Comparing sound absorbers: what really matters

Almost every datasheet quotes an αw value or a sound absorption class, and that one number is precisely the problem. αw condenses the behaviour across five octaves and is essentially read off at 500 Hz. How a material performs at 125 Hz is not in it at all. Two absorbers with an identical αw of 0.90 can differ massively in the bass: one may reach only 0.3 at 250 Hz, the other 0.9. On paper both look the same.

On top of that, αw is not a simple average. It comes from a standardised curve comparison, and a weakness at low frequencies can vanish completely in the process. The American NRC works in a similarly coarse way and leaves out the 4 kHz band altogether. In practice that means: ask for the full table of absorption values per frequency band. Only then can you tell whether an absorber fits your specific problem.

Classes A to E are just a rough pigeonhole for αw. Class A means αw 0.90 and above, class D runs from 0.30 to 0.55. Class D is therefore much wider than it sounds: two products of the same class can differ almost twofold in effect. Anyone comparing by class alone misses that.

A small addition that some datasheets carry is helpful: a letter in brackets after the αw. L, M or H shows in which range the absorber is particularly strong, that is low, mid or high. Unfortunately it is rarely stated, but it says more about the character of an absorber than the bare class.

A porous absorber works best where the air moves fastest, and that is a good distance in front of the wall. Mounted directly on the wall, the same panel delivers noticeably less in the bass. A simple rule of thumb follows: the larger the air gap, the lower the effect reaches. Even a few centimetres of air behind the panel raise the bass absorption noticeably, and a 25 mm absorber with a 25 mm air gap performs roughly like a 50 mm absorber directly on the wall.

An example from measured data shows how large the effect is: the same thin fibre board reaches around 0.09 at 125 Hz directly on the wall and around 0.43 with a 40 cm air gap. That is a factor of 5 without anything about the material changing. This is why a statement such as “class A” is only usable together with the air gap it was measured with. Without that information, two products simply cannot be compared fairly.

The most important material parameter is the flow resistivity, in other words how strongly a material slows the air down. There is an optimum: pressed too densely, the material reflects like a wall; too open, it simply lets the sound pass through. That is exactly why more density is not automatically better. Heavily compressed material can even absorb worse than looser material, and sorting products by kg/m³ leads you astray, especially across different material types.

For low frequencies, what counts in the end is above all the build-up depth. As a rough guide, 50 mm of mineral wool works well from about 1.5 kHz, 100 mm from about 800 Hz and 200 mm from about 400 Hz. Every doubling of the thickness therefore takes you roughly one octave lower. For 100 Hz you would need almost a metre of material depth, and that is exactly why thin foam panels are pure high- and mid-frequency absorbers, whatever brand is on them.

Absorption values are measured in two ways, in the large reverberation room and in the small impedance tube, and both give different numbers for the same material. The tube measures sound only at normal incidence on a small sample, the reverberation room from all directions on a large area, the way it happens in a real room. For room acoustics and reverberation calculations, the reverberation room values are therefore the right basis.

In the reverberation room, values above 1.0 even occur arithmetically, often between 1.05 and 1.20. That is a measurement effect at the edges of the sample and not a sign of a better material. In the tube, by contrast, the value always stays capped at 1.0. What matters: check on the datasheet which method a number comes from, and only compare like with like.

Even the number from the right method is not set in stone. If the same absorber is measured in two different laboratories, the αw can differ by up to 0.1, and that is exactly the gap between class A and class B. The scatter is largest in the bass and small at high frequencies; below about 300 Hz the values are shakiest. The main cause lies in the measurement environment itself, for instance in how evenly the sound field is distributed in the test room. That is precisely why the reverberation room method is being revised.

For practice this means one thing above all: do not overrate small differences on the datasheet. Only from a difference of about 0.1 are you really comparing the material and not the measurement noise.

Before you set two values against each other, it is worth looking at where they come from. A number from a sales brochure and a number from a test report to a standard are not worth the same. A real measurement report to ISO 354 names the laboratory, the test date, the sample size, the mounting type and the full curve across all frequency bands. That lets you trace under which conditions the value came about.

A bare manufacturer’s figure without a test report leaves all of that open. Often there is just a rounded class or a single αw, measured under conditions you do not know. If in doubt, ask for the test report. A supplier who discloses their measurement gives you the basis for a real comparison. That is exactly why Acoustic Index lists the values with standard, measurement method and frequency curve instead of just adopting a marketing number.

One last common mix-up: diffusers scatter sound, they do not absorb it. Rating them by an absorption value makes no sense. Scattering has its own metrics, the scattering coefficient and the diffusion coefficient, which measure different things and are often muddled in the market. Caution is needed above all with geometric mouldings sold as diffusers: many merely redirect the sound instead of scattering it widely. And if a “diffuser” has a high absorption value, then in truth it usually absorbs more than it scatters.

In the end it is about the right reverberation time for the room. A classroom needs a shorter reverberation time than a concert hall, and for typical rooms DIN 18041 (the German standard for reverberation time targets) sets target values. A classroom of 200 m³, for example, lands at about 0.55 s, in the inclusive design rather at 0.45 s. More absorber is not better here, because an over-damped room sounds quiet and dead.

You get the rough quantity by deriving the required absorption area from the room volume and the target reverberation time and dividing it by the αw of your absorber. Remember that furniture and people absorb too; an empty room reverberates much longer than a full one. Our reverberation time calculator takes this calculation off your hands.