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 and 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).
BS EN ISO 717 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.
- if 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 colours 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.
The human ear does not hear all frequencies with the same intensity. It is most sensitive to sounds in the 500Hz-8kHz range. Above and below this range the ear becomes progressively less sensitive. To compensate for this, sound level meters incorporate electronic filtering to correspond with the varying sensitivity of the ear. This filtering is called A-weighting and Sound Pressure Levels obtained with this weighting are referred to as A-weighted and signified as dB(A).