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Sound Testing Explained
Airborne and Impact Sound is transmitted through most walls and floors by setting the entire structure into vibration. This vibration generates new sound waves of reduced intensity on the other side. The passage of sound into one room of a building from a source located in another room or outside the building is termed ”sound transmission”.
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Understanding Sound Testing
Sound transmission loss or Sound Reduction Index, R dB, is a measure of the effectiveness of a wall, floor, door, or another barrier in restricting the passage of sound. The sound transmission loss varies with frequency and the loss is usually greater at higher frequencies. The unit of measure of sound transmission loss is the decibel (dB). The higher the transmission loss of a wall, the better it functions as a barrier to the passage of unwanted noise across the dividing (acoustic) partitions.
Approved Document E stipulates that there are two types of sound insulation in buildings: airborne and impact. Airborne sound insulation is used when the sound produced directly into the air is insulated and it is determined by using the sound reduction index. Impact sound insulation is used for floating floors and it is determined by the sound pressure level in the adjacent room below.
A sound insulation test of a separating partition will be considered as a pass if the airborne sound insulation is equal to or greater than the DnT,w + Ctr value shown for the appropriate dwelling in the table, for airborne testing in new builds properties a figure of 45dB or greater is required and for Impact testing 62dB and below. For conversion properties a figure of 43dB or greater is required and for Impact testing 64dB and below.
What is DnT,w
DnT,w is an in-situ measured performance parameter that demonstrates the level of resistance to sound transmission between two adjacent spaces, such as flats or houses. The measurement will include both direct sound transmission and flanking sound transmission of the construction. Flanking transmission is the effect of sound travelling through the building and may be particularly evident where beams and joists bridge a common partition or along poorly isolated lightweight wall constructions.
The DnT,w of a separating wall or floor will typically be of the order 5 to 7 dB lower than the manufacturers specified Rw (single figure quantity of sound insulation) for the single element, due principally to the contribution from flanking sound transmission around the element when it is built on site – this should always be accounted for in the initial acoustic design.
What is Ctr
Ctr is the spectrum adaption term. It is a correction attributed to the sound insulation quantity to account for urban traffic noise.
What is L’nT,w
L’nT,w is an in-situ measured performance parameter that demonstrates the level of resistance to impact sound transmission between floors. The impact measurement includes both direct sound transmission and flanking sound transmission.
Weighted Sound Reduction Index Rw
When specifying the acoustic performance of an acoustic partition in a more general manner, it can be useful to describe the sound insulation by a single number. The weighted sound reduction index, Rw , is a rating method given in EN ISO 717-1. This standard fits a standard reference curve to the measured sound reduction index curve.
Within the EN ISO 717-1 standard, a rating method is also given where the Rw value is completed by two C-terms which are applied to two models of the noise spectra for various types of noise. These two terms, Rw + C and Rw + Ctr, also include the frequency range 100 – 3150 Hz but can be extended to 50 – 5000 Hz. As industrial and traffic noise often have high sound levels which are also below 100 Hz, it is recommended that the extended frequency area is used.
The summary value, Rw + C, gives the reduction value in dBA for a spectrum with a level that is equally high in all third-octave bands. This can be used for:
• Highway road traffic travelling at speeds in excess of 80 km/h
• Factories emitting mainly medium and high-frequency noise
• Living activities (talking, music, radio, TV)
• Railway traffic at medium and high speed
• Jet aircraft at a short distance
The summary value Rw + Ctr also gives the reduction value in dBA, spectrum with low-frequency dominance such as:
• Disco music
• Urban road traffic
• Railway traffic at low speeds
• Factories emitting mainly low and medium-frequency noise
Both the impact and airborne insulation performance of floors and walls should be carefully considered from the start of the project to ensure that the minimum performance requirements for new and converted developments are met in line with Approved Document E
If you would like more information in regard to our sound testing and/or noise survey services, please contact us now at info@aptsoundtesting.co.uk or call 01525 303 905 today.