Suspended acoustic rafts

Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design

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).

Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design
Brief description of suspended acoustic raft systems

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, the further the rafts are from the hard surface, the higher the sound absorption at mid and high frequencies.

sound absorption below and above the rafts - Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design
Sound absorption below and above the rafts

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).

sound absorptive edges - Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design
Sound absorption of raft edges

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)

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 absorptive edges - Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design

Sound absorption per m² of raft
based on rafts of different sizes
hung at 1000mm from the hard surface above
(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) - sound absorptive edges - Suspended acoustic rafts - sound absorption - acoustic absorption - acoustic consultant - acoustic design - architectural acoustic design

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 thick fibrous or porous sound absorptive materials such as:

  • fibre glass
  • mineral wool
  • wood wool and mineral wool on top
  • polyester fibres
  • wood fibres

Sometimes, the sound absorptive material is wrapped in a fibrous fabric or even painted.