Is it true that the higher the density of sound insulation materials, the better the sound insulation effect?

For materials relying on the "mass law" (e.g., mineral wool, glass wool), higher density does enhance blocking of mid-to-high frequency noise (e.g., voices, traffic). However, this effect plateaus: beyond a certain density (around 60-80kg/m³ for most materials), improvements become negligible, but the material grows heavier, harder to install, and more costly. 

Material type and structure matter equally. Porous sound-absorbing materials (e.g., polyester fiber cotton) lose effectiveness if too dense—clogged pores can’t trap sound waves. Damping materials (e.g., pipeline damping sheets) rely on viscoelasticity, not just density; a medium-density option with good internal friction often outperforms rigid high-density alternatives. Composite structures (e.g., "sound-insulating cotton + damping layer + air gap") are far more effective than single high-density materials, especially for mixed-frequency noise. 

Low-frequency noise (e.g., elevator vibrations, air conditioner hum) can’t be solved by density alone. It travels through structures, requiring damping properties or decoupling designs (e.g., floating floors) rather than just heavy materials. A dense concrete wall may still transmit low-frequency vibrations, while a lighter composite setup blocks them better. 

Poor installation also negates high density. Gaps or cracks create "acoustic bridges," letting noise leak through—even the densest material fails if improperly sealed around doors, windows, or pipes.   In short, density is a "necessary but not sufficient" condition. Choose medium-to-high density materials for air-borne noise, prioritize damping/composite structures for low-frequency noise, and ensure professional installation with proper sealing. Balance density with material type, application needs, and budget for the best results.