When designing a building to mitigate external noise, which property of a material is most critical for reducing the transmission of airborne noise?
- AHigh Porosity
- BLow Density
- CHigh Surface MassCorrect
- DHigh Thermal Conductivity
Solution
Concept
Acoustic mass law for airborne transmission
Formula
TL = 20 × log10(m) + 20 × log10(f) - 47
Step-by-step solution
1. Airborne sound transmission through a homogenous partition causes the partition to vibrate and radiate sound into the receiving space. 2. According to the acoustic Mass Law, sound transmission loss increases with partition inertia. 3. Greater surface mass (kg/m2) increases inertial resistance to vibration, transmitting significantly less sound energy. 4. Therefore, high surface mass is the most critical material property for reducing airborne noise transmission. Answer: C
Explanation
According to the acoustic Mass Law, the airborne sound transmission loss of a single-leaf partition is governed primarily by its surface mass density. Doubling the surface mass per unit area increases the barrier transmission loss by roughly 6 dB across most frequency bands, making heavy, dense materials superior at blocking airborne sound waves.
Why the other options are wrong
A) High porosity provides sound absorption on room surfaces but allows airborne sound transmission through porous paths. B) Low density reduces wall mass and severely degrades airborne sound insulation. D) High thermal conductivity has negligible direct impact on acoustic transmission loss.
Shortcut
Mass Law: Heavier partition (high surface mass) = Higher transmission loss (6 dB per doubling of mass).
Common mistake
Selecting high porosity, which absorbs sound reflections inside a room but allows airborne noise to pass through.
Correct answer: C) High Surface Mass