Noise monitoring & evaluation Study Module 3
Assessment details
Purpose
This unit of competency covers the ability to monitor noise using handheld sound level meters and fixed sound monitoring stations with either data logging or telemetry. It includes the ability to perform noise surveys, process data and report results in accordance with enterprise standards.
Instructions
◗ Read the theory section to understand the topic.
◗ Complete the Student Declaration below prior to starting.
◗ Attempt to answer the questions and perform any associated tasks.
◗ Email, phone, book appointment or otherwise ask your teacher for help if required.
◗ When completed, submit task by email using rules found on last page.
Student declaration
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Details
Student name / Type your name hereAssessor / Marker’s use only
Class code / NME
Assessment name / SM3
Due Date / Speak with your assessor
Total Marks Available / 38
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Final Mark (%) / Marker’s use only
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Weighting / This is one of six formative assessments and contributes 10% of the overall mark for this unit
Sound propagation
This section explains the operational side of acoustic theory. It is here that we introduce the relationship between the absolute and relative measures of sound and the legal and operational aspects of noise measurements and evaluation.
The ‘source-path-receiver’ model
Consider the following image which shows the creation of sound, the travelling of the sound, the interactions of the sound, and finally the receiving of the sound. We need to understand the basics of all of these steps, from source, to path to receiver.
Figure 3.1 – The basic ‘source-path-receiver’ concept. [source]
In the example above, the source of the sounds is created by the train. You learnt earlier how sound pressure waves are produced, and that the exact source of the sound is due to the application of energy to physical media which results in the vibrations that produce a pressure wave which we perceive as sound or noise.
These pressure waves travel outwards from the source in all directions (unless blocked by some object). We don’t measure the sounds everywhere though, we only measure where a receiver is located, such as where people work or live.
The key point here is that there are three basic ‘stages’ in the source-path-receiver model, and each stage has unique properties associated with them.
The source
It takes energy to make physical objects vibrate. The subsequent emission of sounds (which may be perceived as noise by the listener), is referred to as sound power, which is measured in the unit of Watts or Decibel-Watts.
The path
The travelling of the sound is called sound immission and is measured as Pascals or Decibels.
Any interactions with the environment the sounds undergo change the characteristics of the sound before it is received, is measured as sound intensity (again in the unit of decibels), and we need to know about how sound changes (such as adding and subtracting sounds and distance related calculations).
The receiver
Finally the sound is received, and as you receive sound in much the same way you receive medicine, it is measured as a exposure (or dose).
Patterns of sound propagation
There are basically two models of sound propagation termed point source and line source.
Point source
A point source is as simple as it sounds, the noise is coming from one single identifiable source. These sources could be anything, a siren, a television or machinery on a factory floor. They can also be very large, an entire mine site for example, and therefore, the point source is a relative scale when it comes to size and purpose of assessment. Noise emanating from a point source can travel in spherical fields as seen in the figure below;
Figure 3.2 – Image depicting spherical noise field [source]
Line source
The line source is used to describe sources which are not point based and are usually associated with traffic on roads or train lines and the like. Roadway noise is the most common example of a linear noise source, since it comprises the majority of the environmental noise exposure worldwide.
Figure 3.3 – Example of a line source.
Understanding the source fields is very important in understanding the behaviour or effect that the noise should have.
What we won’t study
Note that in most noise theory textbooks, a great deal of consideration is given to the controlling of noise, and as such we would need to cover the concept of reflecting surfaces. We don’t cover that here (at all) because noise control is not part of this unit.
Absolute measures of sound
Source sound power
All noise come from a source which is created through some (usually) mechanical action which involves energy being used and dissipated. An example of this is the ‘whine’ of a bearing in a motor which is getting old and creating noise from friction between two metal surfaces.
The unit of energy is the Joule (J) and expresses an amount of work.
Noise (that bothers us) cannot be a single event, and must therefore be created over a timeframe of some sort, typically from one second to continuous. When we express energy over time, we invent a new unit, power.
Power is the amount of energy per unit time (typically one second).
Formally, it is expressed with the derived unit of J.s-1, but is also expressed in the named unit of Watts (W).
1 J.s-1 = 1W
At the source, we are not interested in the sound pressure as it has not been ‘created’ yet, and as such we must, and can only be interested in the energy that is used to create any future sound pressure. This is contradicted later when we calculate sound power from sound pressure, but remains conceptually accurate for our reasons.
Although sound pressure level is the quantity most directly related to the response of people to airborne sound, and SPL is the quantity that is most often to be controlled. However, sound pressure level is not always the most convenient descriptor for a noise source, since it will depend upon distance from the source and the environment in which the sound and measurement position are located.
This is why a better descriptor is usually the sound power level of the source, as sound always has a source, which could be anything. What concerns us here the amount of sound energy they produce. Sound waves, like other waves, transport energy, which means that the amplitude (in pressure) is proportional to the sound power.
Noise emission
Once the sound has been generated (with an amplitude determined by the sound power) it will form a wave through the process of emission with the characteristic frequency and wavelength as governed by the material from which it is emitted.
Sound pressure
Once the energy has been ‘spent’ on the vibration that creates the noise, a sound pressure wave is produced (see earlier chapters). The sound pressure is measured differently to the sound power.
Pressure is measured in the unit of Pascal (Pa).
The amount of Pascal’s involved in noise that we can hear ranges from 2E-5 Pascal’s at the threshold of hearing to 200 Pascal’s at a plane taking off at an airport. To put that into perspective, atmospheric pressure ranges from 87 000 to 108 500 Pascal’s, so noise does not produce that much pressure relative to the Earth’s atmosphere.
Be careful of units…
Due to the large (and small) numbers involved, we need to employ metric prefixes in the units, so we use the unit of micro Pascal (mPa) for noise pressure.
Sound intensity
Finally, once the energy from the source has produced a vibration, and that vibration has produced a sound pressure wave, we need to acknowledge that the pressure wave travels the atmosphere in a radiating fashion (see figure 3.2 & 3 above).
This means that the sound pressure at any distance from a source passes through a field surface that exhibits an area. When sound pressure passes through an area, the measurement is called sound intensity.
The unit for sound intensity is I, and is reported as W.m-2
Figure 3.4 – Example of sound intensity [source]
The intensity is a measure of flow really as it is a time averaged directional quantity (from the source) that measures the rate of energy that flows through the area in question. Sound intensity measures are somewhat irrelevant for environmental and workplace noise technicians and are usually specialised measures performed by directional intensity probes, as seen in the figure below;
Figure 3.5 – Sound intensity probe [source]
To understand intensity we need to visualise a source and two receiver points at two separate distances from the source. In the figure below, we can see that the source is emitting a sound, which means that it is emitting energy, but how does this energy dissipate?
What is emitted is a fixed amount of energy (the sound power), so if the volume of space is increasing as we move away from the source, then the sound power needs to fill this space,
Figure 3.6 – A source with two receiving points. Consider this as an aerial view (i.e. from above) and that the sound is propagating across the plane of the ground.
which means that sound power must therefore decrease in a proportional amount per unit area as a function of distance.
The specific proportionality that occurs is a special one, the inverse square law, which states that the sound intensity is approximately proportional to the reciprocal of the radius squared, or;
I=1r2
Where;
I = Sound intensity (W/m2)
r = radius (i.e. distance from source in m)
Remember that the Inverse Square Law only describes the change in sound intensity from one point to another, to calculate an actual example we need to employ a slightly different equation;
I2=I1×r12r22
Where;
I1 = Sound intensity at 1st distance (W/m2)
I2 = Sound intensity at second distance (W/m2)
r1 = distance from source to first point (m)
r2 = Distance from source to second point (m)
More on this will be covered in later chapters when we explore distance calculations in more detail, but for now, hopefully you can see that the energy from the source dissipates as the volume the sound has to fill increases over distance away from the source, and does so following the Inverse Square Law.
Relative measures of sound
You need to be reminded of what the term ‘relative’ actually means. One formal definition of relative is;
“considered in relation or in proportion to something else”
In terms of measurements, and especially when applied to sound ‘levels’, the term relative means that all measurements of an absolute value (whether that be power, pressure or intensity) are related to a ‘base’ value.
The reason for this is simple, absolute values are not useful when we are trying to relate the noise to human hearing. As such, we need to ‘scale’ or relate the absolute pressure to the scale of human hearing. This is especially important when the scale you are relating to has a minima and maxima in the ‘middle’ of an absolute range. You learnt from earlier chapters that the range of human hearing is approximately from 5 to 140 decibels, and from 20 Hz to 20000 Hz, and our hearing does not start at zero decibels, nor at zero Hertz, we must make the scales ‘relative’ to the scales of our hearing.
The concept of ‘Levels’
The relevance to ‘levels’ is that the absolute value that noise ‘levels’ are related to is the equivalent to the minimum value to human hearing.
Every absolute noise value is relative to the minimum equivalent value at the threshold of human hearing.
Therefore, the reference values that absolute measures are made relative to for noise studies are;
◗ Sound power = pico Watt (1E-12 W)
◗ Sound pressure = micro Pascal (1E-5 Pa)
◗ Sound intensity = pico Watt (1E-12 W)
But how are these values used? Well, in logarithmic calculations. The logarithm of a number is the exponent to which another fixed value, the base, must be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 10 to the power 3 is 1000 (i.e. 1000 = 10 × 10 × 10 = 103).
If x = by, then y is the logarithm of x to base b, and is written y = logb(x), or y = logb(by), so log10(1000) = log10(103) = 3.
The decibel
To understand the following concepts we need to understand the main unit used in noise assessments – the decibel. The decibel is based on the unit of Bel, whose symbol is B, and the decibel is the decadic version of the Bel.
Many measurement scales in science involves numbers whose range is that of many orders of magnitude, like the pH scale which covers 15 orders of hydrogen concentration (1M to 10-14M, the logarithmic transformation of which covers pH 0 to pH 14 – much easier!).
The unit of decibel was created was to avoid the use of large cumbersome numbers. The good news is that you won’t be assessed on the logarithmic calculations, but just so you know, all logarithmic calculations for determining relative levels (i.e. decibels) work on the same types of calculations;
Level=factor ×log10absolute valuereference value
Sound Power Level (LW)
The acoustic energy emitted by a sound source can be measured in either relative or absolute terms. As mentioned, the absolute measure of emission sound power is performed in units of watts (or milliwatts, microwatts or similar), and the relative measure, as emission Sound Power Level, LW is measured in units of decibels which are made relative to a reference quantity of 1 picowatt (1E-12W), as per the equation below;