---
title: "Common acoustic defects"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:acoustic-defects"
source: https://www.bctengineer.com/q/semester_6/building_technology/acoustic-defects
site: BCT Engineering
---

# Common acoustic defects

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2078 Poush, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/acoustic-defects
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

Explain any four common acoustic defects.

## Short answer

Echo is a distinct delayed reflection cured by absorbent surfaces. Excessive reverberation blurs speech, remedied by adding absorbers. Concave surfaces create sound foci and dead spots, avoided by convex shapes or absorption. Resonance amplifies certain tones, damped by absorbent backing.

## Answer

Four common acoustic defects are as follows.

1. **Echo**: When a reflected sound reaches the listener more than about 0.1 second (some textbooks say 1/15 s) after the direct sound, it is heard as a separate repetition. It occurs in large halls with parallel hard walls far from the source. **Remedy**: cover the reflecting surface with absorbent materials such as acoustic plaster, curtains or panels; make the walls irregular.
2. **Reverberation**: The sound persists in the room after the source has stopped because of multiple reflections. Excess reverberation makes speech unclear and music muddy. Optimum reverberation time is about 0.5 to 1 s for speech halls. **Remedy**: raise the absorption (carpets, curtains, acoustic ceiling, upholstered seats). Reverberation time by Sabine is $T = 0.16V/A$.
3. **Sound foci and dead spots**: Concave surfaces, such as domes and curved walls, concentrate reflected sound at some points (foci) and leave other places with very low sound (dead spots). **Remedy**: avoid concave surfaces, or cover them with absorbing material; use convex diffusers.
4. **Resonance**: Some objects or air spaces vibrate at the same frequency as the sound, increasing certain tones. **Remedy**: damp the panels with absorbent backing, and avoid hollow spaces of resonant size.

Other defects are echelon effect, sound shadow, and external noise.

## Check yourself

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

1. A remedy for echo is:
   A. Make walls smooth
   B. Cover reflecting surfaces with absorbent materials
   C. Add concave dome
   D. Remove curtains
   Answer: B. Absorbers on the reflecting surface remove the repeat. (Key point: Echo remedy)

2. Sabine's reverberation time formula is:
   A. T = 0.16 V/A
   B. T = 0.16 A/V
   C. T = V/A^2
   D. T = 16 VA
   Answer: A. T = 0.16V/A with V volume and A total absorption. (Key point: Sabine)

3. Sound foci are caused by:
   A. Concave surfaces
   B. Convex surfaces
   C. Carpets
   D. Plane walls
   Answer: A. Concave surfaces concentrate reflected sound. (Key point: Foci)

4. Resonance occurs when:
   A. Sound is absorbed
   B. Objects or air spaces vibrate at the same frequency as the sound, boosting some tones
   C. Walls are parallel
   D. Sound is delayed
   Answer: B. Damping with absorbent backing reduces it. (Key point: Resonance)

## Related in Building Technology

- [Acoustics, acoustic defects and remedies](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Aacoustics-defects-remedial) — Building Technology, chapter 1

## Related in other subjects

- [Cast structure and solidification defects](https://www.bctengineer.com/question.md?key=semester_3%2Fmaterial_science%3Acast-structure-and-solidification-defects) — Material Science, chapter 5
- [Defects caused by seasoning](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aseasoning-defects-timber) — Civil Engineering Materials, chapter 7
- [Defects in paints](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Adefects-in-paints) — Civil Engineering Materials, chapter 9
- [Solidification, shrinkage and casting defects](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Asolidification-shrinkage-casting-defects) — Manufacturing and Production Processes, chapter 4