TLDR
Impact sound is noise generated when an object physically strikes a building surface — footsteps on a floor, a chair dragging across tiles, a ball bouncing in a gymnasium above, or a door slamming against its frame. The impact excites bending waves in the structural element, which radiate as audible sound into adjacent spaces, primarily the room below. Impact sound is measured using a standardised tapping machine per ISO 10140-3:2021 (laboratory) and ISO 16283-2:2020 (field), and rated as IIC (Impact Insulation Class) per ASTM E989 in North America or Ln,w (Weighted Normalised Impact Sound Pressure Level) per ISO 717-2:2020 internationally. Unlike airborne sound insulation where higher numbers are better, for Ln,w lower numbers are better — less sound transmitted means less impact noise received. Floating floors, resilient underlays, and soft floor coverings are the primary control strategies.
Real-World Analogy
You live in an apartment below an upstairs neighbour who walks around in hard-soled shoes on a hardwood floor. Every footstep arrives in your living room as a dull thud — not through the air between your apartments (the floor is solid), but because the shoe hitting the floor sends vibration through the floor slab directly into your ceiling. If the neighbour switches to soft slippers or if a thick rubber mat is placed under the hardwood, the thuds diminish dramatically. The vibration energy is absorbed by the soft material before it reaches the structural slab. This is impact sound in its most commonly experienced form — and its most commonly complained-about form.
Technical Definition
Impact sound is a subcategory of structure-borne sound where the excitation mechanism is a transient mechanical force applied to a surface. The standardised measurement procedure uses a tapping machine — a device with five 500 g hammers that fall from a height of 40 mm at a rate of 10 impacts per second, generating a repeatable, standardised impact force.
Laboratory Measurement (ISO 10140-3)
In a laboratory with controlled flanking, the tapping machine is placed on the test floor specimen. The normalised impact sound pressure level is measured in the room below:
Ln = L_i + 10 log₁₀(A/A₀) dB
where L_i is the measured sound pressure level, A is the equivalent absorption area of the receiving room, and A₀ = 10 m² is the reference absorption area. Ln is measured at each one-third octave band from 100 Hz to 5000 Hz.
Single-Number Ratings
- Ln,w (Weighted Normalised Impact Sound Pressure Level) per ISO 717-2:2020: The international metric. Lower values mean better insulation. A bare 150 mm concrete slab might rate Ln,w = 78 dB. Adding a floating screed with 25 mm resilient layer can reduce this to Ln,w = 48 dB.
- IIC (Impact Insulation Class) per ASTM E989: The North American equivalent. Higher values mean better insulation. The relationship is approximately IIC ≈ 110 − Ln,w.
Improvement Rating (ΔLw)
When a floor covering or floating floor is added to a reference slab, the improvement is expressed as ΔLw per ISO 717-2 — the number of decibels by which the impact sound level is reduced. A 10 mm foam underlay might provide ΔLw = 18 dB; a 25 mm mineral wool resilient layer under a floating screed might provide ΔLw = 30 dB.
Low-Frequency Performance
Standard Ln,w evaluation starts at 100 Hz, but footstep noise from adult walking is dominated by frequencies below 100 Hz. The spectrum adaptation term Ci (ISO 717-2) extends evaluation down to 50 Hz. Some Nordic standards (e.g., SS 25267) already require assessment including Ci to capture low-frequency footfall that heavy walkers generate.
Why It Matters for Design
- Multi-storey residential: Impact noise from upstairs neighbours is the single most common acoustic complaint in apartment buildings worldwide. Building codes mandate minimum IIC or maximum Ln,w values for floor-ceiling assemblies between dwellings — typically IIC 50 (IBC 2021 Section 1207) or Ln,w ≤ 58 dB (Approved Document E, UK).
- Floor finish selection: Replacing carpet with hardwood or tile on an existing floor can degrade impact insulation by 15-25 dB. Many strata/condo regulations require impact testing or approved floor coverings for this reason.
- Floating floor systems: The most effective impact noise control is a floating floor — a wearing surface decoupled from the structural slab by a continuous resilient layer. The resilient layer can be foam, rubber, cork, or mineral wool, and its dynamic stiffness (s' in MN/m³) determines the improvement. Lower dynamic stiffness = better impact isolation, but structural load capacity must be maintained.
- Gyms and dance studios: Impact loads from exercise equipment, dance, and ball sports are far more severe than standard tapping machine levels. Sprung floors with discrete resilient bearings are required to prevent impact noise from disrupting spaces below.
- Hospitals and hotels: Guest comfort and patient sleep depend on controlling footfall, trolley wheels, and equipment impacts from corridors and service areas above. Floating corridors with resilient vinyl are common solutions.
How AcousPlan Uses This
AcousPlan's sound insulation calculator includes both airborne and impact ratings for floor-ceiling assemblies in its 52+ assembly library. When you select a floor construction, the engine displays both Rw/STC (airborne) and Ln,w/IIC (impact), and checks both against your selected building code. The improvement calculator lets you layer floor coverings and resilient underlays onto a bare slab and see the predicted ΔLw improvement. The building code compliance report flags whether your assembly meets impact requirements for the specific adjacency — stricter for bedroom-below-corridor than for office-below-office.
Related Concepts
- What is Impact Insulation Class (IIC)? — Deep dive into the IIC rating methodology
- What is Structure-Borne Sound? — The broader category that impact sound belongs to
- What is Airborne Sound? — The complementary transmission mechanism through air
- What is Flanking Transmission? — Indirect paths that limit impact insulation in the field
- What is Sound Insulation Testing? — Field verification of impact insulation performance
Calculate Now
Check your floor assemblies against IIC and Ln,w requirements in AcousPlan — select your slab type, add resilient layers and floor finishes, and see whether the predicted impact performance meets your building code target.