---
title: "Audible sound, reflected-sound defects, insulation"
subject: "Building Technology"
chapter: 1
key: "semester_6/building_technology:sound-audible-insulation"
source: https://www.bctengineer.com/q/semester_6/building_technology/sound-audible-insulation
site: BCT Engineering
---

# Audible sound, reflected-sound defects, insulation

Building Technology · Chapter 1: Functional Requirements of Buildings

- Asked in: 2068 Baisakh (old course), 8 marks
- Video: [Acoustics – Identifying issues and designing solutions](https://www.youtube.com/watch?v=Y583kT-vJfw) — UQ Architecture, Design and Planning
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/building_technology/sound-audible-insulation
- Whole chapter: https://www.bctengineer.com/md/semester_6/building_technology/chapter-1

## Question

What are the characteristics of audible sound? What are defects occurred due to reflected sound? Explain briefly the different methods of sound insulation.

## Short answer

Audible sound has frequency (20 Hz–20 kHz), intensity (dB), timbre, velocity (~343 m/s), and wavelength. Reflected sound causes echo, excessive reverberation, sound foci, dead spots, echelon effect, and resonance. Insulation methods include heavy dense walls, cavity partitions, floating floors, resilient finishes, sealing gaps, and vibration isolation.

## Answer

#### Characteristics of audible sound
1. **Frequency / pitch**: Audible range is about 20 Hz to 20,000 Hz; speech is mostly 250 to 4000 Hz.
2. **Intensity / loudness**: Measured in decibels (dB), from 0 dB (threshold of hearing) to about 120 dB (pain). $L = 10\log_{10}(I/I_0)$ with $I_0 = 10^{-12}$ W/m².
3. **Quality (timbre)**: The tone that tells apart two sounds of the same pitch and loudness; it depends on harmonics.
4. **Velocity**: About 343 m/s in air at 20 °C.
5. **Wavelength**: $\lambda = v/f$.
6. Sound is reflected, absorbed, transmitted and diffracted at surfaces.

#### Defects due to reflected sound
Echo, excessive reverberation, sound foci (from concave surfaces), dead spots, echelon effect, and resonance.

#### Methods of sound insulation
1. **Site planning**: Set the building back from noisy roads, use trees and earth mounds as barriers, and place quiet rooms away from noise.
2. **Heavy and dense construction**: Thick walls and slabs; mass law.
3. **Cavity walls and double partitions** with absorbent filling.
4. **Floating floors, resilient finishes** and suspended ceilings for impact sound.
5. **Sealing** cracks, gaps and service holes, and acoustic doors and windows with weather strips; double glazing.
6. **Vibration isolation**: Flexible mounts for machines and pipes.
7. **Absorbent surfaces** inside the room to reduce noise level.

## Check yourself

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

1. Audible frequency range is about:
   A. 2 to 200 Hz
   B. 20 Hz to 20,000 Hz
   C. 200 kHz
   D. 1 to 10 Hz
   Answer: B. Human hearing is 20 to 20,000 Hz. (Key point: Audible range)

2. Sound level in dB is:
   A. 10 log10(I/I0), I0 = 10^-12 W/m^2
   B. 20 log10(I)
   C. I/I0
   D. 10 log10(I0/I)
   Answer: A. L = 10 log10(I/I0). (Key point: Loudness)

3. Quality (timbre) tells apart:
   A. Two sounds of same pitch and loudness, through harmonics
   B. Two speeds
   C. Two sources of different pitch
   D. Echo
   Answer: A. It depends on harmonics. (Key point: Timbre)

4. A site planning measure for noise is:
   A. Build next to the road
   B. Set building back from noisy roads and use trees or mounds
   C. Remove trees
   D. Face noise source
   Answer: B. Distance and barriers reduce noise. (Key point: Site planning)

## 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
- [Short note: Sound insulation](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Ashort-sound-insulation) — Building Technology, chapter 1
- [Damp proofing and defects due to moisture](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Adamp-proofing-defects) — Building Technology, chapter 1
- [Sound insulating material, barrier systems](https://www.bctengineer.com/question.md?key=semester_6%2Fbuilding_technology%3Asound-insul-poor-absorber) — Building Technology, chapter 1

## Related in other subjects

- [Thin film interference by reflected light](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Athin-film-reflected-interference-theory) — Engineering Physics, chapter 4, asked 2×
- [Audible noise of 750 kV line (20 kV/cm gradient)](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aan-750kv-20m-outer) — High Voltage Engineering, chapter 5, asked 3×
- [Audible noise of 750 kV line (formula with Di)](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aan-750kv-formula-di) — High Voltage Engineering, chapter 5, asked 3×
- [Nature of sound and the human ear](https://www.bctengineer.com/question.md?key=semester_7%2Fenvironment_and_pollution_control%3Asound-nature-human-ear) — Environment and Pollution Control, chapter 6