In many areas installation of noise absorbers will greatly help to reduce the sound energy level in a room, but not least radically improve the reverberation.

This means that even in a room with a relatively high noise level, experience of being in the room is highly improved to a stage where fatigue and physical discomfort is reduced and it becomes possible to understand normal speech.

The simplest and most effective way to obtain this is installing open Rockwool Marine Slab 80, for areas where open insulation is allowed, directly on bulkheads and decks with standard pin’s and washers. Minimum 50 mm and preferably 2 x 50 mm thickness.

In theory you should aim for a thickness of 1/4 of the wavelength for the frequency needed to absorb, but often this is not possible in practice, and by 100 mm insulation very good results will also be achieved.

Investigations and measurements have shown that the Rockwool Marine Slab 80 will be the best compromise in ratio of weight and noise absorption. And to obtain best possible results it is highly recommended to make two layers with an air gab between the layers of approx 5 -15 mm. This enhances both reduction and absorption values.

Please see cut outs from measuring data:

Absorption:
2 x 50 mm Rockwool Marine Slab 80

Reduction:
2 x 50 mm Marine Slab 80, air gap

Absorber Systems with foil cover

For areas with risk of oil contamination of open insulation, SOLAS rules requires the insulation to be covered by liquid and vapour tight facing. In the engine room this is traditionally done by thin galvanised steel sheeting.

But this results in poor noise absorption in the engine room, as the smooth hard surface will refl ect all of the sound energy.

A perfect built-up will therefore be Rockwool insulation covered by a loose fi tted thin vapour resistant foil and mechanical protected by perforated steel sheeting.

Keep in mind to fulfi l SOLAS requirements of Low Flame Spread (IMO res. A653 (16) / IMO FTP Code annex 1 part 5) for the foil.

For the engine room it will be obvious to utilise the normally required A-60 fi re protection to gain maximum noise absorption and reduction in reverberation time. Please note that the approved fi re protection construction is to be installed according to the approved fi re test, but the foil and perforated sheeting can be installed similar to traditional closed steel sheeting, by stud bolts, z-brackets, L-beams, slides, etc.

Measurements of Rockwool Firebatts 100 covered by a 19 my foil have been performed, where it is seen that the foil hardly does not reduce the absorption at all. This specifi c foil has been Low Flame Spread tested by DIFT. Please see comparison graph below and test results in the appendix I.

In areas where a visual nice surface is required it is recommended to use Rockwool products with glass cloth facings.

This is again perfect for the combination solutions. It could be where thermal insulation is required in as example a stores room, and this directly mounted Rockwool will then also act as a very good absorbent and provide reduction to adjacent rooms. The glass cloth will change the absorption coefficient, depending on the type of glass cloth. Thin open woven types will only minimal reduce the absorption.

Thick tight woven types will reduce the absorption by 30-50 %, compared to the pure Rockwool. When the aim is to utilise the absorption, the glass cloth must not be painted. It will therefore allways be a decision between the right mix of mechanical strength and absorption.

Please see measuring results in the appendix I or at our web site www.rockwool-marine.com

Dedicated Absorber Systems

For areas with even more focus on the noise absorption, dedicated absorbers should be fi tted. It will then be possible to take into account the resonance frequencies. The basic principle is to mount the Rockwool Slab in a calculated distance from the bulkhead / deck. Hereby it is possible to combine the principles of porosity absorbers and resonance absorbers.

Mentioned formula below gives us the possibillity to calculate the resonance frequencies fr:

It will be possible to enhance the absorption in the very low frequencies, gain a more fl at absorption curve, with a designed peak frequency.

Such a system for i.e. noise damping in a hi-press room, could be designed towards the machinery’s frequency of the worst noise level.

A system like this could be built-up by Rockwool Marine Slabs and perforated steel sheeting, placed in the calculated distance.

Where necessary a membrane between the perforated steel and the Rockwool can be fi tted, but here great care should be taken not to disturb the ability for the sound energy to pass through the membrane. A foil like aluminium foil glued to the Rockwool will not be the best solution. The membrane has to be an “open” type or a light weight loose foil.

It is also possible to purchase dedicated absorber cassettes complete including bracket system etc from several suppliers. These will of cause also have our non-combustible Rockwool material core.

The same principle is utilised by many ceiling manufactures, again using Rockwool material as the core.