Impact Noise Floor Build-Up: ΔLw Improvement Calculation — ISO 717-2

Impact noise between floors is one of the most common complaints in multi-storey residential buildings. Unlike airborne noise, impact sound is introduced directly into the structure by footsteps, chair scraping, and objects dropped on the floor above. Standard walls and partitions provide no protection because the sound bypasses the air entirely. This article calculates the impact sound level step by step as floor treatments are added to a concrete slab, using the ISO 717-2 method.

Starting Point: Bare 150 mm Concrete Slab

Structural slab: 150 mm reinforced concrete, no finish, no floating layer.

Field measurement (or standard estimate): L'nT,w = 78 dB

This is typical for a bare concrete slab. The ISO 717-2 weighted impact sound level (lower is better):

Standardised impact sound level spectrum (Ln at 1/3-octave bands, Hz):

For this calculation we also work with the six octave-band values for clarity:

L'nT,w of bare slab = 78 dB (ISO 717-2 contour fitting from the 1/3-octave values above).

For a UK Part E comparison: residential separating floor limit is L'nT,w ≤ 62 dB. The bare slab fails by 16 dB. Standard practice requires treatment to reach compliance.

Treatment Step 1: Resilient Mat (5 mm Rubber Crumb)

Description: 5 mm recycled rubber crumb mat, dynamic stiffness s' ≈ 50 MN/m³, laid directly on the slab.

Manufacturer ΔLw data (ISO 717-2): ΔLw = 18 dB

ΔLw spectrum (improvement per octave band):

Note: rubber mats provide little low-frequency improvement (only 5 dB at 125 Hz) because the stiff rubber transmits structure-borne vibration efficiently at low frequencies. The benefit is primarily at mid and high frequencies.

Residual impact level after mat: L_n,step1 = L_n,bare − ΔL (per band):

Estimated L'nT,w after Step 1: Fit ISO 717-2 contour to new spectrum → approximately 65 dB (still 3 dB above the UK Part E limit).

Treatment Step 2: Add 65 mm Floating Screed on Resilient Mat

Description: 65 mm sand:cement screed poured over the resilient mat (separated from walls and all penetrations by resilient edge strip). Total floor buildup above slab: 5 mm mat + 65 mm screed = 70 mm.

Mass of screed: 65 mm × 2100 kg/m³ = 136.5 kg/m²

Resonance frequency of floating floor system: f_r = (1/2π) × √(s' / m)

Where s' is the dynamic stiffness of the resilient layer (N/m³) and m is the mass per unit area (kg/m²):

s' = 50 MN/m³ = 50 × 10⁶ N/m³ m = 136.5 kg/m²

f_r = (1/2π) × √(50 × 10⁶ / 136.5) = (1/2π) × √(366,300) = (1/2π) × 605.2 = 96.3 Hz

The resonance frequency of 96 Hz is just below the 125 Hz octave band. This means the floating floor is effectively isolating from 125 Hz upwards. Below f_r, the system can amplify impact noise.

Additional improvement from floating screed (above the mat alone):

At f > f_r: additional improvement ≈ 40 × log₁₀(f/f_r) dB (mass-spring system above resonance)

The theoretical improvement above resonance grows without bound; practical limits apply due to flanking, stiffness variability, and structural connections. Cap at 35–40 dB above 1 kHz.

Cumulative ΔL (mat + screed) and residual level:

Estimated L'nT,w after Step 2: Fit ISO 717-2 contour → approximately 47 dB (well below UK Part E limit of 62 dB).

The primary improvement from the floating screed is at 250 Hz and above. At 125 Hz, the improvement is small (5 dB from the screed on top of 5 dB from the mat = 10 dB total). Low-frequency footstep impact is still the limiting factor.

Treatment Step 3: Add 8 mm Carpet Underlay + Carpet (Soft Floor Covering)

Description: 8 mm foam underlay + 10 mm loop-pile carpet. Total buildup: 5 mm mat + 65 mm screed + 8 mm underlay + 10 mm carpet = 88 mm above slab.

ΔLw of carpet + underlay: ΔLw = 28 dB (this is measured on a reference concrete floor — it is the carpet's independent improvement).

However, carpet on a floating screed does not stack additively with the screed improvement in a simple linear way — the combined system must be considered. A conservative approach: at high frequencies, the carpet is dominant; at low frequencies, the carpet provides minimal improvement to impact.

Carpet ΔL per octave band:

Residual level after all three treatments:

Estimated L'nT,w after Step 3: The ISO 717-2 contour fitting at this point is controlled entirely by the 125 Hz value of 61 dB. Contour fit → approximately 40 dB.

Four Build-Up Scenarios Compared

IIC conversion: IIC ≈ 110 − L'nT,w (approximate relationship)

• L'nT,w 78 → IIC = 110 − 78 = 32 (approximately, for typical spectrum shape)
• L'nT,w 65 → IIC = 45
• L'nT,w 47 → IIC = 63
• L'nT,w 40 → IIC = 70

The Low-Frequency Problem

All three treatment steps produce diminishing returns at 125 Hz. After all treatments, 125 Hz residual level is 61 dB — still the governing constraint. Low-frequency impact noise from heavy footsteps and dropped objects at 80–125 Hz is very difficult to control because:

1. The resonance frequency of the floating floor (96 Hz) is inside the problem frequency range
2. Soft floor coverings provide minimal improvement at 125 Hz
3. Flanking through walls and columns transmits 125 Hz vibration around the floor system

Softer mat option: s' = 10 MN/m³ (mineral wool resilient layer, 25 mm thick)

f_r = (1/2π) × √(10 × 10⁶ / 136.5) = (1/2π) × 270.7 = 43 Hz

With f_r at 43 Hz, the floating floor is above resonance at 125 Hz by a factor of 125/43 = 2.91:

Additional improvement at 125 Hz = 40 × log₁₀(2.91) = 40 × 0.464 = 18.6 dB

Versus only 5 dB for the stiff rubber mat. The softer mineral wool layer reduces L'nT,w to approximately 35 dB — excellent performance, well below any regulatory target.

The trade-off: mineral wool resilient layers are thicker (25 mm vs 5 mm) and more expensive, and the total floor build-up increases to 100 mm, which has implications for structural and architectural coordination.

Use AcousPlan's Sound Insulation Calculator to calculate the L'nT,w for your specific slab thickness, resilient layer, and floor covering combination, including the resonance frequency prediction.