Sound Transmission Class (STC) is a single-number rating of how well a wall blocks airborne sound. Higher STC means better isolation. STC 30 allows normal speech to be clearly heard; STC 45 makes loud speech barely audible; STC 60 blocks most sounds. The UK equivalent is Rw (weighted sound reduction index).

Why is the received level not just source level minus STC?

Noise reduction also depends on the area of the shared wall and the absorption in the receiving room. A larger wall transmits more sound, while a more absorbent receiving room reduces the build-up of transmitted sound. The formula accounts for both factors.

How can I improve noise isolation?

Add mass (extra plasterboard layers), create air gaps (double-stud walls), break vibration paths (resilient bars, acoustic hangers), seal all gaps (acoustic sealant around edges), and treat flanking paths (floors, ceilings, ducts). Each improvement adds to the overall STC rating.

AudioCalcs

Noise Reduction Calculator — Sound Isolation Between Rooms

Calculate noise reduction between rooms based on wall construction, area and room volume.

Source Noise Level

Sound level in the source room

Wall TypeConstruction of the separating wall (sets its STC/Rw rating)

Shared Wall Area

Surface area of the partition shared between the two rooms (m²)

Receiving Room Volume

Internal air volume of the receiving room (m³)

How We Calculate This

NR = TL + 10 × log10(A / S)

Where NR is noise reduction in dB, TL is the transmission loss (approximated by STC/Rw), A is the absorption area in the receiving room (sabins), and S is the area of the separating wall.

The absorption A is estimated from the receiving room volume assuming an RT60 of 0.5 seconds: A = 0.161 × V / RT60.

This is an idealised, best-case estimate. It uses the laboratory STC/Rw rating as the field transmission loss and ignores flanking transmission (sound travelling via floors, ceilings, ducts and shared structure) plus any gaps or poor seals. Real measured noise reduction is typically several dB lower, so treat the result as an upper bound rather than a guaranteed figure.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard formulas. Always verify results for critical applications.