Basic Design of Sound Insulation Between Two Rooms
The airborne and impact sound insulation of partitions (walls and floors) and of building elements is normally expressed in terms of a single figure value, based on measurement of the sound insulation over a range of frequencies (typically 50 Hz to 3.15 kHz, although this may be extended under special circumstances). The measured values are then compared to a standard reference curve and a single figure “weighted” value of the sound insulation derived. The rating procedure is given in the British Standards BS EN ISO 717-1 and 717-2.
The sound insulation of a built partition is normally different from the sound insulation of the building element when tested in a laboratory because the measured sound levels are affected by the partition area, the volume of the receiver room and the sound absorption in that room (usually defined by its reverberation time). The sound level measured in the receiving room is also influenced by sound travelling through other common elements (walls, the floor, and the ceiling); this flanking sound will reduce the expected sound insulation of the as-built partition. Typical airborne and impact flanking paths are shown in Figure.
Sound Insulation Between Two Rooms
Tables in the local acoustic regulation describe the minimum weighted sound level difference between rooms in terms of DnT,w. This value includes not only the direct sound transmitted through the separating partition itself (the wall or floor), but also the flanking sound. However, manufacturers information tends to be based on laboratory airborne sound insulation data for a sample element on its own, measured according to BS EN ISO 10140-2 and presented as the weighted sound reduction index, Rw.
The test procedure eliminates any significant flanking sound transmission. The Rw value is a fundamental property of the building element itself (like its density), while the DnT,w value specified in the tables of acoustic regulation according to the as-built conditions, that is the common partition area, the receiving room volume, the acoustic conditions (reverberation time) in the receiving room and the flanking sound transmission.
This section provides some basic guidance for the designer on how to use laboratory measured Rw values to choose a suitable separating wall or floor for the initial design. However, specialist advice should always be sought from an acoustics consultant early on in the design stage to assess whether the combination of the separating partition and the flanking elements is likely to achieve the performance standards in the local acoustic regulations. An acoustics consultant can use advanced methods of calculation to predict the sound insulation, such as BS EN 12354-1. The correct specification and detailing of flanking walls, ceilings and floors is of high importance to prevent substantial reductions in the expected level of sound reduction.
The following procedure can be used to choose an appropriate type of separating wall or floor before seeking specialist advice on relevant flanking details.
- From table of the local acoustic regulation determine the required minimum weighted standardized level difference between rooms, DnT,w.
- Estimate the required weighted sound reduction index for the separating wall or floor, as follows:
a) Use the following formula to provide an initial estimate of the measured sound reduction index (Rw,est) that should be achieved by the separating wall or floor in the laboratory:
Rw,est = DnT,w +10 lg ((S*T)/V)+ 8 dB
DnT,w is the minimum weighted standardized level difference between rooms from table of the local acoustic regulation
S is the surface area of the separating element (m2)
T is the maximum mid-frequency reverberation time allowed for the receiving room from table of the local acoustic regulation, applied to all frequency bands for the purposes of the calculation.
V is the volume of the receiving room (m3).
b) Estimate the likely reduction, X dB, in the airborne sound insulation that would occur in the field, to account for less favourable mounting conditions and workmanship than in the laboratory test.
X can be assumed to be 5 dB provided flanking walls and floors are specified with the correct junction details.
However, if flanking walls and floors are not carefully designed, poor detailing can cause the airborne sound insulation to be reduced by a substantial amount. To allow the designer to choose a suitable separating wall for the initial design it is recommended that X is taken as 5dB and an acoustics consultant is used to check the choice of separating element and ensure that the correct flanking details are specified.
c) Calculate the final estimate for the weighted sound reduction index Rw that should be used to select the separating wall or floor from laboratory test data from:
Rw = Rw,est + X dB
The approximate weighted sound reduction index Rw values for typical masonry and plasterboard constructions are shown in Table. The values shown are necessarily approximate and will depend on the precise constructions and materials used. Many blockwork and plasterboard manufacturers provide more accurate data for specific constructions. Sound reduction data should be sought from laboratories that have carried out tests according to BS EN ISO 10140-2. The procedure given in above can then be used to select suitable constructions that are capable of meeting the required sound insulation for a room pair, expressed as the DnT,w.