Acoustic foam or mineral wool: the honest acoustic comparison by use case

Acoustic foam and mineral wool are both porous absorbers. Sound enters the open pores, the oscillating air rubs against the pore walls, and part of the sound energy is converted into heat through viscous friction. The central material property for this is the flow resistivity (flow resistance per metre of thickness). If it lies in the favourable window of roughly 5,000 to 50,000 Pa·s/m², sound can enter and is well damped. If it is too low, the wave passes through almost undamped; if it is too high, the surface already behaves almost like a sound-hard boundary and reflects.

In Acoustic Absorbers and Diffusers, Cox and D’Antonio give a practical rule of thumb: the optimum lies roughly where the product of thickness and flow resistivity is of the order of the characteristic impedance of air (ρ·c around 413 Pa·s/m). For broadband absorption against a rigid backing, the literature quotes a target window of about 800 to 2,400 Pa·s/m for the product of flow resistivity and thickness. It follows directly: a light, very open foam needs more thickness, a denser mineral wool achieves the same effect in less build-up depth. Both materials can reach the same absorption class; the route there differs.

Important for the choice of material: flow resistivity rises with bulk density and falls with larger fibre or strut diameter. Mineral wool in the 30 to 60 kg/m³ range usually lands in the favourable window by construction. Melamine foam at only about 9 kg/m³ (BASF Basotect G+, 9 ±1.5 kg/m³ to EN ISO 845) is very open and light, which gives high absorption at mid and high frequencies but costs thickness at low frequencies. Acoustically, then, what matters most is the build-up depth available per target frequency, far less whether the material is foam or wool.

Porous absorbers work through the particle velocity of the air, and directly at a sound-hard wall that velocity is almost zero. It reaches its first maximum where the distance from the wall equals a quarter of the wavelength. That is exactly where the first absorption maximum of an absorber mounted flush on the wall lies. This gives hard numbers (c = 343 m/s): 50 mm of material corresponds to λ/4 at around 1,715 Hz, 100 mm at 858 Hz, 200 mm at 429 Hz, and λ/4 at 125 Hz would need around 686 mm of solid material, which practically nobody installs.

These corner frequencies mark the first absorption maximum, not the onset of the effect. Useful absorption starts below them, typically from a thickness of about λ/8 to λ/10. In practice that means: 50 mm works usefully from roughly 800 to 1,000 Hz and reaches its first maximum around 1,700 Hz, 100 mm works from a few hundred hertz, and 200 mm remains usable down to below 500 Hz. Towards the low end, the effect of any equally thin panel falls off.

The ISO 354 curve of Basotect G+ shows this by example, although BASF publishes it only as a diagram, not as a table of figures: 50 mm of melamine foam reaches absorption coefficients close to 1.0 at mid and high frequencies and falls off at 125 Hz. Independent reverberation room measurements of 50 mm melamine foam typically lie at about 0.20 to 0.25 at 125 Hz. A 25 mm panel only reaches useful ranges at a few hundred hertz and is correspondingly weaker at low frequencies. This behaviour applies approximately to any porous absorber of the same thickness and similar flow resistance, mineral wool included. Mounting type and exact resistance shift the result, though, so the statement is to be read as an approximation.

The most important lever at low frequencies is the air gap, well ahead of the choice of material. If the absorber is mounted in front of the wall with an air gap, it moves closer to the velocity maximum at λ/4 and its effectiveness shifts downwards. A 50 mm absorber with a 50 to 100 mm air gap often beats a directly glued 100 mm block in the interesting range around 200 to 400 Hz. Real bass traps below about 100 Hz need thick packages or additional resonant principles (panel or Helmholtz absorbers) anyway. For the choice of foam versus wool this means: anyone who wants bass should be talking about thickness and air gap, and there denser mineral wool is often ahead per centimetre and per euro.

This is where the sharpest dividing line runs. Glass and stone wool consist almost entirely of inorganic mineral fibres and reach class A1 or A2-s1,d0 to EN 13501-1, that is, non-combustible. The CWFT rule (Commission Decision 96/603/EC) lists products that count as A1 without individual testing, provided they contain at most 1 percent by weight or volume of homogeneously distributed organic material; mineral wool and glass are among them. Stone wool has melting points above 1,000 °C and does not spread fire; by definition A1 means no contribution to the fire, with no smoke production and no flaming droplets. Be careful with the keyword ceramic fibres: refractory ceramic fibres (RCF) are a special case, listed by the IARC in group 2B, and they do not belong in the same basket either in fire or in health terms.

Melamine foam reaches B1 (flame-retardant) to DIN 4102-1, the German fire classification, without any added flame retardants. The reason is the nitrogen-rich thermoset melamine resin, which is flame-inhibiting in itself: in a fire the material neither melts nor drips, it chars, forming a layer of char, and self-extinguishes. For the EN 13501-1 class, C-s2,d0 is widely documented for melamine foams but is not on every datasheet; BASF states DIN 4102-1 B1 firmly for Basotect G+ and EN 13501 on request. Put precisely: B1 to DIN 4102-1 is established, C-s2,d0 to EN 13501-1 is typical and must be backed in each case by the relevant test certificate.

PU acoustic foam (polyurethane) is the critical group. Untreated flexible foam is easily ignitable, burns fast, drips while burning and releases considerable quantities of toxic gases in a fire (among them CO and hydrogen cyanide/HCN). With flame retardants, PU usually reaches B2 (normally flammable) to DIN 4102, B1 in special grades. B2 corresponds roughly to class E under EN 13501-1. For visible wall and ceiling surfaces in public or commercial rooms with a fire-protection requirement, plain PU foam is therefore often not permitted. The practical consequence: if the building task specifies A1/A2, every organic foam drops out and mineral wool is the given choice. If B1 is enough, melamine is a fibre-free alternative, whereas PU only qualifies in a tested B1 grade and never as a blanket assumption.

The health debate runs along completely different paths for the two material groups. With mineral wool it is about respirable fibres, with foam about emissions and wear.

On the fibre question the regulatory position is clear: in 2002 (Monograph Volume 81) the IARC downgraded glass, stone and slag wool from group 2B to group 3 (not classifiable as to carcinogenicity in humans). In the EU, exemption from the carcinogen classification runs through Notes Q and R of the CLP Regulation (EC) No 1272/2008: Note Q via biosolubility tests (among others, a half-life of fibres longer than 20 µm of under 40 days after intratracheal instillation), Note R via a geometric mean fibre diameter above 6 µm. The German RAL quality mark and EUCEB certification document this biosolubility. Even so, loose fibres irritate skin, eyes and the upper airways during handling, so protective equipment is customary when cutting, and in permanent visible installations open cut edges are practically always encapsulated with fleece, film, acoustic fabric or a frame so that no fibres enter the room air.

Melamine foam is fibre-free, and that is its hygienic trump card. BASF certifies Basotect to OEKO-TEX Standard 100 in product class I, the strictest class (articles with skin contact for babies and toddlers); the material is free of halogenated hydrocarbons and classified as not hazardous to water. The exact class per variant should be checked against the current certificate, since the certification applies above all to the pure white Basotect. Its weaknesses are brittleness (the material crumbles under mechanical stress and attracts dust) and yellowing under UV. Depending on the formulation, PU foam can emit VOCs and, with some flame retardants, additives of concern, so emission test certificates are worth a look here. For food, cleanroom and clinical areas the basic rule is: open porous surfaces, whether foam or wool, are hard to clean. Here encapsulated systems with a wipeable, sealed film or special hygiene-rated solutions are used, otherwise the solution fails HACCP or GMP.

Weight is the clearest practical difference. At around 9 kg/m³, melamine foam weighs about a third to a sixth of a typical acoustic mineral wool (30 to 60 kg/m³). With large ceiling rafts, free-hanging elements or adhesive mounting without a substructure that counts: light foam can often be glued directly, heavy wool boards need load-bearing frames or suspensions.

In handling, foam scores with its formability. Melamine can be cut, milled and thermoformed, so it suits wedges, pyramids and curves. It does crumble, though, and is brittle. Mineral wool is more robust as a block but produces fibres when cut and demands protective equipment. Visually, raw foam (grey, white) is more of a studio and plant-room aesthetic; mineral wool is almost always covered with acoustic fabric or perforated sheet metal and then appears as a textile wall panel or ceiling raft.

On cost, mineral wool is usually cheaper per square metre and per unit of low-frequency absorption, because the raw material is cheap and denser boards deliver more effect per centimetre. Melamine foam costs more than bare mineral wool but buys fibre-free construction, low weight and B1 with no extra effort. Reliable euro figures vary widely with thickness, covering and quantity, so the cost comparison must always refer to the finished, installed solution and never to the bare material. Anyone calculating per sabin (absorbed area) should compare thickness, air gap and covering as a package.

Instead of a blanket winner, here is the honest assignment by building task. Recording studio and bass range: thickness and air gap decide, denser mineral wool (about 40 to 60 kg/m³) as a package with an air gap is usually superior and cheaper at low frequencies; melamine suits light, shapeable first-reflection and ceiling elements where weight and appearance count.

Office and open space: both materials solve the task (reverberation, speech intelligibility, mostly 250 Hz to 4 kHz). Mineral wool with acoustic fabric is the cost standard for wall absorbers and ceiling rafts; melamine scores with light, directly glued ceiling elements without a substructure. Hygiene, food and clinical: fibre-free construction and cleanability take priority. Here fibre-free melamine foam has the edge over open wool, but HACCP and GMP areas additionally need a sealed, wipeable encapsulation; bare porous surfaces are out.

Fire-protection requirement: where the building regulations demand A1/A2 (escape and rescue routes, large assembly venues, many industrial halls), mineral wool is the given choice and every foam drops out. Where B1 suffices, melamine is a fibre-free option, PU only in a tested B1 grade. Budget project: mineral wool with a standard covering delivers the most absorption per euro, especially at low frequencies. For every row of this matrix the selection can be backed with real ISO 354 measured data per thickness and mounting type instead of datasheet promises. That is exactly what a neutral data platform is for: filtering and comparing materials by measured absorption coefficients per frequency band, fire class and build-up depth.