If acoustic panels, bass traps, and ceiling tiles are the visible face of acoustic treatment, mineral wool is what lives inside them. It is the industry's workhorse — the most widely specified acoustic insulation material on Earth. Virtually every professional recording studio, concert hall, hospital, and office building uses mineral wool somewhere in its construction. Understanding its types and properties is foundational to acoustic design.
TLDR
Mineral wool is a family of fibrous insulation materials made by spinning or drawing molten mineral material into fine fibres. The two main types are rockwool (stone wool, made from volcanic basite rock) and glass wool (made from recycled glass and silica sand). Both absorb sound through viscous friction as air oscillates within the fibre matrix. Acoustic performance depends on density (typically 30 to 140 kg/m³) and thickness (25 mm to 200 mm). At 50 mm and 40 kg/m³, mineral wool achieves NRC 0.80 to 0.90. At 100 mm, it reaches NRC 0.95 to 1.05. Mineral wool is non-combustible (Euroclass A1), moisture-resistant, and dimensionally stable — making it the preferred fill for acoustic panels, bass traps, ceiling tiles, wall cavities, and floating floors. Major brands include ROCKWOOL (rockwool), Knauf Insulation and Isover (glass wool), and Owens Corning (fibreglass, a glass wool variant).
Real-World Analogy
Think of mineral wool as a dense forest of microscopic glass or rock threads. Sound entering this forest is like wind entering a real forest: the trees slow the wind through friction, and the deeper the forest, the calmer the air becomes on the other side. Mineral wool fibres slow and absorb sound waves through the same principle — friction between air particles and the thousands of tiny fibres per cubic centimetre. The denser the forest (higher density mineral wool) and the deeper the forest (thicker slab), the more sound energy is converted to heat.
Technical Definition
Manufacturing Process
Rockwool is made by melting basalt rock and slag at approximately 1,500°C, then spinning the molten material on a rotating drum or disc to draw it into fibres 3 to 7 micrometres in diameter. A binder (typically phenolic resin, 2 to 5% by weight) is added to hold the fibres together, and the mat is cured in an oven at 200°C.
Glass wool is made by melting silica sand and recycled glass at approximately 1,100°C and drawing fibres using a centrifugal spinner. Glass wool fibres are typically 4 to 9 micrometres in diameter and slightly more flexible than rockwool fibres.
Acoustic Properties
Both types work as porous absorbers per the model described in ISO 354:2003. The sound absorption coefficient depends on:
- Flow resistivity (ISO 9053): The resistance to airflow per unit thickness. Optimal range for acoustic applications is 8,000 to 40,000 Pa·s/m². Too low (loose, low-density wool) and sound passes through with insufficient friction. Too high (very dense, compressed wool) and sound reflects off the surface.
- Thickness: Determines the lowest frequency effectively absorbed. The rule of thumb is that a porous absorber becomes effective at frequencies whose quarter-wavelength is less than or equal to the material thickness. For 125 Hz (wavelength 2.7 m), the quarter-wavelength is 0.69 m — which is why bass traps need to be very thick or use an air gap.
- Density: Higher density increases flow resistivity, improving low-frequency absorption but potentially reducing high-frequency absorption if the surface becomes too reflective.
Rockwool vs Glass Wool: Acoustic Comparison
| Property | Rockwool | Glass Wool |
|---|---|---|
| Fibre diameter | 3-7 µm | 4-9 µm |
| Typical density | 40-140 kg/m³ | 12-80 kg/m³ |
| Fire class | Euroclass A1 (up to 1000°C) | Euroclass A1 (up to 600°C) |
| Moisture resistance | Hydrophobic | Absorbs moisture if unfaced |
| Sound absorption (50mm, 40 kg/m³) | NRC 0.85 | NRC 0.80 |
| Rigidity | Self-supporting at 60+ kg/m³ | Requires support at all densities |
| Cost | Higher | Lower |
For acoustic applications, both perform comparably at the same density and thickness. The choice often comes down to secondary factors: rockwool's higher fire rating and rigidity (self-supporting panels) versus glass wool's lower cost and lighter weight.
Why It Matters for Design
Mineral wool is the active ingredient in most acoustic products. When you specify a fabric-wrapped acoustic panel, the fabric is decorative — the mineral wool core does the absorbing. When you install a suspended ceiling tile, the mineral fibre inside is doing the work. When you build a stud wall for sound insulation, the mineral wool in the cavity absorbs sound that would otherwise resonate between the plasterboard leaves.
Understanding mineral wool density grades helps you specify the right product. For acoustic panel cores, 40 to 60 kg/m³ provides the optimal balance of absorption and weight. For bass traps, 60 to 80 kg/m³ extends low-frequency performance. For wall cavity insulation (sound insulation between rooms), 30 to 45 kg/m³ is sufficient because the cavity depth provides the air gap.
How AcousPlan Uses This
AcousPlan's material database includes mineral wool products from ROCKWOOL, Knauf, Isover, Owens Corning, and other manufacturers with full octave-band absorption coefficients at multiple densities and thicknesses. When you assign these materials in the room builder, the calculator uses the specific product data — not generic estimates — to compute RT60 and STI across all octave bands.
Related Concepts
- What is NRC? — How mineral wool absorption is rated
- What Are Bass Traps? — Mineral wool as bass trap fill
- What Are Acoustic Wall Panels? — Mineral wool as panel core
- What is Acoustic Foam? — Foam alternative to mineral wool
- How Do Acoustic Panels Work? — The physics of porous absorption
Calculate Now
Model rooms with mineral wool-based treatments and compare densities and thicknesses. AcousPlan shows the frequency-dependent absorption impact at every octave band.