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When an SFS wall needs high acoustic performance — separating walls between dwellings, hotel rooms, offices, or music rooms — the choice is usually between a single frame with heavy boards, or a twin frame with two independent stud lines. Both can work. This guide explains what each is, what each achieves, and how to decide which to use.

What is a single frame wall?

A single frame SFS wall has one row of studs, with boards fixed to both sides. The cavity between the two board faces is shared — that is, the same cavity sits behind both the left board and the right board. This is the standard partition detail used in most construction.

The studs are continuous vertical members spanning from the base track to the head track. Boards are fixed to the flanges of the studs on both sides. Insulation is typically full-fill in the cavity, or partial-fill depending on the acoustic target.

Single frame walls are cheap, fast, and simple. For most acoustic targets (Rw 40–55), they work well.

What is a twin frame wall?

A twin frame SFS wall has two independent rows of studs — one supporting the boards on the left side, the other supporting the boards on the right side. The two frames are separate, with a gap between them and no structural connection between the two stud lines (except at the top and bottom tracks, which may be shared or separate depending on the detail).

This is often called a "twin frame" or "double stud" wall. The key feature is that the studs on the left side are not the same studs as the studs on the right side — there's no rigid connection between the two faces of the wall.

The cavity in a twin frame wall is much wider than in a single frame wall — typically 200mm to 300mm — because it includes the two stud depths plus the gap between them. Insulation fills the entire cavity.

Why twin frame is better for acoustics

Sound moves through a wall in two ways:

• Through the air in the cavity — reduced by insulation and by the mass of the boards
• Through the structure — sound vibrations travel through the studs from one side to the other

In a single frame wall, the studs are a direct structural path from the left face to the right face. Sound vibration can travel through the studs, bypassing the acoustic insulation. This limits the maximum achievable performance.

In a twin frame wall, there's no direct structural connection between the two faces. The two frames are independent, so the structural path is broken. Sound has to travel through the air gap (and through the insulation, if specified), which is much harder to do — particularly at mid and high frequencies.

The result is that twin frame walls achieve significantly better acoustic performance than single frame walls, especially at low frequencies. Rw 60+ is achievable with twin frame, but rarely with single frame — no matter how much board you add.

Acoustic performance comparison

Rough comparison of achievable acoustic performance for common SFS wall build-ups:

The key takeaway: single frame tops out at around Rw 55 with careful build-ups. Twin frame goes up to Rw 65+ with similar effort per side.

Cost comparison

Twin frame is more expensive than single frame — but the cost depends on how you measure it.

Material cost

• Studs: Twin frame needs twice as many studs (or wider studs)
• Tracks: Twin frame may need double base and head tracks (or a wide single track)
• Insulation: Twin frame needs much more insulation (the cavity is twice as wide)
• Boards: Same per side, so total board cost is the same
• Overall: Twin frame typically costs 1.5–2× the material cost of a single frame

Labour cost

Twin frame takes longer to install — more studs, more insulation, more care. Labour cost is typically 1.3–1.5× a single frame.

Space cost

Twin frame walls are 200–300mm thick, versus 100–125mm for a single frame. That means losing 75–175mm of floor area along the length of the wall — which in a small flat or hotel room can be significant.

Cost per decibel

Because twin frame performs significantly better, the cost per dB of improvement is often similar or better than adding more board to a single frame. If you need Rw 55+, twin frame is usually the more cost-effective route than trying to push single frame harder.

Space and weight

Twin frame walls take up more space than single frame walls. On a typical hotel room, twin frame might take 250mm of the room width, versus 100mm for single frame. Over a 5m room, that's 750mm of lost floor space — which is significant.

Twin frame walls also weigh more than single frame walls (more studs, more insulation, plus the wider cavity). For floor loading, this may need to be considered — though the weight difference is usually small compared to other building elements.

When twin frame is worth it

Twin frame is worth the extra cost and space when:

• The acoustic target is Rw 55+ — single frame struggles above this
• The wall separates very different uses — e.g. hotel room to corridor, hotel room to hotel room, dwelling to dwelling
• The wall separates a noise source — e.g. a music room from a quiet room
• Flanking transmission is well-controlled — twin frame gains are wasted if flanking is poor
• Space allows — the wall thickness (200–300mm) has to fit
• The building has a long service life — twin frame justifies its cost over decades of use

Common applications where twin frame is standard:

• Separating walls between dwellings (Building Regs require DnT,w 45+, often 50+ in practice)
• Hotel guest room separating walls
• Office meeting rooms to open-plan areas
• Schools, hospitals, and studios
• Cinema rooms and music practice rooms

When single frame is enough

Single frame is enough when:

• The acoustic target is Rw 55 or lower — single frame achieves this easily with the right boards
• The wall separates similar uses — office to office, bedroom to bedroom in the same dwelling, corridor to room
• Flanking transmission is not controlled — twin frame gains would be wasted anyway
• Space is limited — a 100mm partition vs a 250mm partition makes a big difference
• Budget is tight — single frame costs 50–60% of twin frame
• The building has a short service life — temporary accommodation, office fit-outs that may change

For most residential projects and standard commercial fit-outs, single frame with careful build-ups is sufficient. Twin frame is reserved for critical acoustic separations.

Summary

Single frame and twin frame SFS walls are both valid choices for acoustic walls. Single frame is cheaper, thinner, and faster, and works for acoustic targets up to Rw 55. Twin frame is more expensive, thicker, and slower, but achieves Rw 60+ reliably.

The choice is driven by acoustic target, available space, and budget. For most projects, single frame with the right boards and insulation is sufficient. For critical separations (dwellings, hotels, studios), twin frame is the standard solution.