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Sound and Noise

Sound and Noise

Sound intensity, Sound pressure and Sound pressure level ..

Loudness is an intuitive concept, a loud noise usually has a larger pressure variation and a weak one has a smaller pressure variation. Depending on what we are looking for – the cause, the effect or the perception of noise – we use different variables and units:

  • Sound intensity refers to the cause of noise (not of our concern, only of interest for acoustic engineer). It measures energy flow at the source, so its unit is W/m2.
  • Sound pressure refers to the effect of noise as a wave impacting any given surface, that is, noise as energy being transferred through air. Not of our concern either, more for physicists. Its unit is the Pascal or N/m2 (1Pa = 1N/m2). 
  • Sound pressure level or SPL refers to the perception of noise in humans as it can be “read” by our ears. So SPL is what matters to us. For ease of numbering SPL is measured in decibels (dB). A dB is a dimensionless unit used to express logarithmically the ratio of a value (the measured sound pressure) to a reference value (the lower threshold of hearing). Decibels are used since sound pressure level expressed in Pa would be too wide. 0 dB (the lower threshold of audition for humans) equals 0,00002 Pa; whilst 140 dB (the upper human threshold or threshold of pain) equals 200 Pa. This is a range of 140 against 10 million. But logarithms are not “natural” to understand, so some examples will be of help.

Sound intensity, sound pressure and sound pressure level are obviously related, but they measure different things and they should not be confused. The table below, taken from the very useful Sengpiel audio webpage provides some tips for getting it right, at least conceptually :

Sound and Noise

SPL variation (left column) related to sound pressure (field quantity) and sound intensity (energy quantity) Source: Sengpiel Audio

Lessons from the table above:

  • A raise in sound pressure level (SPL) of 3 dB equals an increase in sound pressure (field quantity) of 1.414 times, and (everything else being equal) it comes as a result of doubling the sound intensity (the source of sound).
  • A reduction in sound pressure level measured inside a room of 10 dB equals a reduction in sound pressure of 3.16 times, and it comes as a result of dividing the sound intensity (noise generated on the outside) by ten.

A typical opaque façade (not glass) can have a sound reduction index (a reduction of SPL) of around 40 dB. This means that if the SPL measured at the street is 70 dB, inside the façade one would perceive only 30 dB. Up to here, just arithmetic. 

Now, if the sound reduction index of the façade could be raised from 40 to 43 dB, the perceived noise coming from the street would equal that of reducing the source of noise by half. Even more, if the façade could be acoustically improved so that its sound reduction index raised from 40 to 50 dB (difficult but it can be done), the perceived noise coming from the street would equal that of reducing the source of noise (sound intensity) by ten: ten times less cars in the street, ten times less people celebrating the victory of their football team outside.

Sound intensity

Expected sound pressure levels for different noises and their equivalent sound pressure and sound intensity. Source: Sengpiel Audio.

We got the point : Sound pressure level measured in dB (sometimes indicated as dB-SPL) is critical for architectural physics – a small variation can make a lot of difference. But loudness (sound expressed as pressure variation) is not the only story. Noise – what we want to avoid inside our buildings – is the mixture of sounds of different “quality”, some are bass, some are treble. Is our façade or our glass pane capable of stopping each of these “noise qualities” in the same percentage ? Could an envelope act as a barrier for bass and a filter for treble ? What do bass and treble have to do with noise ? Keep looking at our blog pages.

Sound and noise

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