Amogh N P
✦ In loving memory of Amogh N P — Architect · Designer · Visionary ✦
A recording-studio control room with thick wall treatment, corner bass traps and a double-glazed isolation window — insulation, not just absorption.
Unit IVAcoustics in Architecture

Noise Control & Insulation

Keeping unwanted sound out — and the absorption-vs-insulation trap.

≈ 40 min + studio task

Reverberation is sound inside a room; noise is sound from elsewhere, and stopping it is a different craft. Noise is airborne or structure-borne, and it is blocked by insulation — transmission loss — which the mass law says rises ~6 dB per doubling of mass. Crucially, absorption is not insulation: foam tames echo inside but does nothing to block sound between rooms. Learn the STC, the NRC and the CPCB limits.

Learning objectives

By the end of this lesson, you will be able to — mapped to the course outcomes for Acoustics in Architecture:

1
CO4 · Understand

Distinguish airborne and structure-borne noise.

2
CO4 · Apply

Apply sound insulation, transmission loss and the mass law.

3
CO4 · Understand

Distinguish absorption (NRC) from insulation (STC).

4
CO6 · Apply

Apply source-path-receiver control and the CPCB noise standards.

Block it with mass

Insulation & the mass law

Airborne noise is blocked by transmission loss, which rises ~6 dB per doubling of mass — and absorption is NOT insulation.[2, 5]

Airborne vs structure-borne noise airborne → through the partition impact → through the structure
DiagramTwo paths of noise — airborne through the partition, and structure-borne impact through the floor and fabric

Block it with mass

NOISE travels two ways: AIRBORNE (through the air — speech, traffic) and STRUCTURE-BORNE / IMPACT (vibration through the fabric — footfall, machinery). Airborne noise is blocked by INSULATION, measured as TRANSMISSION LOSS (TL). The MASS LAW says a single partition's TL rises about 6 dB for every doubling of its mass per unit area (and per doubling of frequency) — so a heavy wall blocks far better than a light one. Double-leaf cavity walls beat the mass law.[2, 5]

Mass law — heavier blocks better light wall — much passes little TL heavy wall — little passes high TL +6 dB transmission loss per doubling of mass
DiagramThe mass law — a heavier wall blocks more sound, +6 dB transmission loss per doubling of mass
Absorption ≠ insulation ABSORPTION — tames echo INSIDE INSULATION — mass BLOCKS between foam absorbs but does NOT block — soundproofing needs mass + sealing + isolation
DiagramAbsorption tames reverberation inside a room; insulation with mass blocks sound between rooms
STC, NRC, CPCB

Ratings & the law

STC rates insulation (between rooms), NRC rates absorption (inside); control noise source-path-receiver, and meet the CPCB limits.[2, 6]

The two single numbers

Two single-number ratings recur. STC (Sound Transmission Class) rates a partition's airborne INSULATION — roughly: a stud partition ~33–39, a single brick wall ~45, masonry ~48–55, a decoupled double wall 60+. STC 25 lets normal speech through intelligibly; 50+ gives good privacy. NRC (Noise Reduction Coefficient) rates a material's ABSORPTION — the average α at 250–2000 Hz. FLAG: STC is for blocking (between rooms), NRC for absorbing (inside) — don't confuse them.[2, 5]

The noise-control facts

At a glance

AspectOneThe other
Two kinds of noiseAirborne: through the airStructure-borne: through the fabric
Block vs tameInsulation (TL/STC): blocks between roomsAbsorption (α/NRC): tames inside a room
How to blockMass law: ~6 dB per doubling of massCavity / double-leaf beats the mass law
Soundproofing mythFoam/egg-cartons only absorbNeed mass + sealing + isolation to block
CPCB day/nightResidential 55/45 · commercial 65/55Industrial 75/70 · silence zone 50/40
Vocabulary

Key terms

Airborne noise

Noise travelling through the air — speech, traffic, music.

Structure-borne noise

Vibration through the building fabric — footfall, machinery; needs isolation.

Transmission loss (TL)

The reduction in sound level (dB) across a partition.

Mass law

A single partition's TL rises ~6 dB per doubling of its mass per unit area.

STC

Sound Transmission Class — a single-number airborne insulation rating (between rooms).

NRC

Noise Reduction Coefficient — average absorption (α) at 250–2000 Hz (inside a room).

Absorption ≠ insulation

Absorption tames reverberation inside; insulation blocks sound between rooms — different physics.

CPCB noise limits

India's day/night ambient limits — residential 55/45, commercial 65/55, industrial 75/70 dBA.

Apply it

Studio task

For a bedroom beside a noisy road, design the wall and window to block the noise — name the mass, the cavity or double glazing, and the seals — and state the CPCB night limit you must meet. Note why acoustic foam alone would fail.

Check your understanding

Self-assessment

1. Sticking acoustic foam on a wall to stop your neighbour's TV will —

2. By the mass law, doubling the mass per unit area of a single wall raises its transmission loss by about —

3. The CPCB night-time ambient noise limit for a residential area is —

In a nutshell

Recap

Noise is airborne or structure-borne; airborne noise is blocked by insulation (transmission loss), which rises ~6 dB per doubling of mass (the mass law).
Absorption is NOT insulation — foam tames reverberation inside a room but does nothing to block sound between rooms; blocking needs mass, sealing and isolation.
STC rates insulation (between rooms), NRC rates absorption (inside) — don't confuse the two single numbers.
Control noise source-path-receiver, isolate impact noise, and meet the CPCB limits (residential 55/45, commercial 65/55, industrial 75/70 dBA).
The evidence

References & further reading

  1. [2]M. David Egan, Architectural Acoustics. McGraw-Hill / J. Ross Publishing.
  2. [5]National Building Code of India 2016, Part 8, Section 4 — Acoustics, Sound Insulation and Noise Control. BIS.
  3. [6]CPCB — Noise Pollution (Regulation and Control) Rules, 2000. Central Pollution Control Board. https://cpcb.nic.in/noise-pollution-rules/

Further reading

  • Cyril Harris, Handbook of Acoustical Measurements and Noise Control. McGraw-Hill.
  • M. David Egan, Architectural Acoustics. J. Ross Publishing.
  • NBC 2016 Part 8 §4; CPCB Noise Rules 2000.

Sources gathered and fact-checked June 2026. Published values vary by source, sample and method — treat as indicative and confirm against the cited standard before structural use.