Noise may be simply defined as excessive or unwanted sound and occurs when it is deemed to be loud or unpleasant and/or it causes disturbance. There can often be a significant subjective response to noise, however, with many factors affecting the way humans react to it.
For this reason alone, it becomes important to be able to objectively assess noise and to quantify and evaluate its potential impact as opposed to describing it subjectively, i.e. what did it sound like to you.
‘Environmental noise’ addresses all types of noise that could potentially affect people in their homes and in public places such as urban areas. Other terminology is applied, such as ‘community noise’ and ‘neighbourhood noise’ and very often a precise legal definition is required, depending on the circumstances. The key element is that environmental noise typically applies to the cumulative effect of noise emitted from many sources, but it specifically excludes occupational noise or noise in the workplace.
In the past decade, the significance of the noise environment has assumed greater priority and environmental noise is now regarded as a worldwide problem. This is because of the scientific evidence which concludes that noise exposure can induce hearing impairment, hypertension and ischemic heart disease, annoyance, sleep disturbance, and decreased intellectual/academic performance. The need to assess and manage environmental noise in a systematic manner has, therefore, never been greater.
In a 2011 report, the World Health Organization (WHO) concluded that: “There is sufficient evidence from large-scale epidemiological studies linking the population’s exposure to environmental noise with adverse health effects. Environmental noise should therefore be considered not only as a cause of nuisance, but also a concern for public health and environmental health.”
Recent data from the EU estimates that approximately 20% of the population (circa 80 million people) suffer from unacceptable noise levels that cause sleep disturbance, annoyance and adverse health effects.
Standardising noise measurement
The ability to meaningfully measure and describe noise has evolved significantly over the past decades and many individuals and companies have access to a range of noise monitoring equipment. In fact, many of us carry phones which can incorporate smartphone sound measurement apps and studies have been undertaken to examine the accuracy of such measurements. In addition, some recent research studies have involved environmental noise monitoring utilising mobile phones, and ongoing research addresses their suitability to assess occupational noise.
Modern smartphone manufacturers use microelectromechanical systems’ (MEMS) microphones in their devices, and developments in MEMs have had a significant benefit to the range of noise monitoring equipment which is available.
We may take for granted some of the capabilities of modern instrumentation, but it is not so long ago that noise monitoring was an esoteric pursuit involving bulky, unreliable and non-standardised apparatus, which was particularly limited in comparison with modern instrumentation. In a 1957 Acoustical Society of America paper, Hermon Hosmer Scott traced the history of the sound level meter and alluded to a 110 pound unit dating back to 1932 that was ‘housed in four wooden carrying cases’. He also addressed the lack of standardisation and the fact that most SLMs were custom built in very small quantities, making the instruments extremely expensive. He referred to a sound level meter costing a thousand dollars or more in 1929, “when you could buy many automobiles for well under this figure.”
As the pioneers of acoustics and in particular those involved in developing instrumentation became aware, some form of consensus and standardisation was essential in order for the science to progress. One of the earliest published references on the development of standards for instrumentation dates back to a meeting of The American Institute of Electrical Engineers (AIEE) in 1935.
The logic and justification for standardisation more than 80 years ago is equally as applicable today. According to McCurdy (1936): “The purpose of a set of standards for sound level meters is to bring about a condition such that, if a given noise of a general character is measured with any meter designed in accordance with the standards, the result will be substantially the same as that which would be obtained with any other similarly designed meter.”
Modern instrumentation incorporates sophisticated circuitry and developments in microprocessors have rapidly increased the functionality of sound level meters (SLMs). The key elements in a SLM are as follows:
1. Microphone – converts the variations in sound pressure into voltage variations.
2. Preamplifier – conditions or amplifies signal from the microphone prior to it being processed by the meter.
3. Weighting network – conditions the pressure signal by the application of a frequency weighting scale, most commonly A, Z and/or C scale.
4. Frequency filters – separate and transmit within a certain band of frequencies while attenuating all other frequencies. The most common types are octave bandwidth, one-third octave bandwidth and narrowband.
5. Output amplifier – amplifies the signal before it passes on for further processing.
6. RMS detector – converts to a root mean square signal by squaring the pressure and averaging square over a defined time constant and determines the square root. Two time constants (time weightings) are found in most SLMs, fast and slow, with time constants of 0.125 and 1.0 seconds, respectively. Impulse time weightings are also found on some SLMs.
7. Display – gives a visual (generally digital) readout of RMS and peak levels.
There are three types of microphone – free-field, pressure and random incidence – and strictly the free-field should be pointed at the noise source. Microphones are particularly sensitive and should be handled very carefully.
The SLM must be calibrated before and after each use, so each set of measurement data can be confirmed to be within specification. An accredited laboratory must also periodically verify or calibrate SLMs and calibrators. Most standards require that SLMs should be ‘verified’ every two years, while calibrators must be verified/calibrated at least once a year.
With most SLMs, the readings can be taken on either slow or fast response. The response rate is the time period over which the instrument averages the sound level before displaying it on the readout. Some standards specify that noise level measurements should be taken using the slow response and regard should be had to the relevant criteria which apply to the appropriate jurisdiction.
An ‘A-weighting’ filter is generally built into all SLMs and can be switched on or off, and it is important for the operator to understand the instrument settings. Some SLMs provide only dB(A) measurements and as such the A-weighting filter is permanently applied. A ‘basic or simple’ SLM measures only instantaneous noise measurements, but most modern meters will have an integrating facility that enables them to calculate electronically the average noise level (i.e. the energy average) over a period of time. Thus, an integrating sound level meter determines equivalent sound levels over a measurement period, even when the noise levels are constantly changing. Another major difference between the basic and the integrating SLM is that the latter generally operates over a much wider dynamic range.
Combining the A-weighted Network and the integrating facility allows the instrument to provide measurements of the Equivalent Continuous Sound Pressure Level (LAeq,T). The units of time (T) can be specified by a statutory requirement or based on a standard specification (e.g. a 24-hour or one-hour value) and LAeq is universally used to describe both workplace and environmental noise. It is defined as the equivalent steady sound level in dBA containing the same acoustic energy as the actual fluctuating sound level over a given period.
Recent innovations in SLM technology include the ubiquitous touch screen, extended battery life, intuitive user interfaces incorporating graphical icons, LAN, USB, WLAN, GPRS, 3G or 4G communication options, audio recording facilities – either on a specified trigger or on demand – video camera options, and voice, text, GPS and camera note facilities. Along with developing the user friendliness of the SLM, manufacturers are providing instruments whose capabilities are designed to be expanded and upgradeable. Thus someone can purchase an SLM with limited options based on their current requirements and rather than replacing the instrument, in some cases they can purchase an additional option or module as the need arises.
While a high quality SLM will go a long way to facilitate an assessment of most environmental noise issues, another innovation has been in the development of acoustic cameras. These contain vast numbers of microphones (in some cases up to 225 individual units) and these can enable the user to perform noise analysis with a clear view of the spatial distribution of the sound. Such technology can be used to identify acoustic leakage from buildings and/or the likely source of significant noise emission from a facility. Occasionally there may be localised issues, which complicate the identification of such noise in close proximity to the source and/or using standard acoustical equipment and subjective assessment techniques.
Acoustic cameras will typically be utilised in conjunction with an SLM, both of which now come with a raft of post-processing software options, which allow the assessor to interrogate and analyse the collected data. In the case of acoustic cameras, sound signals from every microphone and video data from the integrated optical camera are recorded and stored. During the data analysis phase, user-friendly software packages can be used to examine the ‘pictures’ e.g. by examining recorded noise levels and listening to the recorded sound in the selected directions/positions.
Noise monitoring networks can also be designed to automatically notify certain parties when specified criteria are exceeded and this allows prompt intervention as and when required. In addition, monitoring stations can incorporate sound and video recording triggers and so an audible (and sometimes visible) log of the noise source along with the measurement data can be assessed. This can be helpful to identify when extraneous or spurious noise causes an exceedance and/or when building up evidence either to prove or disprove a case.
Many equipment suppliers will attest to the adept user-friendliness of their instrumentation and will claim that intuitive features and/or minimal guidance will allow users to run measurements once the equipment is out of its packaging, using their ‘point and click’ set-ups. The reality, however, is that standardised and quality instrumentation is but one of the prerequisites for effective noise monitoring.
In many cases an untrained individual may take measurements and recordings, but a competent noise assessor may be required and in some cases the qualifications of the assessor will become critical to the case. In some scenarios and jurisdictions, the concept of ‘a competent person’ is addressed in law and/or in certain standards. Typically, the competent person is defined in terms of the suitability of their training, experience and knowledge, so as to provide informed and appropriate advice to a certain set of circumstances. The complexity of the environment, the presence of tonal or impulsive noise and/or the selection of appropriate noise controls may in some instances have a major bearing on the question of competency. This is particularly the case in many jurisdictions where there are no strict statutory limits for the control of environmental noise.
In practice noise limits and criteria have been established for industrial and commercial activities, having regard to local circumstances and the need for environmental protection. In addition many different standards can be used to measure and assess environmental noise and various ‘authorities’ and agencies have issued ‘guidance and guidelines’ to assess environmental noise. However, the context and circumstances of the noise emission will largely dictate how the monitoring will proceed and/or which standards apply.
It is seldom ever a case of simply reading a number from an instrument and comparing it with a noise criterion or a specified noise limit. Very often the assessor will need to consider a wide range of factors before he/she even commences any measurements.
According to British Standard BS 4142 (2014), for example, the assessor should gain a sufficient understanding of the situation (context) to be assessed by conducting an appraisal, as appropriate, in order to:
• Identify and understand all the sounds that can be heard and identify their sources
• Identify which measurement methods, instruments and metrics would be most appropriate for the assessment
• Identify potential measurement locations
• Identify the necessary measurement frequencies, durations and timings
• Understand, where a new development is to be assessed, what kind of sound a new industrial source would introduce or what potential impact would be imposed from an existing source on a new sensitive receptor
During the investigation of noise complaints it may be critical to objectively evaluate the circumstances so as to determine whether reasonable grounds for complaint exit. In some cases, a noise complaint may arise due to an acute sensitivity (hyper-sensitivity) and/or there may be instances where vexatious complaints arise without any apparent justification. Sometimes when one out of a number of households complains about environmental noise, the presumption is that it cannot be justified as no one else complains. This, however, is misleading and the absence of complaints from other parties does not necessarily indicate the absence of legitimate grounds for complaint.
A range of factors, both acoustic and non-acoustic, will affect the tolerance and or reaction of an individual to noise and the overall impact of a noise source. BS 4142 gives priority to the degree by which the noise exceeds the pre-existing noise levels.
Other pertinent factors in assessing or predicting noise impacts include:
• The absolute sound pressure level of the noise source and variations over time
• Frequency of the noise (spectral components)
• Extent of the impulsive elements and special features or characteristics of the noise
• Variations in the hearing sensitivity of individuals
• Site location and local land use
• Nature and character of the locality
• Activities underway when noise is audible
• Local attitudes to the source or sources of the noise
• The likely duration of the noise and the ability and/or willingness to control its impact
The above list is by no means exhaustive and many social, psychological and economic factors will affect the sensitivity of an individual or a situation over time.
A competent assessor should be able to provide a definitive and considered opinion as to whether a noise complaint is justified – often their experience and knowledge of a wide range of different standards will be brought to bear, along with their noise measurements. Given that the assessor’s considered opinion may form the basis of a defence and/or may form a blueprint for the development of noise attenuation or control measures, it is important that their assessments are well-informed and reliable.
It is not just the instrumentation that has been evolving over recent years. Very often standards are periodically reviewed and updated and ‘guidelines’ can also be superseded and/or withdrawn. While effective noise monitoring requires an understanding of acoustics, codes of practice, pertinent standards as well as practical skills – it is essential that these skills are updated and that the noise assessor participates in Continuing Professional Development (CPD).
Probably one of the best ways of demonstrating CPD is to adhere to or exceed the CPD requirements of a professional body. The Institute of Acoustics (IOA) Certificate of Competence in Workplace and Environmental Noise Measurement courses, for example, have been specifically developed to provide participants with the necessary training to ensure that they can competently perform their duties. Given the evolving nature of acoustics and its legislative framework, refresher courses are also seen as an important element of (CPD) even for seasoned practitioners.
Published: 16th Feb 2016 in AWE International