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R,w = The measurement used to rate airbourne sound insulation of a material or building element in a laboratory.
CTr = The spectrum adaptation term to take account of specific sound spectra, which are predominantly low frequency – only used as a correction to airbourne measurements.
D’nT,w = The measurement used to measure the airbourne sound insulation between two rooms (on site)
D’nT,w + Ctr = See above, but with the low frequency correction factor included.
L’n,w = The measurement used to measure the impact sound insulation of floors. L’n,w = laboratory testing.
L’nT,w = As above but tested on site.
Decibel. A unit for measuring the relative strength of noise. Usually expressed as the logarithmic ratio of the strength of a transmitted signal to the strength of the original signal.
A decibel is one tenth of a ‘bel’ which is also a unit used for measuring the intensity of noise.
Sound travels in different ways depending on the medium. The medium is the material the sound is using to travel. Air is the most common. When sound travels through the air it normally originates from somewhere in space, like a speaker or a person’s mouth, and then the waves travel in spherical ripples, this is similar to water waves: those travel in circular ripples because the surface of water is flat.
Transmission Loss (TL) is a figure which rates the ability of a material to block sound. It is usually measured in 1/3 octave band intervals. Mathematically it is defined as the ratio of the sound energy transmitted through a material to the sound energy incident on the material.
The Transmission Loss (TL) of a material is measured by mounting a sample of the material in an opening of a wall separating two reverberant test rooms. Broadband noise is played in one room (source). The difference between the sound levels in the source room the the other (receiving) room is defined as the Noise Reduction (NR). As the frequency and/or density increases the Transmission Loss also increases. The density of the material is directly related to Transmission Loss.
The ASTM (American) 1/3rd octave Sound Transmission Losses measured above are referred to by the European ISO standards as Sound Reduction Indices (R).
ISO 717/1 defines a standard contour and a procedure for fitting the contour to the measured sound reduction indices to determine a single-number rating of a sound transmission loss spectrum. This rating is called the Weighted Sound Reduction Index (Rw).
Unlike the STC contour, the Rw contour is defined over a slightly lower frequency range of 100 Hz to 3.15 kHz.
The contour fitting procedure requires that:-
• The sound reduction values be determined to one decimal place
• The contour be raised in 1 dB increments to a point where the average deficiency over the contour frequency range is as close to, but not exceeding, 2.0 dB.
• I f an 8dB or larger deficiency exists in the sound reduction index data, then the deficiency amount in dB and the frequencies at which they occur must be reported.
The average deficiency is the sum of all deficiencies in all frequency bands divided by 16, the number of 1/3rd octave frequency bands spanned by the contour.
The actual Rw value is equal to the fitted contour value at 500 Hz.
It should be noted that the SRI value has been developed to approximate the performance of a material in reducing the transmission of speech. The SRI value obtained from the TL data is useful for a quick comparison of materials but does not give a true idea with respect to non-speech sounds such as music, traffic, trains, aircraft etc.
STC is the American ASTM, standard E413, equivalent of SRI and is based on the averaged sound insulation achieved between 125Hz and 4kHz. As before, the standard defines a procedure for determining the STC rating for a TLoss spectrum by fitting a contour to the 1/3rd octave data. This procedure involves raising or lowering the contour following these rules:-:-
• The contour may not be raised above the point at which the Tloss in any 1/3rd octave band falls more than 8dB below the contour.
• The contour may not be raised above the point at which the total number of deficiencies is greater than 32.
A deficiency occurs when the TL data in any 1/3rd octave band falls below the contour by 1dB.
The STC rating resulting from the contour fitting procedure is the TL value of the contour at 500Hz.
It should be noted that the STC value has been developed to approximate the performance of a material in reducing the transmission of speech. The STC value obtained from the TL data is useful for a quick comparison of materials but does not give a true idea with respect to non-speech sounds such as music, traffic, trains, aircraft etc.
The Noise Reduction Coefficient, defines how much sound specific materials absorb. It is the average sound absorption between 250Hz – 2kHz rounded to the nearest 0.05.
This is analogous to a room’s finishes. Just as various colors of paint, or textures, visually alter a room, various materials with different NRC ratings, such as carpet or tile, audibly alter a room. A material with a low NRC rating (tile) absorbs little sound and a material with a higher NRC rating (carpet) absorbs more sound.
For those of us that do not know what flanking noise is, it is both airborne and impact noise that usually transmits through the fabric of a building. This is mostly a problem through walls that penetrate floors or walls that timber suspended flooring are usually fixed to.
Part E puts the onus on the builder to demonstrate that the stated acoustic rating of separating floors and walls have been achieved. The requirement of the Part E document is, therefore, that 10% of all new dwellings should be Pre-Completion Tested on-site. Testing applies to separating walls and floors between dwellings only and is not required between living spaces within dwellings, nor for corridors, stairwells or hallways. Testing needs to be carried out by a test body with appropriate third party accreditation.
To avoid the need for Pre-Completion Testing the industry and, in particular, the housebuilders led by the HBF, have sought approval of a series of performance tested solutions which will be deemed to satisfy Part E. This is in relation to separating walls and floors using existing construction products and systems. These solutions are called Robust Standard Details (RSD’s)
Robust Standard Details are performance tested and proven to work in the field, and have since been given approval by the Government. As previously mentioned a delay in the implementation of Pre-Completion Testing for New Build dwellings to 1 July 2004 was granted to facilitate the proper consultation and completion of the process leading to the agreement of Robust Standard Details.
For more on Robust Standard Details click here
Proprietary Systems are those developed and tested by manufacturers using specified components that achieve a certain level of performance.