Acoustic foam or a stone wool panel? An honest comparison

Grey pyramids and waves on the walls are many people's first picture of acoustic treatment. Foam is cheap, light and easy to buy. Stone wool panels cost more, so the natural question is whether they are worth it. In this guide we compare the two honestly – including the cases where foam is enough.

How both materials work

Acoustic foam and stone wool work on the same principle. Both are porous: sound enters them, air molecules rub against the walls of the pores or the fibres and lose energy, which turns into a tiny amount of heat. The thicker the layer, the lower the sounds it can absorb.

The differences come from how much material there is, what its structure is like, and what happens to it over the years.

Absorption: thickness decides

The most popular foams are 2–5 cm thick and profiled – pyramids, wedges, waves. They look good, but the profile means the average thickness of material is less than the nominal one. A "5 cm" pyramid foam has roughly as much material as a flat sheet half as thick.

A layer like that handles high frequencies well: hissing consonants, rustles, the sharp echo after a clap. In the mid frequencies, where most of the energy of speech lies, it absorbs noticeably less, and in the low frequencies almost nothing. The result can be surprising: after covering a room with foam, the top end sounds quieter, but voices still boom and lack clarity, because the mids and lows bounce around as before.

A stone wool panel has its full 5 or 10 cm thickness across the whole surface. Wool of the right density absorbs practically the whole speech range, and a 10 cm panel reaches noticeably lower. We explain this in more detail in 5 or 10 cm. On top of that, the panel hangs with a small gap from the wall, which further improves absorption of low frequencies.

Fire and safety

This is where the difference is biggest, and it is worth knowing. Ordinary polyurethane foam is combustible and gives off dense, toxic smoke when it burns. There are foams with fire retardants and melamine foams with better fire performance, but many cheap products carry no declared fire classification at all. If you choose foam, look for that information on the product sheet – especially if it is going to cover a large part of a wall in a child's room, a bedroom or an office.

Stone wool is non-combustible – it has the highest reaction-to-fire class, A1. It is the same material used to insulate the walls and floors of buildings. In our panels it is enclosed in a wooden frame, under a veil and fabric, so it does not shed and you never touch it in everyday use. We describe what is inside a panel on How we make it.

Durability and looks

Polyurethane foam ages. Light and time make it yellow, harden and start to crumble, especially at the edges. It collects dust that is hard to remove, and when it is peeled off after a few years it often takes glue and paint with it.

A panel with a solid wood frame and acoustic fabric keeps its shape. The fabric can be vacuumed with a soft brush, and the whole panel can be taken down and hung elsewhere – after a move, for example. It also looks like part of the interior rather than studio equipment, which matters in a living room, a bedroom, or an office where you talk to clients on camera.

Mounting

Foam is usually glued to the wall. It is quick but hard to undo – a real problem in a rented flat or a freshly painted room. Double-sided tapes often fail to hold foam for more than a few months.

Our panels hang on Z-clips or a French cleat fixed with a couple of screws. When you take them down, all that is left are a few small plug holes. They can also stand on feet or hang from the ceiling on cables. You will find the details in the installation guide.

Price

Foam is clearly cheaper per square metre. A fair comparison, though, should take into account how many square metres of each material you need for the same effect in the speech range, and how long it will last. You need more thin foam, and after a few years it often has to be replaced. A panel with a frame and fabric you buy once.

Even so, if your budget is very tight, a few well-placed panels in the most important spots do more than covering a whole wall in foam. We explain where those spots are in the guide to panel placement.

When foam is enough

Foam has its uses, and there is no reason to hide that:

• inside a small booth or a microphone box, where weight matters and high-frequency absorption is enough,
• as a temporary fix for a few weeks, for example in a room rented for a single project,
• as a top-up in places where only a sharp, high echo bothers you,
• inside cabinets and enclosures, where nobody sees it.

If, however, you want clearer speech in an office, calmer conversation in a living room or better voice recordings, you need absorption across the whole speech range – and here a thicker porous material such as stone wool has an advantage that thin foam cannot make up for.

Summary

• Absorption – 2–5 cm foam: mainly high frequencies; 5–10 cm wool panel: the whole speech range, 10 cm also lower frequencies.
• Fire – ordinary polyurethane foam: combustible; stone wool: non-combustible, class A1.
• Durability – foam yellows and crumbles over time; a framed fabric panel keeps its shape and looks.
• Mounting – foam is usually glued; a panel hangs and is easy to take down.
• Price – foam is cheaper up front; a panel costs more, but you buy it once for years.

Not sure how many panels you need? Read how many acoustic panels do I need or ask for our free advice.