Soundproofing Materials: Three Mainstream Acoustic Building Materials

1. Rockwool: Fibrous Structure and Airborne Sound Impedance

Rockwool is made from minerals such as basalt, melted at high temperatures, and spun into fibers. In acoustic physics, its internal interlaced fibrous structure and pores create frictional resistance to incoming sound waves, converting sound energy into minute heat energy, thereby reducing the transmission and reflection rates of sound waves^[1].

• Acoustic Data: Possesses a relatively high surface density, providing attenuation for blocking low-to-mid frequency noise (such as machinery operation and traffic sounds).
• Fire Safety Regulations: Due to its inorganic mineral composition, rockwool possesses non-combustible properties, complying with Hong Kong fire safety standards, and is widely used in wall interlayers of commercial and residential buildings.

In practical applications, such as the soundproofing project for the core building of the Hong Kong Polytechnic University, ENA Soundproof Cotton containing rockwool components was used within partition walls to reduce sound transmission between adjacent classrooms.

2. Acoustic Foam: Indoor Reverberation Control

Acoustic foam (or porous sound-absorbing material) is primarily used to alter the reflection paths of sound waves within a space, reducing indoor echoes and Reverberation Time, rather than blocking sound from penetrating walls.

• Sound Absorption Data: The Noise Reduction Coefficient (NRC) depends on the material's thickness and porosity. For example, the tested NRC value of ENA Soundproof Cotton is 0.3, suitable for wall application in meeting rooms and recording spaces^[2].
• Limitations: Based on physical properties, lightweight porous materials cannot effectively block low-frequency sound penetration. If thorough sound insulation is required, they must be paired with mass-loaded materials.

The International House at Hong Kong Baptist University applied porous acoustic cotton on meeting room walls, enhancing speech intelligibility within the space by reducing the reflection of indoor sound waves.

3. Mass Loaded Vinyl (MLV): Application of the Mass Law

In architectural soundproofing, according to the "Mass Law", the greater the mass per unit area of a material, the higher its impedance to airborne sound^[3]. Mass Loaded Vinyl is a high-density flexible building material developed based on this law.

ENA Soundproof Blanket NE200 is a representative MLV material, with a density reaching 2400 kg/m³ and a Sound Transmission Class (STC) of 23. Compared to bricklaying or grouting, MLV features a thin profile and flexible bendability, allowing it to be directly adhered to existing walls or floor slabs, or sandwiched between two layers of gypsum board. The material has passed SGS testing, complies with Hong Kong Indoor Air Quality regulations, and has been applied in the guest room soundproofing project of the Mandarin Oriental Hotel.

4. Selection of Acoustic Materials and Architectural Planning

When implementing indoor acoustic interventions, the choice of materials must correspond to the propagation paths and frequencies of the noise:

• Frequency Correspondence: For low-frequency and airborne sound (such as traffic noise or next-door TV sounds), the high-density ENA Soundproof Blanket NE200 should be selected; for indoor echoes and mid-to-high frequency sounds, ENA Soundproof Cotton is suitable.
• Treatment of Openings: Residential windows and entrances are soundproofing weak points. Sound diffraction can be cut off by configuring soundproof curtains or replacing them with airtight soundproof windows and soundproof doors (STC 35-45).
• Outdoor and Construction Site Applications: For construction sites or temporary facilities, engineering soundproof fabric provides a mobile temporary soundproof barrier.