Weighted Difference level (Dw)
The most basic index is the Weighted Difference level Dw. This index is defined by measuring in decibels (dB), the noise level produced on each side of a building element under test (e.g. a wall) when noise is produced in a room on one side (or outdoors) and measured both in the room where the noise is produced and in the room on the other side of the element under test. This measurement may be carried out by measuring the levels in octave bands, or in 1/3 octave bands. (the latter is normally used for most applications). The minimum requirements of the standards require for the frequency range from 100 Hz to 3.15 kHz to be measured (16 octave bands). In some situations measurements may be carried out in the bands down to 50 Hz and/or up to 10 kHz. The measured levels in each 1/3 octave band (or octave band) from the source room (or area) (S) are then compared to the measured levels in the receiving room (R), and the difference is taken (S-R). this produces a measured difference level 'D' for each frequency band in the measured spectrum. To produce a singleSound Reduction Index (R)
The Sound Reduction Index is expressed in decibels (dB). It is the weighted sound reduction index for a partition or single component only. This is a laboratory-only measurement, which uses knowledge of the relative sizes of the rooms in the test suite, and the reverberation time in the receiving room, and the known level of noise which can pass between the rooms in the suite by other routes (flanking) plus the size of the test sample to produce a very accurate and repeatable measurement of the performance of the sampled material or construction.Apparent Sound Reduction Index (R')
This is a field measurement which attempts to measure the sound reduction index of a material on a real completed construction (e.g. a wall between two offices, houses or cinema auditoriums). It is unable to isolate or allow for the result of alternate sound transmission routes and therefore will generally produce a lower result than the laboratory measured value. The calculation method used to produce the Sound Reduction Index takes into account the relative size of the tested rooms, and the size of the tested panel, and is therefore (theoretically) independent of these features, therefore a 1×1 panel of plasterboard (Normalized Level Difference (Dn)
This is an index which is measured in field conditions, between "real" rooms. It is a measurement which deliberately includes effects due to flanking routes and differences in the relative size of the rooms. It attempts, however, to normalize the measured difference level to the level which would be present when the rooms are furnished by measuring the quantity of acoustic absorption in the receiving room and correcting the difference level to the level which would be expected if there was 10m2 Sabine absorption in the receiving room. Detailed, accurate knowledge of the dimensions of the receiving room are required.Standardized Level Difference (DnT)
Similar to the normalized level difference, this index corrects the measured difference to a standardized reverberation time. For dwellings, the standard reverberation time used is 0.5 seconds, for other larger spaces longer reverberation times will be used. 0.5 seconds is often cited as approximately average for a medium-sized, carpeted and furnished living room. Due to not requiring detailed and accurate knowledge of the dimensions of the test rooms, this index is easier to obtain, and arguably of slightly more relevance. Once the difference level or sound reduction index is obtained, the weighted value may be obtained from the corrected spectrum as described above from the reference curve.How does the Weighted Sound Reduction Index (Rw) relate to a Weighted Level Difference (Dw)?
The calculation to convert from Rw to Dw has to account for: * The area size of the separating partition, a bigger wall means more area of sound energy to transmit through. A bigger surface area means a higher Rw is required to achieve the same Dw. * The volume of the ‘receiving’ room. The smaller the space, the greater the concentration of sound energy, the higher the sound pressure level. A smaller volume means a higher Rw is required to achieve the same Dw. * The reverberation time of the ‘receiving’ room, which is the time taken for sound to decay by 60 dB. A high reverberation time means a build up in noise levels from sound reflecting around the room, with energy being dissipated slowly from a lack of absorbent materials. A higher reverberation time means a higher Rw is required to achieve the same Dw. * Possible flanking transmission on-site due to a potential downfall in construction quality or inattention in design to junction and penetrating detailing. The higher the risk of flanking, the more compensating in the Rw rating to achieve the same Dw.References
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