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Answers

Is terrazzo noisy underfoot?

Like any hard floor, it transmits impact sound rather than absorbing it, and the rating it ends up with belongs to the assembly rather than to the terrazzo. Impact Insulation Class is defined by ASTM E989 and measured to ASTM E492; IIC 50 is the figure most codes ask for, and a bare hard floor on a slab does not reach it.
Six terrazzo processes, drawn to one scaleEach column shows the layers of one terrazzo process. Precast and cement-bound are thick; epoxy is thin; porcelain is a fired body. Dimensions are published ranges, not our measured figures.substratePrecast20 mmsubstrateCement-based12 mmsubstrateEpoxy5 mmsubstratePoured10 mmsubstrateLiquid3 mmsubstratePorcelain9 mmSections drawn to one scale from published ranges. Our measured figures are listed separately.
The six processes drawn to one scale. Layer thicknesses are published ranges.

The figures, and where each one comes from

IIC What is heard in the room below
about 25 footsteps clearly audible, as if in the same room
about 50 the minimum most codes require
about 55 normal walking inaudible; research finds complaints fall sharply here
60 and above only extreme impacts perceptible
Term Standard Direction
IIC, impact insulation ASTM E989 rated, E492 tested higher is better
STC, airborne sound ASTM E413 rated, E90 tested higher is better
Ln,w, the European equivalent ISO 717-2 ⛔ lower is better

Why the processes are not interchangeable

The reason terrazzo cannot solve this by itself is stated plainly in the acoustic literature: a concrete floor covered directly with a non-resilient material such as tile or stone has a low IIC because there is no give in the system. Terrazzo is exactly that — a hard, dense, ground surface bonded to a slab — so the rating belongs to what is under and over it, not to the finish. That is why the isolation has to be designed before the terrazzo arrives: a floating screed or resilient layer between the slab and the finish, and a decoupled ceiling below. The sand-cushion terrazzo system is the one terrazzo assembly that arrives with an isolation sheet already in it, which is why a specification that has no acoustic design in it will not acquire one when the terrazzo arrives.

What these figures are, and what they are not

IIC is defined by ASTM E989 and measured to ASTM E492; STC by ASTM E413 and E90; the European equivalent is ISO 717-2. The rating scale and the statement that a hard floor on a slab has a low IIC come from the acoustic engineering literature. ⛔ NO IIC VALUE FOR TERRAZZO IS QUOTED HERE. The figure that circulates for a terrazzo floor is one manufacturer's measured value for one product, which is a product data point and not an industry figure, and ⛔ it is not reproduced. ⛔ The numbers in the first column describe the scale, not a specification for any floor; we have not tested the acoustic performance of a floor we supplied.

Measured data sheets and test reports

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