---
title: "Sound absorption and noise control"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:absorptive-materials-noise-control"
source: https://www.bctengineer.com/q/semester_6/building_technology/absorptive-materials-noise-control
site: BCT Engineering
---

# Sound absorption and noise control

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2067 Asar (old course), 4+4 marks
- Video: [14.1 Noise Evaluation and Sound Absorption and Noise Reduction](https://www.youtube.com/watch?v=fU0OmY1nfeg) — SUT Building Physics with Prof. Lv
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/absorptive-materials-noise-control
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

How do absorptive materials absorb sound? Write down the general considerations for noise control in buildings.

## Short answer

Porous materials like mineral wool convert sound to heat via air friction in pores; panel absorbers vibrate and damp low frequencies; cavity resonators absorb narrow bands. Noise control considerations: site planning with buffers, separating noisy and quiet zones, heavy insulation, interior absorption, source silencing, and isolating services.

## Answer

#### How absorptive materials absorb sound
Sound absorbers are porous, soft or flexible. Absorption works by three mechanisms:
1. **Porous absorption**: Sound enters the connected pores of materials such as mineral wool, glass wool, acoustic tile, carpets and curtains. The air in the pores moves, and friction with the fibres changes the sound energy into a little heat.
2. **Panel (membrane) absorption**: Thin panels of plywood or hardboard over an air space vibrate when sound strikes them, and absorb mostly low frequencies by internal damping.
3. **Cavity (Helmholtz) resonators**: Perforated boards with an air space behind; air in the holes resonates and loses energy, mostly in a narrow band.

The absorption coefficient $\alpha$ is the fraction of incident sound absorbed (0 for a perfect reflector, 1 for an open window).

#### General considerations for noise control in buildings
1. **Site selection and planning**: Set back from highways, railways and airports. Put noisy zones away from quiet rooms. Use trees, fences and mounds as buffers.
2. **Building layout**: Group noisy and quiet spaces separately; a service core between.
3. **Insulation**: Heavy walls, floors, double glazing, sealed doors.
4. **Absorption** inside rooms using acoustic ceilings and carpets.
5. **Control at the source**: Silent machines, vibration mounts and enclosures for generators and pumps.
6. **Services**: Isolate ducts, pipes; lined ducts, silencers.
7. **Legal regulation**: Noise limits and horn restrictions.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. In porous absorbers, sound energy is lost by:
   A. Reflection
   B. Friction of moving air in pores with fibres, turning it to heat
   C. Resonance of panels
   D. Diffraction
   Answer: B. Air in pores moves and friction produces heat. (Key point: Porous absorption)

2. Panel (membrane) absorbers mostly absorb:
   A. High frequencies
   B. Low frequencies
   C. All equally
   D. None
   Answer: B. Vibrating panels damp low frequencies. (Key point: Panel absorbers)

3. Cavity (Helmholtz) resonators absorb:
   A. A narrow band of frequencies
   B. All frequencies
   C. Only highs
   D. Heat
   Answer: A. Resonating air in holes absorbs a narrow band. (Key point: Helmholtz)

4. Absorption coefficient of an open window is:
   A. 0
   B. 0.5
   C. 1
   D. 10
   Answer: C. An open window absorbs everything: 1. (Key point: Absorption coefficient)

## Related in Building Technology

- [Sound insulation versus sound absorption](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asound-insul-vs-absorption) — Building Technology, chapter 1
- [Floor insulation and sound insulation methods](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Afloor-insulation-sound) — Building Technology, chapter 1

## Related in other subjects

- [Sound insulating materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Asound-insulating-materials) — Civil Engineering Materials, chapter 10, asked 2×
- [Noise control methods](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Anoise-control-methods) — Environment and Pollution Control, chapter 6
- [General energy equation for control volume](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Ageneral-energy-equation-control-volume) — Fundamental of Thermodynamics & Heat Transfer, chapter 4
- [General block diagram of a control system](https://www.bctengineer.com/question.md?key=semester_3%2Fcontrol_systems%3Ageneral-bd-control-system) — Control Systems, chapter 1