Navigation Techniques for Environmental Technicians
Noise monitoring & evaluation Study Module 4
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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◗ I know how to get assistance from my assessor if needed… ☐
◗ I have read and understood the SAG for this subject/unit… ☐
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Details
Student name / Type your name hereAssessor / Marker’s use only
Class code / NME
Assessment name / SM4
Due Date / Speak with your assessor
Total Marks Available / 59
Marks Gained / Marker’s use only
Final Mark (%) / Marker’s use only
Marker’s Initials / Marker’s use only
Date Marked / Click here to enter a date.
Weighting / This is one of six formative assessments and contributes 10% of the overall mark for this unit
Introduction
So far, we have learnt what sound is, the physical properties of sound, how we hear sound, how sound travels and how we represent sound through the different stages of sound propagation and travel from the source to the receiver. But how exactly do we measure it?
A quick recap of what we know;
The source of the sound, or emission, is measured absolutely as sound power in unit Watts, but expressed as a sound power level relative to the 1E-12 Watts reference.
The travelling sound waves, or immission, is measured absolutely as sound pressure in unit Pascal but expressed as a sound pressure level relative to the 2E-5 Pascal reference.
The surface of the immission sound wave field is measured absolutely as intensity in Watts per meter squared but relatively as sound intensity level relative to 1E-12 W reference.
The sound pressure received by the ear, exposure or dose, is measured absolutely as Pascals squared times time (Pa2.h), but expressed as sound exposure level as decibels.
This is a lot of information, fortunately for the environmental technician, we can (in most cases) either get away without measuring all of these aspects, or we can use just one or two pieces in equipment.
Types of sound measurement devices
The different types of measuring device are classified based on what they actually measure but generally speaking, we have Sound Level Meters (SLM), Sound Intensity Probes (SIP) and personal sound exposure meters (PSEM). Of key importance to this unit of competence is the Sound Level meter.
Sound Level Meters (SLM)
The SLM is the key device used by environmental and WHS field technicians (in conjunction with a data logging versions for field studies). These devices form the major part of these notes so won’t be discussed here in any great detail.
Data loggers
A data logger is a device that captures data. For our purposes, it is an ugly computer that requires another computer to read it! The point of a data logger is space, as in space for the data, they also need to be very rugged to put out in the field.
Data loggers are designed to monitor noise levels in remote or unattended environment for long periods of time (say, for periods of up to 2 weeks). The internal computer of the data logger will typically compute the same measurements as the handheld instruments including percentile noise statistics as well as the equivalent noise levels for time intervals ranging from one minute to one hour (typically 15 minutes). Modern data loggers can also operate as a sound level meter as they display the current noise level on either the loggers LCD screen or a connected PC. You will explore data loggers more in later modules.
Sound Intensity Probes (SIP)
These are very specific probes that are used to determine many qualities of sound such as sound power requirements emitted from machinery, or to determine the source of a noise.
Although, formally speaking, sound intensity is the product of sound pressure and particle velocity, we have ignored the particle velocity concept as it has little relevance to the field technician, but massive relevance to the overall filed of acoustics.
Commonly, there are two different probe set-ups used in sound intensity measurements, but all set-ups use probes that measure the sound intensity using two microphones.
The p-p type of sound intensity probe measures the sound intensity using two phase-matched microphones positioned face-to-face with a known distance between them. These microphones determine a pressure gradient, from which the particle velocity is calculated. The sound pressure is determined from the average from both microphones output.
The p-u type of sound intensity probe measures both the sound pressure with a microphone and the particle velocity directly with a particle velocity probe.
Figure 4.1 – Example of a SIP [source]
Personal Sound Exposure Meters (PSEM)
Another common sound or noise measurement device is the Personal Sound Exposure Meter or PSEM. The latest version of these things are literally tiny little ‘badges’ that people wear during their working day.
The PSEM has, like all technology, undergone significant change throughout its relatively short history, and they used to be clunky devices hooked on to your belt, with a microphone on a cable was pinned to your collar to receive noise as close to your ear as possible.
You will learn more about the PSEM in the next module where noise studies are applied to the Workplace Health & Safety environment.
Industry standards
The practice of noise measurements is both heavily regulated and standardised. As mentioned several times, the reason for the regulation is to protect human hearing and loss of amenity. Standardisation (i.e. the use of the same techniques worldwide) is a result of the physical nature of sound (it is a universal property) and is and the requirement for results to be reportable in similar units worldwide.
Sound measurements can be done for a variety of reasons, but generally they can be qualitative (informative) or quantitative (legal). For this reason, different classes or types of noise measurement equipment have been developed.
Australian & International Standards
In Australia, we use the Australian Standard series of documents (from Standards Australia) which cover three main areas of implementation and monitoring;
Compliance requirements of equipment
All acoustic equipment is designed in accordance with the International Electrotechnical Commission (IEC) specifications (or equivalent). The IEC develop the ‘overarching’ electrical standards for most of the electrical equipment used worldwide and is heavily involved in standardising the electrical function and calibration of noise monitoring equipment.
Workplace health and safety
The series of standards that apply in Australia for workplace monitoring of noise is the AS/NZS1269:2005 series, with standard 1 being the most significant as it deals with the operational aspects of the monitoring. This forms the majority of the next module.
Environmental
The Australian Standard for environmental monitoring of noise is covered by the AS 1055:1-3 series. Note that there are many other non-regulatory documents used in environmental monitoring but these are dealt with by module 6.
Legal categories of instrument
Unfortunately even with standardisation, there is a multitude of different terms for the same thing as a result of ‘sovereign differences’, but the only two legal classifications we need to worry about is the qualitative and quantitative classes, which are explained below;
◗ Type 0 equipment (factory calibration and the like)
◗ Class 1 / Type 1 SLM (high accuracy and precision, for legal use)
◗ Class 2 / Type 2 SLM (lower accuracy and precision, for common use)
The Integrated Averaging Sound Level Meter (IASLM)
What is a Sound Level Meter (SLM)?
A Sound Level Meter (SLM) is a measurement device used to measure various properties of sound waves that enables us to relate the measured property for either the protection of hearing (WHS applications) or the protection of loss of amenity (community and environmental applications). A typical Class 1 / Type 1 SLM can be seen in figure 4.2 below;
Figure 4.2 – Typical Sound Level meter (SLM). This is a Bruel & Kjaer model.
These types of meters are typically classified as Class 1 / Type 1 sound level meters. We know it is a SLM, so it measures the sound pressure from a source, but what do the other terms mean?
Integration is a mathematical term which implies that the area under a curve is calculated. In our case, the curve is created as a graph of the sound pressure versus frequency, so it is the frequency domain that is integrated. The specific area calculated varies depending on how the frequency octaves are used (1/1 octave or 1/3 octaves).
Averaging is applied to the decibels in the time domain (from a graph of decibels versus time or frequency) and is typically performed early in the electrical process by use of a Root Mean Square (RMS) circuit. You will learn about this circuit later. The typical electrical ‘process’ that occurs in a IASLM can be seen in figure 4.5 below;
Figure 4.3 – Construction of a typical Sound Level Meter (Rion NA 27 SLM).
Admittedly, this looks a little confusing, but the whole process can be broken down into its key constituents (which are listed below and explained in the following sections);
◗ Microphone receives sound pressure and converts to electrical signal
◗ Signal is pre-amplified (sometimes more than once, depending on the instrument)
◗ Frequency weighting is applied (or not, if linear pass (un-weighted) is requested)
◗ Analogue to digital signal conversion occurs
◗ Digital signal processing to determine RMS sound pressure occurs
◗ Digital signal processing of frequency analysis occurs
◗ Displaying the final information on the readout
The microphone
When an object vibrates in the presence of air, the air molecules at the surface will begin to vibrate, and this vibration will travel through the air as oscillating pressure waves at frequencies and amplitudes determined by the original sound source power. Microphones are mechanical transducers that are designed, like the human ear, to transform pressure waves into useful electrical signals which we can use to determine the properties of the sounds. Like the human ear, microphones are designed to measure a very large range of amplitudes, typically measured in decibels (dB) and frequencies in hertz (Hz).
Microphone type & construction
Measurements of sound pressure level can be carried out with a variety of microphone types. Most IASLM’s employ the condenser microphones because they are compact and delivers stable and reliable response, but other microphone types (such as resistance) are sometimes used.
A condenser microphone is a type of ‘capacitance’ microphone. The housing of a condenser microphone utilises basic transduction principles to transform the sound pressure to capacitance variations, which are then converted to an electrical voltage. This is accomplished by taking a small thin diaphragm and stretching it a small distance away from an insulated stationary metal backplate. A voltage is applied to the backplate to form a pre-polarised capacitor by using an electret plate with permanently charged particles attached to the backplate.
In the presence of oscillating pressure, the diaphragm will move which changes the gap between the diaphragm and the backplate. Using a load resistor, this produces an oscillating voltage from the capacitor, proportional to the original pressure oscillation.
Figure 4.4 – Construction of a typical SLM condenser microphone (RION NA 27)
Microphone characteristics
The key operational characteristics of microphones used for SLM’s define the responses of the microphone and are directly related to the quality of the transduced sound pressure. The key characteristics for IASLM include the frequency response characteristics, the directional characteristics, the thermal characteristics and the humidity characteristics.
The frequency response as well as the temperature and humidity characteristics of a pre-polarized microphone depend considerably on the type and properties of the materials used. The frequency range is determined by the resonance frequency of the diaphragm assembly.
Voltage Amplification
The voltage generated from the microphone is incredibly small, so small in fact that it has little practical purpose, and therefore it requires ‘enlarging’ so that a practical voltage is achieved for use by later componentry. This process is called amplification.
Preamplifier
Since the condenser microphone is a small-capacity transducer, it has high impedance, especially at low frequencies. Therefore a very high load resistance is required to ensure uniform response extending to the low frequency range. The relationship between the microphone capacitance and the low-range cut-off frequency can be expressed as follows.
If the output of the microphone were directly routed through a long shielded cable, the capacitance between the cable conductors would cause a sharp drop in sensitivity, as is evident from the following equation.
For the above reasons, a preamplifier is connected directly after the microphone, to provide a low-impedance output signal. To reduce measurement deviations due to refraction effects and the acoustic influence of the operator, the microphone/preamplifier assembly can be detached from the main unit and connected via an extension cable.