Acoustic treatment placement and panel dimensions

Acoustic treatment is sold as a quantity, so many square feet of panel, and used as a position. The quantity sets how dead the room becomes. The position decides whether the treatment removes the reflections that smear dialogue or the ones nobody was ever going to notice. Two panels in the right places beat ten in the wrong ones, and the right places can be found with a mirror and a tape measure in ten minutes.

The datum: where a reflection point actually is#

A first reflection point is not on the wall beside the speaker and it is not on the wall beside the seat. It is where the angle of incidence equals the angle of reflection, and for a listener and a speaker at different distances from the same wall, that point sits much closer to the speaker than to the listener.

The formula is one line. With the speaker a from the wall, the listener b from the same wall and D between them along the room, the reflection point lies D x a / (a + b) along that path from the speaker.

Work it for the layout in speaker placement dimensions: a 15 ft (4.57 m) room, front left 21 in (53 cm) from the side wall, the seat on the centreline 90 in (229 cm) from it, 120 in (3.05 m) apart. The reflection point is 120 x 21 / 111, which is 22.7 in (58 cm) from the speaker plane. Not half way. Not by the sofa. Two feet in front of the speaker, where almost nobody puts a panel.

The mirror test finds the same point without arithmetic: sit in the seat, have somebody walk a mirror along the wall, and mark the spot where the speaker appears in it. Do it for both walls, the ceiling and, if the floor is hard, the floor.

The reflection points in a real room#

Two rows deserve a second look. The centre channel reflects off the side walls at the mid point of its path, because it sits on the centreline and so does the listener: 60 in (152 cm) rather than 23 in. A 24 in panel cannot cover both, which is the honest argument for a 48 in (122 cm) wide panel centred around 40 in (102 cm) from the speaker plane. It spans 16 to 64 in and catches every front channel on that wall.

The ceiling point falls at exactly half the distance whenever the tweeter and the ear are at the same height, which they should be. Move the tweeter down to 36 in (91 cm) and the point shifts to 63 in (160 cm), a change of 3 in. The ceiling cloud position is therefore insensitive to almost everything, which is useful when the ceiling has downlights in it and something has to move. The depths available for a recessed cloud are the same ones that govern ceiling heights and datum lines.

The vertical position of a side wall panel follows the same weighted average. With the tweeter at 42 in and the ear at 42 in the point is at 42 in; with the tweeter at 36 in it drops to 37 in, because the point sits close to the speaker and therefore close to the speaker's height. Centre the panel at 40 in (102 cm) and a 48 in tall panel covers 16 to 64 in (41 to 163 cm), which absorbs the path for a reclined ear and a sitting-up ear alike.

Thickness is a frequency, not a quality grade#

A porous absorber works on the velocity of air moving through it. Velocity is zero at a hard wall and greatest a quarter of a wavelength out from it, so a panel of depth t reaches its full absorption above the frequency at which t is that quarter wavelength, and falls away steadily below.

Read the first row and the problem with foam tiles becomes obvious. One inch of anything is a treble absorber. Cover a third of a room in it and every reflection above 1,700 Hz disappears while everything from 100 to 800 Hz, the band that actually makes a small room sound like a small room, carries on exactly as before. The result is dull and boomy at the same time.

The air gap row is the useful one. Two inches of mineral wool on a 2 in (50 mm) batten behaves, for this purpose, like 4 in of solid material, because what matters is the distance from the wall to the front face. Gaps are free. Material is not.

Density is a much weaker variable than either. Mineral wool between 30 and 80 kg per cubic metre (2 to 5 lb per cubic foot) all performs similarly, and denser boards are slightly worse, not better, because sound has to get into the material before it can be absorbed.

How much of the room needs treating#

Sabine's equation gives the absorption a target reverberation time requires: RT60 = 0.049 x V / A with the volume in cubic feet and the absorption in sabins. For a 15 x 18 x 8 ft (4.57 x 5.49 x 2.44 m) room the volume is 2,160 cu ft (61 m3) and the total surface area is 1,068 sq ft (99 m2).

Start with what is already there. Carpet and underlay over 270 sq ft at a mid-band coefficient of 0.35 gives 95 sabins. Painted plasterboard over the remaining 798 sq ft at 0.05 gives 40. Six upholstered seats give roughly 18. The total of 153 sabins puts the untreated room at 0.69 s, which is about twice what a cinema wants.

Thirty-eight panels is the number people do not believe. It is 304 sq ft (28 m2) of treatment, and it is why a dedicated room ends up with panels on the front wall, both side walls, a ceiling cloud and the whole rear wall. The last row shows the other end: 58 per cent coverage is unreachable in a normal room and would sound wrong if you got there. To work out your own surface area, measure the room the way the paint quantity calculator does, walls plus ceiling plus floor, then take a quarter of it.

Thicker panels change the count more than extra panels do. Sixteen panels at 6 in (150 mm) buy more usable absorption below 400 Hz than 24 panels at 2 in.

Corners, traps and the depth that matters#

Every axial, tangential and oblique mode in a rectangular room has a pressure maximum at a corner, which is why a corner is the one place a single device intercepts all of them. A trap that spans 16 in (406 mm) down each wall presents a face of 22.6 in (57 cm) and reaches a maximum of 11.3 in (29 cm) into the room. That depth puts its quarter wavelength at 300 Hz.

Front corners come first, because they hold the subwoofers and the front speakers. Rear corners come second. Wall to ceiling edges are the third rank and are usually where a soffit already runs, which makes them cheap to fill during a build in a way they never are afterwards.

The rear wall and the limits of diffusion#

Diffusers are commonly specified for the wall behind the seats. They need room. A diffuser scatters properly only once the listener is far enough away for the scattered wavefronts to combine, and the rule of thumb from the literature is about three wavelengths at the design frequency. For a diffuser designed down to 500 Hz, a wavelength is 2.26 ft (69 cm), so the nearest ear should be 6.8 ft (2.1 m) away.

Very few domestic rooms deliver that. If the back row sits 36 in (91 cm) off the rear wall, as it does in most of the layouts in theatre seating rows and risers, a diffuser there is an ornament. Absorb instead: 4 in (100 mm) across the full width, corner to corner, and accept that the room will be slightly drier at the back than the front.

Keep diffusion for the upper rear wall above a back row, for the rear half of a long room, or for a room over 22 ft (6.7 m) deep. The proportions that make one possible are set out in home theatre room dimensions.

Building the panels: sizes that come off a standard slab#

Mineral wool comes in 24 x 48 in (610 x 1,220 mm) slabs in the US and 1,200 x 600 mm in metric markets, and that is the reason acoustic panels are the size they are. A frame in 3/4 in (18 mm) timber around a 24 x 48 in slab finishes at 25.5 x 49.5 in (648 x 1,257 mm), which is the dimension to check against the wall, the skirting and any wall panelling already there.

The 24 in and 600 mm slabs are not the same size, and a frame built to one will not take the other: 24 in is 609.6 mm, so a metric slab rattles in an imperial frame and an imperial slab will not go into a metric one. This is the usual imperial and metric collision, and it decides which slab you can buy locally more often than performance does.

Cover with a fabric you can breathe through with the fabric held to your mouth. If you cannot, neither can the sound. Heavy upholstery weave, blackout lining and rubber-backed cloth all turn a 4 in absorber into a 4 in reflector with soft edges. The same test applies to the curtains covering a false window, though fabric hung in folds is doing a different job, and the fullness that governs it is in curtain width, fullness and stack-back.

Under the panels, the floor matters more than any of them. Carpet on a resilient underlay is the single largest absorber in the room and also the main structural break for footfall, and the build-ups are covered in underlay and subfloor thickness. The terms used above, from sabin to first reflection, are defined in the glossary of interior dimensions, and the rest of the room is in the media room dimensions hub, including how the treatment interacts with media room lighting when both want the same ceiling.