The Floor Beneath the Footsteps

Footsteps, a dropped weight in a hotel gym, a chair dragged across an apartment floor: impact noise does not travel through the air between rooms, it travels through the structure itself. The slab that carries the building also carries the sound, which is why a floor that looks solid can still be the loudest element in the room below.

Impact noise has its own rating

Impact sound is measured differently from airborne sound. In the laboratory, ASTM E492 places a standard tapping machine — a box of five hammers that fall independently and repeatedly — on the floor of the upper room at prescribed positions and measures the sound arriving in the room below. The result is converted to a single-number Impact Insulation Class (IIC) using ASTM E989, where a higher IIC means better performance. ASTM E1007 is the field version of the same procedure, used on real floors in finished buildings.

The International Building Code gives a sense of the benchmark. Section 1206.3 of the 2021 IBC requires floor-ceiling assemblies between dwelling or sleeping units, or between those units and public or service areas, to achieve an IIC of not less than 50 when tested to ASTM E492, or a Normalized Impact Sound Rating of not less than 45 if field tested to ASTM E1007.

Mass on a spring

A floating floor works by resting a heavy upper layer — typically a concrete or board topping — on a resilient layer or isolators, so the finished floor is separated from the structural slab by a spring. The physics of that spring cuts both ways. Vibration isolation theory shows that transmissibility only drops below 1 once the forcing frequency exceeds roughly 1.4 times (the square root of two) the system's natural frequency. Below that crossover lies the amplification region, where the floated layer can transmit more vibration than a bare slab; above it lies the isolation region. This is why the natural frequency of a floating floor has to be designed deliberately, well below the frequencies it needs to stop.

Where the laboratory and the building disagree

Real buildings add complications a single spring model misses. A 2013 study in the Journal of Sound and Vibration compared laboratory and field measurements and found negligible differences for bare concrete slabs but notable differences once floating floors were installed, concluding that low-frequency impact sound depends on in-situ matched resonances that conventional single-degree-of-freedom models do not capture. A 2026 field case reported by Noise News International found a similar lesson from a different cause: where a concrete topping was poured over the perimeter isolation and the air between isolators was sealed in, the trapped air added its own stiffness and raised the system's natural frequency. Only when that air stiffness and the structural slab's own response were included in a two-degree-of-freedom analysis did predictions match what was measured.

Why the details decide the result

Both findings point to the same conclusion: a floating floor is a system, not a product laid on a slab. The resilient layer, the topping mass, the venting of the air cavity, the perimeter isolation at every wall and the stiffness of the structure beneath all shape the natural frequency — and the natural frequency decides whether the floor isolates or amplifies.

Understanding soundproofing cost can help you plan an effective solution for reducing unwanted noise.

The acouStick approach

acouStick supplies floating floor build-ups from 30 to 100 mm for apartments, hotels, cinemas and gyms. Each is selected from the loads the floor must carry, the impact sources it must stop and the structure it sits on, with perimeter isolation treated as part of the system rather than an afterthought.

Learning what is soundproofing is a useful first step toward reducing unwanted noise.

If your project places apartments, hotel rooms, a gym or a cinema above occupied space, acouStick can help you select a floating floor build-up before the topping is poured.

Email: info@acoustick.co | Call: +966 51 130 809 | Web: www.acoustick.co

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REFERENCES

• International Code Council. 2021 International Building Code, Section 1206.3 "Structure-Borne Sound" — as quoted in Allen, D. "Acoustical Requirements for Ceilings and Floor-Ceiling Assemblies." Association of the Wall and Ceiling Industry (AWCI), July 2023: awci.org

• ASTM International. ASTM E492 (laboratory impact sound), ASTM E989 (IIC classification) and ASTM E1007 (field impact sound) — as described in "ASTM E492 – Impact Noise Measurement for Floor-Ceiling Assemblies in the Lab," Noise News International: noisenewsinternational.net

• Masterson, P.A. "The Basics of Vibration Isolation Using Elastomeric Materials." E-A-R Specialty Composites — transmissibility, crossover frequency at √2 and the isolation region: vibrationdata.com/tutorials_alt/vib_iso.pdf

• Cho, T. "Experimental and numerical analysis of floating floor resonance and its effect on impact sound transmission." Journal of Sound and Vibration, Vol. 332, Issue 25, 2013 (ScienceDirect).

• Byrick, "Floating floor design in 2026." Noise News International, 2026 — field case on air stiffness, perimeter isolation and 2DOF analysis: noisenewsinternational.net/floating-floor-design-in-2026