---
title: "Sound insulating material, barrier systems"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:sound-insul-poor-absorber"
source: https://www.bctengineer.com/q/semester_6/building_technology/sound-insul-poor-absorber
site: BCT Engineering
---

# Sound insulating material, barrier systems

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2079 Asoj, 6 marks
- Video: [Sound Insulation | Soundproofing | Acoustics | Materials | Ideas |](https://www.youtube.com/watch?v=8TkhIG7OLXs) — NIVIP Information
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/sound-insul-poor-absorber
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Can a good sound insulating material be a poor sound absorber? Explain. Describe various systems for sound insulation including detail on vertical and horizontal barrier system.

## Short answer

Yes, a dense concrete wall is a good insulator due to mass and airtightness but a poor absorber because its hard surface reflects sound. Vertical insulation uses heavy walls, cavity walls, and sealed gaps; horizontal insulation uses floating floors, resilient finishes, and suspended ceilings to reduce impact and airborne sound.

## Answer

#### Can a good sound insulator be a poor absorber?
Yes. Sound insulation depends on the mass and airtightness of a material, which reflect sound. Sound absorption depends on porosity, which lets sound enter and lose energy. A dense concrete or brick wall is an excellent insulator (high transmission loss), but its hard, smooth surface reflects almost all sound back into the room, so its absorption coefficient is very low (about 0.02 to 0.05). On the other hand, a mineral wool blanket absorbs well but, being light and porous, insulates poorly. So the two properties are not the same and may even be opposite.

#### Systems of sound insulation
Sound travels as airborne sound (voice, TV) and as impact or structure-borne sound (footsteps). Insulation is provided by barriers.

**Vertical barrier systems (walls and partitions)**
1. Heavy, dense walls: thick brick or concrete (mass law: doubling the mass gives about 5 to 6 dB more).
2. Cavity (double) walls with a gap of 50 mm or more, no rigid ties, and the gap filled with sound-absorbing wool.
3. Staggered-stud or double-stud partitions with separate frames.
4. Plastering on both faces to seal pores and gaps; sealing of all cracks and service holes.
5. Acoustic doors, and double-glazed windows with panes of different thicknesses.

**Horizontal barrier systems (floors and ceilings)**
1. Heavy RCC slab (for airborne sound).
2. **Floating floor**: a screed and floor finish laid on a resilient layer (rubber, cork, mineral wool) over the slab, and separated from the walls by a gap or edge strip, so that it does not touch the structure.
3. Resilient floor finishes: carpet, rubber or cork tiles to reduce impact noise.
4. Suspended (false) ceiling hung on resilient hangers, with absorbent material above it.

```text
 Floating floor:
   finish
   screed
   resilient layer (wool)  <- edge strip at wall
   RCC slab
```

## Check yourself

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

1. Why can a dense concrete wall be a good insulator but a poor absorber?
   A. Its mass blocks transmission but its hard surface reflects sound back into the room
   B. It is porous
   C. It is light
   D. It has high absorption
   Answer: A. Mass blocks sound; the smooth surface reflects, so absorption is only 0.02 to 0.05. (Key point: Can insulator be poor absorber)

2. Doubling mass raises sound reduction by about:
   A. 1 dB
   B. 5 to 6 dB
   C. 20 dB
   D. 50 dB
   Answer: B. Mass law gives about 5 to 6 dB per doubling. (Key point: Mass law)

3. In a floating floor the finish rests on:
   A. Directly on slab
   B. A resilient layer, separated from walls by edge strip
   C. Water
   D. Steel plates
   Answer: B. The resilient layer decouples the finish from the slab. (Key point: Floating floor)

4. For a sound-insulating cavity wall:
   A. Use rigid ties
   B. Gap of 50 mm or more, no rigid ties, filled with absorbent wool
   C. No gap
   D. Gap with metal ties
   Answer: B. Rigid ties would carry sound across. (Key point: Cavity wall)

## Related in Building Technology

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

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