Frequently Asked Questions (FAQ)

How to combine noise levels? What is different about a sound level calculation?

Sound levels (decibels) are logarithmic so they do not scale the same as other units of measurement.  You will need a better understanding of this to perform mathematical calculations yourself, but some useful examples:

• 10dB + 10dB = 13dB
• 10dB + 100dB = 100dB
• 3dB increase = double the energy, but
• 10dB increase = appears twice as loud.

How to calculate sound level at a distance? How does distance affect sound?

Sound energy propagates outwards in a sphere and therefore reduces in energy based on the "inverse square law". 

This is easily understood as a reduction in 6dB per double of distance.  So the reduction from 1m to 2m distance is the same as from 100m to 200m distance.

How to model sound level reduction

There are many aspects to consider when modelling sound level both regarding the laws of physics and environmental effects on the sound.  Some of the things to consider are:

• Distance attenuation
  • This is a relatively straightforward physical phenomena as mentioned above.Screening and diffraction around screens
  • This is frequency dependent and is mostly based on path difference (the difference between the direct path and diffracted path)
• Reflections from surfaces
  • Any flat surface can be reflecting.  The extent of this depends upon the qualities of the surface and the frequency of the sound.
  • A perfect reflection will increase the level by 3dB.  This is sometimes called the "façade level".
• Absorption from the ground and reflecting surfaces
  • This depends on the softness of the surface and the frequency of the sound.
  • To understand ground absorption better, check out the section in the noise mapping tool guide.
• Air absorption
  • This is energy lost by friction in the atmosphere and considers the temperature and  humidity of the air and the frequency of the sound.
• Meteorological conditions
  • Wind speed and direction can considerably affect sound levels.  It is usually necessary to consider downwind conditions to minimise these affects.
  • Temperature inversion, where the temperature varies considerably at different heights, can also bend sound paths over long distances.

What is ISO 9613?

It is a international standard that describes a method for calculating the attenuation of sound during propagation outdoors in order to predict the levels of environmental noise at a distance from a variety of sources. The method predicts the equivalent continuous A-weighted sound pressure level (as described in ISO 1996) under meteorological conditions.

How accurate are sound level calculations and noise mapping?

The accuracy is always going to be limited to the details of the model, which will never quite match the real world conditions perfectly. 

Common problems:

• If the noise source has inherent directivity this is often not correctly modelled. 
• Sound power level calculations may not be accurate as the source of noise changes over time or due to wear.
• If ground height is quite variable, this may not be possible to model accurately. 
• Changing seasons will affect foliage and ground absorption.
• Changing wind and meteorological conditions will affect levels, especially over long distances.
• Complicated reflected paths.  There are many other aspects to consider when dealing with multiple reflecting surfaces.  Absorption of each surface, reverberation or "canyon effect", standing waves.  This accuracy is mostly important when there is no direct line of sight.

There is more information about accuracy in the noise mapping tool guide.

What is A-weighting?  Why is it so common?

The human ear doesn't respond to all frequencies equally.  For example: a sound at 1kHz will appear much louder than a sound of equal energy at 50Hz.  A-weighting is a frequency-based adjustment to the decibel level to account for this variance in audibility.  It is common to use a-weighted levels dB(A) when referring to noise impact as .

What is sound power?  How do you calculate a sound power level?

When you see a decibel level for a source of noise it is usually accompanied by a distance from the source, this is a sound pressure level and indicates how much sound energy is present at this distance. e.g. A compression unit is 70dB at 1m distance or 50dB at 10m. 

The sound power level does not contain distance information and instead represents the total acoustic energy of the source of noise, e.g. The same compression unit has a sound power of 81dB. Using this figure we can calculate the sound pressure level at any distance.  We can also estimate the sound power level by using a pressure level with the distance from the source.  All of our calculators provide methods for doing this.

How can I calculate sound transmission from inside to outside?

We are considering adding a calculator for this. If you think it would be beneficial please let us know.