Profiled decks present a hard surface that reflects sound back into the internal environment. This can lead to longer reverberation times. The longer the reverberation time, the more ‘echoey’ a room sounds. Reverberation time performance of a room can be calculated from the areas of walls, floors and ceiling, and their absorption coefficients. Typical requirements for schools, recommended by BB93, fall between 0.6 seconds and 1.5 seconds. 

Sound absorption is the reduction of sound energy. The sound absorption coefficient indicates the fraction of energy absorbed on striking any surface, stated as values between 0 and 1.0. If a surface absorbs no sound its coefficient of absorption is 0. If 100% of sound is absorbed the coefficient is 1.0. Absorption is frequency dependent, and is tested over a range from 125 to 5000 Hz.

Ratings for Sound Absorption classifies materials into bands, Class A to Class E. Insulation achieves Class A, the highest level of absorption, whereas plain steel, or aluminium, sheet reflects sound with no absorption and is unclassified.

To enhance acoustic absorption decks can be supplied perforated. Fully perforated this would reduce the structural strength of the deck; therefore perforation areas are limited to the side webs.

Perforating the liner sheet, allows sound to penetrate into the cavity, to be absorbed into the soft insulation, formed from web inserts, or rigid slab, at 45 Kg/M³ minimum density. Insulation to be tissue faced, to prevent fibres detaching and penetrating the internal environment. By allowing sound to escape into the cavity this does have the effect of worsening the over all reduction value. However this can be compensated for by the addition of dense acoustic membranes.

Structural Trays offer a wider flat soffit thus enabling a greater perforation area, which achieves the best acoustic absorption result.

Where perforated decks, or trays, are installed these cannot act as a vapour control check, therefore a separate vapour control membrane layer must be installed.

A range of acoustic absorption tests have been conducted on our perforated decks, and trays, at Salford University Acoustics Laboratory, with either open troughs or with the troughs in filled with preformed insulation. The test results are indicated below.