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 intensity level
L1 = 10 log l/I0
Sound pressure level
L1 = 10 log P2/ P02= 20 log P/P0
The decibel (dB), is the unit
of measurement. Although the acoustic intensity of two or more sources increases by addition, the same is not true of the acoustic power.
2 trumpets each producing a level of 80 dB together produce 83 dB, and not 160 dB.
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 20000 Hz. Architectural acoustics generally concerns itself with the 50 Hz to 5 000 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 absorbent 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 a window or partition) for each 1/3 octave band centred between the values 100 and 3 150 Hz (16 values). Further measurements may be taken at frequency ranges of 50 to 100 Hz and 3150 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 :
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.