Acoustic Testing

Field vs. Laboratory

It is always important to remember that published acoustic performance data is normally derived from laboratory tests, under controlled conditions. When applying this data, there are several factors that need to be remembered, so that expectations of performance are not unrealistic.

Sound Absorption

Unless stated otherwise, sound absorption data will always relate to the basic ceiling system, based on a sample size of around 10 m². This sample will include the relevant amount of ceiling grid as well as the basic ceiling tiles, although it will not include any secondary ceiling features, such as light fittings, air diffusers or loudspeakers. When these are added they will clearly affect the overall level of sound absorption offered by the entire ceiling surface, and any negative effect may need to be taken into account.

For an open plan office ceiling, the effect is probably negligible and can be ignored; however, for ceiling area calculations to BB93 or the Building Regulations, the absorbent surface area is critical and the effect should be accounted for by omitting the relevant surface area of feature from the calculation.

Sound Attenuation

Unless stated otherwise, published sound insulation data is derived from a laboratory test to a defined standard under controlled conditions, and applies only to the element of concern, in this case the suspended ceiling. When this ceiling is installed as part of a system including partitions and raised floors, the composite effect of the elements together needs to be considered. In addition, any ceiling features such as light fittings or air diffusers may contrive to lower performance.

Specific advice on this matter should be sought form an acoustic consultant, however as a rule of thumb, installed ceilings on site offer between 3–5dB less performance than laboratory test figures state. This accounts for sound flanking paths and potential installation tolerances.

Sound Insulation

Where partitions abut the underside of a suspended ceiling, the quality of acoustic seal at the junction between the two will be critical. Remember that in the laboratory test this weakness is avoided by use of a massive partition and a level of head seal that would be visually unacceptable in an occupied environment.

There may be small sound paths through the ceiling grid to be sealed, as well as shadow gaps against plaster margins. In any event, the partition head must be very well sealed against the ceiling grid (NB: Self adhesive tape may be not be sufficient).

Where partitions are placed “off-grid”, (i.e. the partition junction runs across the centre of a tile as opposed to along a ceiling grid line), the level of sound insulation offered may not match the laboratory test data even after applying the 3–5dB field performance allowance. This is because sound can pass through the body of the ceiling tile via the perforated face, acting as a sound flanking path over the top of the partition head junction.

If such junctions are unavoidable, and sound insulation is important, then you should replace the partition head tiles with non-perforated plain tiles, or replace the tile insulation with a solid sheet material into which the partition head can be positively fixed.