We’re proud to be part of the Merford family, and here we take a another look at a fanstastic project delivered by Merford Noise Control at Amsterdam Centraal Railway Station in 2018.
A Guide to Low Frequency Noise
Low frequency noise (500 Hertz and lower) from various sources is an increasingly common form of environmental noise pollution in urban environments and it can also be quite a challenging problem to treat effectively.
Sonobex has seen a marked increase in enquiries relating to the identification and mitigation of this troublesome low frequency noise.
This blog article aims to help set out some information on the; common sources of low frequency noise, ways to identify and measure it, the health issues associated with it, as well as the specialist low frequency noise control solutions Sonobex can offer to mitigate it.
Causes & Sources
There are many possible sources of low frequency noise, but it is most often associated with some form of machinery. It could be industrial noise from nearby heavy industry, factories and plants.
In homes and offices themselves, common sources are heating and ventilation systems, boilers, and fans and pumps in appliances. In larger buildings the plant room for central services like HVAC and power can be a particular issue as well, generating relatively high levels of low frequency noise.
Elements that make up the electrical power grid are also quite prevalent sources of low frequency noise, including transformers in substations, generators and wind farms.
A major complicating factor with low frequency noise is that it can travel long distances with relatively little attenuation compared to higher frequency components noise. Typically, noise levels fall over distance due to geometric spreading and absorption by the ground or air.
Also, constructions like walls or barriers and buildings will help to block transmission from the noise source to sensitive receivers. All of these attenuation and noise control mechanisms are frequency dependent and are generally less effective at lower frequencies.
This means that as sound travels, its relative frequency content alters making the low frequencies more prominent at greater distances, creating low frequency noise problems. As a result of this it is not uncommon for complaints to be received from residences located far away from a problem source and over quite a wide area.
Transformers in electrical substations are a particularly common source of low frequency noise complaints as they are found in the vicinity of residential areas and workplaces. They produce quite a distinctive low frequency hum, which consists of tones at multiples of 100 Hz. The tonal nature of the noise they produce often increases the perceived annoyance.
The team at Sonobex have experience with transformer noise issues and have developed some specially tailored noise control solutions for them.
In terms of household low frequency noise sources, heat pumps are a growing issue, particularly with their increased usage across Europe.
The fans and compressors in many models of heat pump produce a significant amount of low frequency noise; some of the issues and solutions we can offer for heat pump noise are covered in a previous blog article. https://www.sonobex.com/blog/heat-pump-noise-enclosures-noisetrap-blox-application
How to identify
Low frequency noise is typically perceived as a low throbbing, beating, rumbling, or even as a pressure on the ears. A person’s response to low frequency noise can also be quite individual due to differences in the frequency sensitivity of their hearing, which can vary considerably from person to person and with age.
When these perception factors are combined with the fact that low frequency noise can travel relatively easily with little attenuation, it means that the identification and location of low frequency noise sources can sometimes be challenging without specialist measurement equipment.
Sound level meters can be used to measure and quantify low frequency noise. Class 1 sound level meters (as defined in the standard IEC 61672-1) will provide more accurate measurements at low frequencies as they are required to meet stricter tolerances and have a wider frequency range.
It can also be particularly useful to use a meter with octave band or FFT analysis functions when measuring low frequency sound, as this will help to give you a much clearer breakdown of the frequency content of the noise. In general though, extra care should taken when making low frequency measurements. Taking repeat measurements in the vicinity of the noise source and/or using a longer averaging time can help to accurately measure the sound pressure level.
Typically, sound pressure level measurements and levels in environmental noise regulations are expressed in dB(A) or A-weighted decibels. The ‘A’ frequency weighting applies a filter which reflects the frequency response of the human ear. Large weightings are applied to low frequency components, reducing their contribution to the total sound pressure level. This means measurements in dB(A) may not capture or highlight the presence of low frequency noise very well.
Alternatively, measurements made using either ‘C’ weighting (dB(C)) or ‘Z’ weighting (dB(Z) or simply dB) can be useful to help identify the presence of low frequency noise. The ‘C’ weighting filter is also designed to account for the response of the human ear, but with smaller weightings at low frequencies when compared to the ‘A’ weighting filter. The ‘Z’ or zero weighting is simply a filter with a flat frequency response, so it effectively counts all frequencies equally.
Most sound level meters are capable of displaying results in dB(A), dB(C) or dB(Z). A useful rule of thumb to help confirm the presence of a low frequency noise problem is when the total sound pressure level measured in dB(C) is considerably larger than in dB(A), i.e. a difference of 15 dB or more.
Some environmental noise regulations include criteria based on noise levels in dB(C) to specifically account for low frequency noise. They may also include specific penalties for low frequency noise sources, where an additional penalty factor, e.g. 5 dB, is added to the measured noise level in dB(A) during the assessment to account for the more problematic nature and higher perceived annoyance of low frequency noise.
It is also worth noting that some noise regulations also include penalty factors for multiple other complicating noise issues, such as tonality and impulsiveness. This means particularly problematic noise sources may be eligible for multiple penalties, for example a power transformer in a substation may have penalties applied for producing noise that is both low frequency and tonal.
In the UK, the Department for Environment, Food and Rural Affairs have recognised the prevalence of low frequency noise issues and some of the technical complexities with it’s assessment. As a response to this they commissioned the development of a special “Procedure for the assessment of low frequency noise complaints” by the Acoustics Research Centre at the University of Salford which can be found here http://usir.salford.ac.uk/493/ and provides useful practical guidance.
The team at Sonobex have a large amount of experience investigating, identifying and providing solutions to low frequency noise problems.
We have access to a number of sophisticated class 1 sound level meters as well other advanced acoustic measurement equipment to aid in this process, including sound intensity probes and an acoustic camera.
More information on the noise survey and measurement services we offer can be found here https://www.sonobex.com/noise-surveys-and-mapping and will hopefully also be discussed in a future blog post.
Health effects & problems
Exposure to relatively high levels of environmental noise, including low frequency noise, is considered a risk because of the adverse effects on health it can lead to. The World Health Organization (WHO) has supported a number of works that look at these issues in detail in order to raise awareness of the health risks.
The report titled “Burden of disease from environmental noise: quantification of healthy life years lost in Europe” from 2011 provides a good overview of the situation and can be found freely available at https://apps.who.int/iris/handle/10665/326424.
The health effects caused by noise can broadly be broken into two main categories; auditory and nonauditory effects. Auditory effects, such as tinnitus and hearing loss, are caused by injury to a person’s hearing system through direct exposure to high levels of noise. The nonauditory effects can be more subtle and are often associated to more long-term exposure to levels of noise which may not be as high in absolute terms.
Nonauditory effects highlighted by the WHO include; high blood pressure and cardiovascular diseases, cognitive impairment of children, sleep disturbance, and annoyance (when considering a broader definition of health accounting for physical, mental and social well-being).
The WHO estimate that the total impact of all these adverse health effects leads to the loss of at least 1 million health life years per calendar year in Western Europe. They point out that road traffic related noise is one of the most common sources related to these health effects, but note that a large proportion of low frequency components in a noise may increase considerably the adverse effects on health.
At Sonobex we have developed our range of NoiseTrap® products which are specifically engineered for the mitigation of low frequency noise
NoiseTrap® Solutions – NoiseTrap® Panels, NoiseTrap® Active & NoiseTrap® Blox
- NoiseTrap® Panels – These unique metamaterial based acoustic panels were developed with a focus on power transformer noise abatement, but can also be applicable to other low frequency noise control applications. They can be used to construct either noise barriers or full free-standing enclosures depending on requirements. They consist of coupled resonator elements designed to ‘trap’ the sound, resulting in both high levels of absorption and transmission loss. - https://www.sonobex.com/technologies/noisetrap-acoustic-panels
NoiseTrap® Active – This active noise control system has been developed for noise control situations which require acoustic attenuation with minimal impediment to airflow. The system consists of a sensor or microphone which monitors the noise, which is then connected to a feedback control system and speaker which generates a counter wave to cancel the outgoing sound wave from the noise source. It is particularly suited to low frequency and tonal noise issues. More details on the concepts behind active noise control and our solution have been covered in a recent blog article https://www.sonobex.com/blog/active-noise-cancellation-anc-technology-noisetrap-active-application - https://www.sonobex.com/technologies/noisetrap-active
- NoiseTrap® Blox – This innovative modular acoustic ventilation system has been developed specifically to provide increased low frequency noise attenuation. It consists of modular moulded plastic blocks which incorporate coupled resonators to target low frequencies and a weather resistant foam liner to attenuate higher frequency components. It can be used as an alternative to conventional louvres or silencers to provide ventilation in enclosures and barriers around low frequency noise sources. - https://www.sonobex.com/technologies/noisetrap-blox