Basotect vs stone wool: absorption coefficient, bass and fire safety compared head to head

Both materials are porous absorbers. Sound sets the air in the open pores in motion, viscous friction arises at the pore walls, and part of the sound energy is converted into heat. How well that works depends essentially on three quantities: the flow resistivity of the material (the flow resistance per unit thickness), the layer thickness and the distance to the acoustically hard wall.

Flow resistivity (unit Pa s/m2, formerly Ns/m4 or Rayl/m) is the most important single quantity. In “Acoustic Absorbers and Diffusers”, Cox and D’Antonio give a favourable target range of about 5,000 to 50,000 Pa s/m2 for a porous absorber standing free in front of the wall. Practically equivalent is the rule of thumb that the product of flow resistivity and thickness (sigma times d) hits the optimum at around 1,000 Pa s/m. Both are approximations, not a sharp limit. Below that value, sound enters easily but is slowed too little. Well above it, there is an impedance jump at the surface and part of the sound reflects before it gets into the material.

Basotect, at around 10,000 Pa s/m2, sits at the lower edge of this window; stone wool of the usual acoustic densities (40 to 60 kg/m3) sits well above it, usually over 15,000 Pa s/m2. The exact values depend on product and density and scatter more than fixed numbers suggest: for Basotect roughly 8,000 to 15,000 Pa s/m2 is measured, for mineral wool of these densities about 10,000 to 40,000 Pa s/m2. The behaviour in the bass follows from this difference.

A first orientation comes from the empirical Delany-Bazley model. It calculates characteristic impedance and propagation constant from the ratio f/sigma alone, but it was derived on fibrous insulation materials with porosity close to 1 and is valid for roughly 0.01 < f/sigma < 1.0. For foams such as Basotect it is only an approximation. Physically more accurate are multi-parameter models such as Johnson-Champoux-Allard, which additionally account for porosity, tortuosity and characteristic lengths. For practice the core statement is enough: flow resistivity and thickness together fix the absorption curve.

Basotect G+ is an open-cell melamine resin foam with a density of 9 ±1.5 kg/m3 (EN ISO 845) and a thermal conductivity of around 0.035 W/mK at 10 °C. On temperature, the frequently quoted figure of 240 °C is a short-term peak. In continuous service the working range is more like 150 to 200 °C. The very low density is the reason for the low weight and at the same time for the low flow resistivity.

Acoustically, Basotect shows the typical foam profile: weak at low frequencies, very strong at mid and high frequencies. For 50 mm directly on the wall, published measured values to EN ISO 354 lie roughly at αs 0.15 to 0.20 (125 Hz), 0.5 to 0.6 (250 Hz), around 0.95 (500 Hz) and 0.9 to 1.0 from 1000 Hz upwards. These are indicative values; the specific figures depend on product, batch and test set-up. At 25 mm it is above all the lower bands that drop off markedly, and the absorber only becomes really effective above about 1000 Hz. The NRC to ASTM C423 for older 50 mm boards is around 0.95.

The rule of thumb behind this is the quarter-wavelength condition: a porous absorber becomes effective (αs above 0.5) where its thickness corresponds to about a quarter of the wavelength. At 50 mm this limit works out at around 1715 Hz (c = 343 m/s); because of the finite flow resistivity, usable absorption sets in somewhat lower. That does nothing to change the fact that 125 and 250 Hz are barely reachable with thin foam boards. Anyone who wants to hit 125 Hz with pure porous absorption needs material depth in the order of 60 to 70 cm (lambda/4 for 125 Hz is around 0.69 m), which in practice is solved with an air gap rather than with massive thickness.

Important for interpretation: reverberation room measurements to ISO 354 regularly give values above 1.0 because of diffraction at the specimen edges. For classification into a sound absorption class (A to E), the single-number value αw to ISO 11654 is formed by fitting the reference curve in steps of 0.05 to the practical values αp (octaves 250 to 4000 Hz) until the sum of the unfavourable deviations does not exceed 0.10. Basotect boards of 50 mm thus reach class A or a high class B, depending on build-up and air gap.

Stone wool in acoustic grade typically lies at 40 to 60 kg/m3. A published ISO 354 data set for 50 mm stone wool (60 kg/m3) directly on a solid backing gives αs 0.11 (125 Hz), 0.60 (250 Hz), 0.96 (500 Hz), 0.94 (1000 Hz), 0.92 (2000 Hz), 0.82 (4000 Hz). A lighter variant at 33 kg/m3 reaches 0.15 (125 Hz), 0.60 (250 Hz), 0.90 (500 Hz) in 50 mm and stays at 0.85 to 0.90 above that. These are example data sets of specific products; other products of the same density scatter, especially at 125 Hz and at high frequencies.

Two points here are not obvious. First: higher density does not automatically mean more absorption. Once the flow resistivity rises beyond the optimum, low-frequency absorption can decline slightly, because the impedance jump at the surface reflects earlier. That is why medium densities (about 40 to 60 kg/m3) often do better in the bass than very dense 90 kg/m3 boards. The effect is real but moderate. Second, thickness decides more than density: 100 mm stone wool (33 kg/m3) lifts the 250 Hz value from around 0.60 to about 0.90 to 0.95 and so delivers the decisive step in the lower mid range that no 50 mm board manages. Doubling the thickness shifts the effective absorption about an octave lower.

In a direct 50 mm comparison, stone wool and Basotect lie close together from 500 Hz upwards, both near 0.9 to 1.0. The difference arises in the two lowest bands.

The core point of this comparison: at the same thickness, Basotect and stone wool perform practically identically at high frequencies, but not at low frequencies. There is a transition frequency below which stone wool overtakes the melamine foam. According to the published 50 mm data, this crossover frequency lies in the range of about 250 to 500 Hz. That is a defensible span, not a fixed point; its position depends on the specific products and on the flow resistivity.

The reason is the flow resistivity. At low frequencies the wavelength is large compared with the layer thickness, so the material sees only a small fraction of the wave. In this range friction alone determines how much energy is retained. Basotect, at around 10,000 Pa s/m2, slows the slowly oscillating air too little; stone wool with its higher value hits the optimum better and gets more out of the same thickness. Above the crossover frequency this no longer matters, because there both materials swallow almost all of the sound anyway.

The practical consequence: if you have 50 mm available and are tackling speech intelligibility or reverberation at mid and high frequencies (office, first reflections in a studio, ceiling), you get equally good results with both materials and can decide by weight, fire safety and looks. If you have to hit the lower mid range or the bass (room modes, drum room, low-frequency machine noise), you win with stone wool at the same thickness, or with foam you have to compensate through more build-up depth and an air gap. An air gap behind the absorber shifts the effective absorption of both materials to lower frequencies, because the absorber then sits closer to the particle velocity maximum, where friction yields the most. That improves Basotect noticeably, but rarely closes the gap to stone wool completely at the same total depth.

In reaction to fire the difference is clear-cut. Stone wool is non-combustible and reaches the highest Euroclass, A1, to EN 13501-1; it only melts above around 1000 °C, does not contribute to a fire and produces no significant smoke. The A1 rating applies to the pure mineral wool product. With an organic binder or a facing the class can change, for instance to A2. Basotect, a thermoset, is flame-retardant with C-s2,d0 to EN 13501-1 (evidenced by a Müller-BBM test certificate) or B1 to DIN 4102 (the German classification). It does not melt or drip but chars, though it develops some smoke (s2) and in continuous service can only be used up to around 150 to 200 °C. For applications with strict fire safety requirements (escape routes, tall assembly halls, industry with flying sparks) A1 is often mandatory, which settles the choice on stone wool.

On weight the picture reverses. At 9 kg/m3, Basotect weighs only about a fifth to a sixth of a 50 to 60 kg/m3 stone wool. A 50 mm board weighs around 0.45 kg per square metre instead of 2.5 to 3 kg (9 kg/m3 times 0.05 m against 50 to 60 kg/m3 times 0.05 m). For suspended baffles, large-area ceiling rafts and mounting on delicate substrates that is a real advantage, often without a substructure.

Hygienically, Basotect is fibre-free. Cutting with a sharp blade produces hardly any dust, it does not itch and it releases no mineral fibres in service, which makes it practical for exposed applications and sensitive areas. Stone wool releases fibres and dust during handling that can mechanically irritate skin and airways, which is why it is usually installed enclosed or faced with a fleece and handled with protective measures. To put the often-raised cancer concern in context: mineral wool that meets the criteria of Note Q and is certified through EUCEB (in Germany effectively since 2000) is exempt from the carcinogen classification. The IARC lists modern bio-soluble fibres as Group 3 (not classifiable as to carcinogenicity), while older, bio-persistent fibres count as Group 2B (possibly carcinogenic). The old criticism of fibres concerns material from before this changeover.

Stone wool is the better choice when fire safety demands A1, when the lower mid range or the bass counts with limited build-up depth, or when the absorber disappears behind a cladding, an acoustic fabric or a perforated panel anyway. It is cheaper per square metre and available in many thicknesses.

Basotect is the better choice where weight is a problem (ceiling rafts, baffles, light fixing), where fibre-free is a requirement, for visible shape-milled components and where the material is meant to be exposed. Acoustically it plays to its strength at mid and high frequencies, that is, where speech intelligibility and reverberation time in offices and meeting rooms are decided.

The honest short version: at high frequencies the two are interchangeable. In the bass, stone wool wins at the same thickness. On weight and freedom from fibres, Basotect wins. On fire safety, stone wool wins. Anyone who wants to compare the actual αs values of specific products should look at the ISO 354 test reports per frequency band instead of manufacturer prose, because build-up, density and air gap shift the curve more than the material type itself.