Many customers ask us why some acoustic products have absorption coefficients greater than 1 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. This figure ranges from zero (no sound absorption) to 1.0 where 100% of the sound is absorbed.
If the figure is less than 1 it means that only half 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 midrange 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.
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 Mounting).
It is not actually possible for a material to have an absorption coefficient of more than 1 but sound measurements
can yield figures greater than 1. 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 of 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 against another but as a generalisation it's 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 the 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.