Suspended acoustic rafts are free-hanging elements used to absorb sound in spaces, and therefore reduce the sound reverberation internally.
They are quite popular for education spaces, office spaces, assembly spaces and public spaces (such as receptions, entrance halls, etc).
This page briefly explains:
what suspended acoustic rafts are exactly
which sound absorption performances acoustic rafts achieve and, most importantly, how they should be specified
which materials are usually used for acoustic rafts
You will also find some product examples at the end of the page.
What are suspended acoustic rafts?
Suspended acoustic rafts are flat elements made with a thick fibrous or porous material with sound absorption qualities. You hang them horizontally at a certain distance from a hard surface (usually a soffit).
Although acoustic rafts are mostly square or rectangular, some of them are also round, oval or any other bespoke shape.
Some raft manufacturers/suppliers give the option to include lighting and other electrical systems within the rafts.
Sound absorption performance of suspended acoustic rafts
The sound absorptive materials used for suspended acoustic rafts generally achieve sound absorption Class A, Class B or Class C.
However, you should know that sound absorption classes are generally for a materials fixed to a hard surface with only one side visible. This side is the only one that absorbs sound. The other on doesn’t.
For acoustic rafts, it is a little bit different.
Suspended acoustic rafts – Sound absorption below and above
Not only the underside of the rafts absorbs sound, but also upper side indirectly.
Sound will first hit the hard surface above the acoustic rafts and then the upper side of the rafts.
Therefore, the distance between the rafts and the hard surface above has an influence on the sound absorption of the raft systems. Below approximately 1m, thefurther the rafts are from the hard surface, the higher the sound absorption at mid and high frequencies.
Suspended acoustic rafts – Sound absorption of the edges
The edges of the rafts also absorb sound.
So a couple of large acoustic rafts absorb a little bit less sound than a few smaller acoustic rafts (with the same amount of absorptive materials).
Suspended acoustic rafts – Examples of sound absorption performances
To illustrate this, the graphs below shows examples of sound absorption performance for various raft systems.
Note: for rafts, the sound absorption performance is qualified in terms of equivalent absorption area per unit. Not in terms of sound absorption coefficient per m² of material.
Equivalent absorption area (m²) of 1200mm x 1200mm acoustic rafts
located at different distances from the hard surface above 200 mm, 400 mm and 1000 mm
and spaced 500 mm apart
(Courtesy of Ecophon)
Sound absorption per m² of raft
based on rafts of different sizes
hung at 1000mm from the hard surface above
(Courtesy of Ecophon)
Suspended acoustic rafts – Specification
Based on the above, you can now understand that the sound absorption characteristics of raft systems don’t just depend on the material of the rafts.
Therefore, in specifications, it is important to include the following information:
the sound absorption of the material itself (when measured in line with ISO 354:2003 Acoustics — Measurement of sound absorption in a reverberation room)
the size of the rafts
the spacing between the rafts
the distance between the rafts and the hard surface above.
Note: the above is application to any other suspended acoustic system.
Materials used for suspended acoustic rafts
The materials preferred for acoustic rafts are usually thickfibrous or porous sound absorptive materials such as:
wood wool and mineral wool on top
Sometimes, the sound absorptive material is wrapped in a fibrous fabric or even painted.