Measuring noise levels in Montreal using a SPL meter
Recently several newspapers have written about a study on the noise levels in Montreal. The study was conducted by the Director of Public Health at the Agence de la santé et des services sociaux de Montréal and it states that, in certain areas on the island of Montreal, transport generated noise pollution exceeds the recommended norms set by the World Health Organisation. As in many large cities, the factors that contribute to noise pollution are motorized vehicle traffic, trains and airplanes. The main effect of noise pollution on public health is disturbed sleep. This can lead to cardiovascular problems and high blood pressure. Here, in Part 1, we will look at the SoundPressureLevel meter, which is the device used in the study. In “Noise and Health Part 2” we will see how the SPL meter was used and how it should be used to measure noise pollution.
The SPL meter, decibels and Pascals
As sound propagates in the environment it creates pressure variations somewhat like a wave. The SPL meter measures these pressure variations and translates the variations into decibels. The decibel is the favoured unit of measure in the sound industry.
It must be said that the unit used to measure air pressure is actually the pascal (or Pa) and it is used mainly in meteorology to measure atmospheric pressure (with the help of a barometer). The problem is that measuring sound pressure levels in pascals is unpractical as it doesn’t correspond to the way our ears perceive sound. The pascal is a linear unit but our ears perceive sounds in a logarithmic way: a sound source which causes a pressure of 2 Pa is not perceived as being twice as loud as a sound source that causes a pressure of 1 Pa.
We must also make a distinction between the constant atmospheric pressure of the earth (even when there is no sound) and the pressure which is created when the sound is made, in order to measure the level of pressure variation of a sound using pascals. The SPL meter is able to make that distinction.
Fletcher and Munson curves
When it comes to measuring a complex sound like noise, the ear, as complex as it is, has its limitations. The ear is not equally sensitive to low, medium and high frequencies. To complicate matters further, this difference in sensitivity changes with a soft, medium or loud sound source.
Bell Laboratories broached the subject in the 1930’s, which resulted in 1933 with the elaboration of equal loudness contours, curves bearing the names of the engineers that devised them, Harvey Fletcher and Wilden A. Munson. The equal loudness curves indicate the level of sound pressure necessary for a sound with frequency X (between 20Hz and 20kHz) to be perceived as being at the same level as frequency Y (between 20Hz and 20kHz).
Summing up the work of Fletcher and Munson we can observe two things. First, more air needs to be moved (higher pressure variation) in order to hear high and low frequencies than to hear frequencies between 1kHz and 3kHz. Second, the higher the sound level, at a rock concert for example, the lesser the difference between the frequencies to which the ear is not very sensitive (the highs and lows) and the frequencies to which the ear is very sensitive (between 1kHz and 3kHz for example). The ear has a more linear response to frequencies when in a high sound level setting.
The weighting of the SPL meter
When the Sound Pressure Level meter is used instead of another type of pressure indicator (like the barometer for example) it is because it indicates a measurement that corresponds with the human ear’s perception. A SPL meter should be able to integrate the results of Fletcher and Munson’s research. Depending on the sound level that is measured, certain frequencies must weigh more than others in order to obtain a frequency response that is comparable to what the human ear perceives.
The SPL meter usually allows modifying the weight of some frequencies compared to other frequencies of the same level. These are called the weighting curves. They are in fact filters at the input of the SPL meter that attenuate the highs and lows to various degrees depending on the chosen weighting curve.
These filters are inverted equal loudness curves at different volume levels. The three most common weighting curves are A (high reduction of the lows and highs), B (medium reduction of lows and light reduction of highs), C (slight reduction in the lows and light reduction of the highs), which you can see in the diagram below. There is also a D curve, which is used essentially to measure the perceived sound levels of airplanes.
Now that we understand the SPL meter and its parameters, we can explore the use of these parameters when measuring sonic annoyances in Noise and Health Part 2
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