Acoustic comfort means having the right level and quality of noise to use the space as intended. We can create a comfortable environment by controlling parameters like Sound Level Reduction, Reverbration Time and Sound Reflection.
Acoustic power can be expressed by its intensity I or its pressure P, measured in Watts per square metre or Pascals. It is common practice to measure sound by its level of pressure or intensity, against a logarithmic scale which starts at the threshold of hearing (I0, P0)
Sound arises from molecules vibrating in a gas, liquid or solid. The number of vibrations or soundwaves emitted per second is known as the frequency and is expressed in Hertz (Hz). The human ear is sensitive to sounds in the frequency range 16 Hz to 20,000 Hz. Architectural acoustic concerns itself with the 50 Hz to 5000 Hz range, which is divided into bands of frequencies or octaves. At each octave, the frequency doubles. For more detailed analysis, 1/3 octaves may be used.
To account for the subjective nature of the human ear at different frequencies (low, medium and high-pitched sounds), sound pressure levels are measured against an A-weighted curve.
These levels, expressed in A-weighted decibels dBA, more accurately reflect noise or unwanted sound. Sound exposure meters can measure levels in dB or dBA.
The Sound Reduction Index (R)
This index allows the sound absorpent properties of materials to be calculated and is measured in a laboratory. Measurements are taken in accordance with EN ISO 140, and represent the acoustic properties of an element (such as window or partition) for each 1/3 octave band centered between the values 100 and 3150 Hz (16 values). Further measurements may be taken at frequency ranges of 50 Hz to 100 Hz and 3150 Hz to 5000 Hz.
Calculations based on the 1/3 octave frequency bands enable the acoustic properties of an element to be expressed in different ways. These are defined in globally accepted values, by EN ISO 717-1, and adapted to two given noise spectra:
- “pink noise” index, where equal levels of sound power are applied across the entire spectrum of frequencies
- “road traffic noise” index is a good representation of how glazing attenuates road noise
Using the Rw index (C, Ctr)
Environmental noise must be reduced by building materials from levels which are uncomfortable to levels which are acceptable to the occupants and users.
The acoustic insulation property of a building material is defined by an index representing the difference between internal and external noise levels.
The acoustic insulation performance of a building element is thr R sound reduction index. Designers use the R index for building materials to help achieve the specified noise reduction level.
Weighted reduction index Rw
This is the most common method of rating sound insulation in buildings and building elements. It incorporates a weighted correction for the human ear and is expressed in dB. Calculation methods use a standard noise spectrum and are detailed in EN 717-1.
Spectrum Adaptation terms – C and Ctr
Depending on how a window is assembled and installed, it may under-perform at low, medium or high-frequencies.
Optimum performance can be achieved from a unit when it provides good acoustic insulation at the frequencies where the noise is at its greatest.
Good acoustic insulation against specific types of noise can also be achieved by modifying the type and composition of the glazing.
Until recently, glazing specification has been based on a single performance figure, without taking all the characteristics of the noise source into account, which can sometimes lead to unwise investment and over-specification.
For this reason, a common index has been created: Rw (C;Ctr).
The correction Ctr should be used when the source of the noise in question is road traffic. For outside background noise it is better to use the correction C. Both corrections are generally negative and are deducted from the Rw to determine the noise reduction properties of a building element. They are provided by test laboratories and appear alongside the value for Rw.
According to EN 717-1 the formula is: Rw(C;Ctr)=37(-4,-9)
This means that the sound insulation value for a façade is 37 dB and is reduced by 9 dB for traffic noise. In some countries, the result can be indicated directly:
RA,tr=28 dB, that is = 37 – 9
The same calculation can be applied for term C(pink noise):
RA=33dB, that is 37 – 4
This approach enables the designer to select the optimum window specification for the required application.