Measuring Acoustic Absorption in Rooms

Measuring the Acoustic Absorption of Acoustic Foams

Many customers ask us why some acoustic products have absorption coefficients greater than 1.00 and how profiled acoustic foam differs from flat sheet type acoustic foams.

Generally, acoustic products have what is called an 'absorption coefficient' which gives details of how effective they are over a range of frequencies of taking in sound energy, converting it to heat, and radiated less energy back into the room. This figure ranges from zero (no sound absorption at all) to 1.0 where 100% of the sound that hits the material is absorbed. If the figure is less than 1 it means that only part of the sound is being absorbed at a particular frequency with the remainder either being reflected back or passing through the material.

Sometimes product specifications also include an NRC (noise reduction coefficient) figure. This is an average figure of the mid-range absorption coefficients between 250Hz and 2Khz. Occasionally absorption figures are stated in units called 'Sabins'. These can be rather misleading as this refers to total absorption over an area. To convert Sabins into coefficient figures you need to divide the Sabin figure by the surface area of the material in question. It is usually only 'objects' that quote Absorption in sabins (things like suspended cubes or rafts).

To obtain absorption figures, a sufficient area of material is placed in a reverberant room and the decay time is measured at different frequencies before and after the test material is placed in the room. The difference between these two measurements allows the absorption to be calculated.

Most absorption measurements are taken with the test material mounted directly on a wall but when you space absorbent material away from a wall or other hard surface the low frequency performance is improved so sometimes absorption figures that include spacing are often included in product specifications (i.e. E-250 Mounting which means the product is mounted 250mm away from the surface). It is not actually possible for a material to have an absorption coefficient of more than 1.00 but sound measurements can yield figures greater than 1.00. The reason for this is that all materials have a certain thickness but the edges, which are not included in the surface area calculations, also absorb some of the sound. As an example a 1200mm x 600mm x 100mm F/G Absorber has a front surface area of 0.72m² but including the four edges the overall 'surface area' increases to 1.08m² which is a 50% increase in surface area.

The other problem is that a smaller panel will have test figures better than that of a larger panel because proportionally the smaller panel has more 'edge surface area' than that of a larger panel even if the thickness is the same for both.

It gets worse. One last problem is that most acoustic foam panels are mounted next to each other on a wall or ceiling etc. This means that the edges are not absorbing any sound 'on-site' but were absorbing sound when the material was tested so the actual stated absorption figures will be around 1/3 down on any published figures which were arguably inflated by 50% in the first place when tested.

It is basically a slight 'flaw' in the test methods which produces figures above 1.0 as the test method does not take into account the edges of a panel. This can cause confusion and can make it difficult to compare one product with another but as a generalisation it is often wise to assume that when absorption products are mounted 'in-situ' they will provide less than the measured absorption figures so you should always go for the thickest foam your budget allows.

When it comes to triangular type 'foam corner wedges' this disparity is even worse and you may get 50% less absorption than the stated figures especially at the lower frequencies.

Another thing to bear in mind is that quite often manufacturers of profiled acoustic foams use the acoustic data from the same thickness of material then proceed to cut away lots of material to achieve the 'profile' which means it's highly unlikely the data will be anywhere near accurate because half of the initial material is missing..

Amongst other things, British Standard BS EN ISO 11654 requires absorption information to be rounded to the nearest 0.05 with anything over 1.00 to be rounded down to 1.00.

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