Audible sound, reflected-sound defects, insulation

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.