed evenly for symmetry, because the ones at the reflection points are intercepting the specific energy that is causing the problem.
Bass is a different problem with a different panel
Thin panels do not absorb bass. A two inch fibrous panel is effective in the midrange and treble, tails off through the low mids, and does close to nothing below 100 Hz, because absorbing a wavelength requires material thickness on the order of a useful fraction of that wavelength — and a 60 Hz wave is nearly nineteen feet long. This is why a room can have a dozen panels on the walls and still boom.
What works on low frequencies is depth and position. Thick absorbers, or panels mounted with a substantial air gap behind them, reach lower. Corners are the highest-value position, because pressure builds at room boundaries and the vertical corners are where the largest number of modes meet. A floor-to-ceiling absorber in each corner of a small room typically does more for the perceived bass problem than every other panel combined. Getting genuinely flat bass response is a measurement job, not a coverage-percentage job.
The order to do it in
Move the speakers and the seat first, because it is free and it can eliminate problems that treatment can only reduce — the speaker placement calculator covers that geometry. Then treat the first reflection points on the side walls and ceiling, which buys the largest single improvement in imaging. Then the front wall behind the speakers. Then the corners, for whatever bass control the budget allows. The rear wall comes last and is the one place where diffusion rather than absorption is often the better choice, because a completely dead wall behind your head can make the room feel closed in.
Measure if you can. A calibrated microphone and free software will tell you more in one afternoon than a month of reading, and it will occasionally tell you that the thing you were about to spend money on is not the thing that is wrong.
Questions people ask
Will acoustic panels stop my neighbours hearing the TV?
No, and this is the single most common misunderstanding in the subject. Acoustic panels are porous and lightweight by design, because that is what makes them absorb sound energy inside the room. Stopping sound passing through a wall requires the opposite properties: mass, airtight sealing, and mechanical decoupling between the two sides of the assembly. Adding panels to a wall changes its transmission loss by an amount so small you would struggle to measure it. If the goal is isolation, the work is in the wall assembly itself, and it is construction rather than decoration. Foam sold with the word soundproof on the packaging is absorption regardless of what the packaging says.
How do I find the first reflection points without doing the maths?
The mirror trick, and it is more reliable than the calculation because it uses your actual room. Sit in the listening seat and keep your head still. Have someone hold a hand mirror flat against the side wall and slide it slowly along, roughly at the height between your ears and the tweeters. Wherever you can see a speaker driver in the mirror, that spot on the wall reflects that speaker straight into your ear. Mark it. Repeat for the other wall and, with a mirror held up on a pole or a broom, for the ceiling. The marks you end up with are the positions the first panels go, and they account for a bay window or an offset seat that no rectangular formula knows about.
Is thicker always better for acoustic panels?
Thicker reaches lower, which is usually what a room needs, but it is not free. A 2 inch panel is effective through the midrange and treble; a 4 inch panel or a 2 inch panel mounted on a 2 inch air gap extends useful absorption meaningfully further down. Below roughly 100 Hz you need serious depth, which is why bass absorbers are built as thick corner units rather than wall panels. The cost of thickness is that a very absorbent room across the whole spectrum can end up sounding dead, and thick panels intrude into the room and are harder to mount. A common approach is thin to moderate panels at the reflection points and much thicker units in the corners, rather than the same thickness everywhere.
What is RT60 and should I be chasing a number?
RT60 is the time it takes sound to decay by 60 decibels after the source stops, and it is the standard single-figure description of how live a room is. For a domestic listening or media room, figures in the range of roughly 0.3 to 0.5 seconds are commonly considered comfortable, with smaller rooms at the lower end. The reason not to chase it too hard is that it is one number describing something that varies enormously with frequency. A room can measure a respectable average while having a bass decay three times longer than its treble decay, which is exactly the problem most untreated rooms have. The Sabine estimate on this page shares that limitation and should be read as a rough indicator of direction, not a specification.
Can I use egg cartons, carpet or moving blankets instead of panels?
Partly, with honest limits. Heavy moving blankets and thick curtains hung with an air gap do genuinely absorb, mostly in the treble and upper midrange, and they are a legitimate low-cost first step in a room where nothing is being done at all. Carpet absorbs treble and nothing else. Egg cartons do essentially nothing at any frequency and the belief that they work is one of the durable myths in the topic. The pattern is that anything porous, thick, and mounted with space behind it will do something useful in the highs, and nothing improvised will touch the bass, because bass absorption needs depth that improvisation rarely provides. If the room is bright and echoey, soft furnishings help. If the room is boomy, they will not.