Many around the world are experiencing life with very low noise levels due to restrictions as we are confined to our home and there is a decrease in the industrial, transportation and leisure activity. This provides a wonderful opportunity to quantify and record for the future the lower noise levels of our soundscapes. With the reduction in shipping there is also a change in the underwater soundscapes.
Nowadays there are a high number of noise monitoring systems (noise monitoring terminals, city wide systems, underwater systems etc.) installed all over the world which will capture this information for the future. However, there are many acousticians working from home with access to a sound level meter that can be used to capture the soundscape from their balcony or from their garden and compare the before and after the restrictions.
The IYS 2020 committee has provided a central contact between a number around the world who were thinking similarly that there would be some benefit in coordination and a little standardization in the capture of the data. Marçal Serra from CESVA has taken a lead to set up a LinkedIn group COVID-19 Noise Reduction (at www.linkedin.com/groups/13844820/) and with hashtag #COVID19NoiseReduction for any posts.
The following is a general structure for those who wish to participate and share their data in the future. But do not break your confinement to report this data!
Place: Country and city (e.g., Spain village near Barcelona)
Primary noise source: (e.g., Traffic noise: note number of lanes per direction or Social noise: note if café/bar/restaurant/sporting)
Noise measuring system: The noise measuring system used to measure Lduring, Lbefore, and Lafter
Noise level during COVID-19 confinement:Lduring, expressed as a weighted overall level (preferably LAeq,1 hour), spectrum or psychoacoustic metrics as Loudness. It could also be reported as an image of the noise time history or a weekly color map and/or compiled into a report/article/conference paper with the measurement details and the comparison data
Noise level before & after COVID-19 confinement:Lbefore & Lafter, expressed in the same way as Lduring and over the same time period.
Most industrial activities create noise that can be harmful to the environment as well as to their workers. To minimize this effect, governments, associations, and companies have created regulations, standards, and codes to set the allowable noise both inside the sites, that can be harmful to the workers, as well as to the environment. In a lot of cases, during the planning phase, the plant owner and project management want to be sure that the noise levels are acceptable. Since the plant is not built yet, what can be done is creating a noise model to simulate the plant, so that the noise levels can be predicted. In this article, we will explore how we can do so.
The first thing we must know is how much noise does the noise sources inside of the plant will emit. The noise source is usually described in two ways which is Sound Power Level (Lw or SWL), and Sound Pressure Level (Lp or SPL) in certain distance, most commonly Lp in 1 m distance. There are multiple ways to get this information for certain noise sources. First, if the equipment type and model have been chosen, the equipment manufacturer will normally report the noise level in their datasheet. However, this is not usually the case with most of noise predictions since the noise study is normally done before the equipment suppliers are appointed. So, the second way to be able to predict the noise emission is by following empirical formulas that are developed by researchers. You can find such formulas in some textbooks, journals, and papers. For rotating parts, you will need its rated power and rotational speed to be able to estimate the noise emission.
For example, in the speed range of 3000-3600 rpm, the noise level of a pump with drive motor power above 75 kW can be predicted using the following equation:
Suppose a pump with rotational speed of 3000 rpm and 100 kW, according to the formula, it can be estimated that the noise level at 1 m from the pump would be 92 dB. And suppose the noise source can be considered as point source on the ground (hemisphere propagation), the sound power level of the pump can be calculated using the following formula:
Where r is the distance from source to receiver
And in this case, the predicted Lw would be 100 dB.
Thirds, noise measurement to a similar equipment can also be an option to be able to determine the noise level of the new equipment. Another option, in some countries, there are noise emission limit for certain equipment, you can use that limit if it is applicable for your project.
After the Lw of all noise sources is obtained, we want to calculate the noise levels (the Lp) at the receivers. There are some standards which procedure can be followed to calculate this. Few of which are ISO 9613-2, NORD 2000, CNOSSOS EU, and many others. Most of the standards consider some factors to the calculation such as distance, atmospheric absorption, ground reflection, screening effect (from barriers and obstacles) and other factors such as volume absorption from vegetation, industrial site, etc. Most consultants and projects will require a software such as SoundPLAN to do this calculation.
Depending the project, there are few types of noise limit which compliance will need to be ensured. The most common ones are environmental noise limit, noise exposure limit, area noise limit and absolute noise limit. Besides, the noise level during emergency is also modelled so that the information can be used for safety and PAGA (Public Address and General Alarm) study.
Environmental noise limit is usually calculated for the plant’s contribution to the plant’s boundary as well as to the nearest sensitive receiver such as residential and school near the plant. How this is accessed depends on the regulation applicable on the plant area. In Indonesia for example, the noise limit for residential area is Lsm 55 dBA and industrial area is Lsm 70 dBA. Lsm is a measure like Ldn, but the night noise level addition is 5 dB instead of the 10 dB addition that most other countries, especially Europeans use. To ensure compliance with this regulation, the noise level at fence should be less than Lsm 70 dBA, and suppose there is a residential area nearby, the contribution from the site should be less than 55 dBA. It is also advisable to measure the existing noise level at the sensitive receivers to make the study more relevant to the situation.
Noise exposure limit is the maximum exposure to noise that the workers get during their working period. In Indonesia, the noise exposure limit is 85 dBA for 8 working hours. To change the working hours, 3 dB exchange rate is used. For example, if the noise level in the plant is 88 dBA, then the workers can only work there for 4 hours, if it is 91 dBA, then the time limit is 2 hours, and so on. To extend the working hours on a noisy area, the options are to actually reduce the noise level by reducing the noise emission from the source or noise control at transmission (for example using barrier), or by usage of Hearing Protection Device (HPD) for the workers such as ear plugs and ear muffs. The noise exposure of workers after usage of HPD can be calculated using the following formula:
Where NRR is the noise reduction rating of the HPD in dB.
Different area might have different noise level limits, and therefore area noise limits are useful. For example, in an unmanned mechanical room, the noise level can be high, for instance 110 dBA. However, inside of the site office, the allowable noise level is much lower, for example 50 dBA. This noise level shall be calculated to ensure compliance with the noise limit. Different companies might have different limits for this to ensure their employees’ health and productivity. If the area is indoor and the noise source is outdoor, then the interior noise level can be estimated using standards such as ISO 12354-3.
The absolute noise limit is the highest noise level allowable at the plant, and shall not be exceeded at any times, including emergency. In most cases, the absolute noise limit for impulsive sound is 140 dBA. To ensure compliance with this requirement, potential high-level noise shall be calculated, for example safety valves.
During emergency, different noise sources than normal situation will be activated, such as flare, blowdown valves, fire pumps, and other equipment. In such cases, the sound from the alarm and Public Address system must be able to be heard by the workers inside of the plant. Normally the target for the SPL from the PAGA system should be higher than 10 dB above the noise level. Therefore, the noise level during emergency in each area should be well-known.
The COVID-19 lockdown could become an unprecedented natural experiment in noise pollution. Some of the world’s most vocal animals — birds and whales — might already be benefiting from a quieter environment.
According to the World Health Organization (WHO), noise pollution affects over 100 million people across Europe and, in Western Europe alone, road traffic accounts for premature deaths equivalent to the loss of roughly “1.6 million healthy years of life.”
Take the disturbance to human health out of the equation, and noise remains a big source of pollution for the other inhabitants of the planet as well, namely, animals.
But how much have animals in countries on lockdown really benefited from the drop in noise levels? Turns out, that’s a very difficult question to answer.
Birds will benefit the most
Birds — by far the most visible animals found in cities, and the most vocal — stand to be among the biggest beneficiaries of quieter streets and parks.
The signals birds send each other through song is a means of survival. Without the ability to sing, hear and be heard, birds would have a difficult time finding a mate or defending their territory from predators.
Human activity influences bird behavior, even prompting them to communicate at less ‘busy’ times of day
The swift rise of human-made noise — also known as anthropogenic noise — over the past century has made this harder for birds.
Just like humans who have to speak up in a loud setting, birds, too, have to sing louder to communicate properly in today’s noisy world, according to ornithologist Henrik Brumm, who heads the research group for the communication and social behavior of birds at the Max Planck Institute for Ornithology near Munich.
“This happens really fast,” Brumm told DW. “We found out that it takes roughly 300 milliseconds, so less than 1 second, for birds to readjust when the level of noise rises. So, when their surroundings become louder, they sing louder, too.”
Are birds getting quieter? Maybe.
Birds are already known to sing more quietly in the early morning hours of the weekends, says Brumm. The reason: there’s less traffic to compete with.
With Europe on lockdown, Germany for its part, has seen passenger air travel slashed by over 90%. Moreover, car traffic has dropped by more than 50% and trains are running at less 25% their usual rates.
A recent study from the Max Planck Institute also suggests that chronic traffic noise can have a negative effect on embryo mortality and growth in zebra finches. This, in turn, could mean that the current lockdowns coinciding with mating season could lead to not only more, but also healthier hatchlings. That is, as long as their parents choose a spot that’s still safe from humans after the lockdown ends.
Though it’s difficult to speculate without real-time data, Brumm says, it stands to reason that the current period of quiet could mean birds might be singing more softly than usual, which would already be a huge benefit.
At land or sea, noise is bad news for animals
Birds aren’t the only animals that stand to benefit from less noise. According to a recent study published in the journal Biology Letters, noise pollution affects any number of creatures ranging from frogs, to shrimp, to fish, mammals, mussels and snakes.
In fact, another habitat garnering more and more attention for noise pollution is the ocean. As bioacoustics expert Christopher Clark described it in with Yale’s environmental magazine, the din from oil and gas activity, for example, is filling entire ocean basins with “one big storm of noise.”
While research on noise pollution and marine life, just like with ornithology, is in its early stages, a landmark study conducted in the days after 9/11 found that less shipping traffic seemed to make whales calmer.
Examining the feces of right whales — a species of baleen whale that can reach 15 meters in length and weigh up to 70 tons — researchers found that fewer ships in the waters along the US-Canadian coast correlated with lower stress hormones.
The noise levels from shipping traffic, whose 20–200 Hz hum disturbs sea life despite being a low frequency, decreased by 6 decibels, with a significant reduction below 150Hz .
An unprecedented time for researchers
Just like ornithologists, marine life researchers have also found correlations between noise and interruptions in behaviors like foraging and mating. Whales, like birds, also “mask.” That is to say, they sing louder to be heard over noise disturbances, be they high or low frequency sounds.
“It’s really a huge footprint that these activities have in the ocean,” according to Nathan Merchant, an expert on noise and bioacoustics at the UK’s Centre for Environment, Fisheries and Aquaculture Science (CEFAS).
And the sources of noise pollution — ranging from shipping, to wind farms, to the sequence of powerful blasts from seismic air gun tests used to locate oil and gas deposits in the ocean deep — are even harder to escape in the ocean than on land.
“It has a lot to do with how sound travels under water. Sound can travel much further and much faster than in air,” Merchant told DW.
Instruments off the coast of North America, for example, can detect seismic air gun testing as far away as the Brazilian coast.
With many cruises suspended, oil freighter traffic impacted by an oil price crash and rig activity being run by skeleton crews to curb the spread of COVID-19, marine biologists could potentially find a treasure trove of data once they’re allowed to go back into the field.
“We have underwater noise recorders at sea as we speak, but they aren’t cabled to land. So, we’ll find out when get out on a ship in several months’ time and get the data back,” Merchant said.
The more interesting question by that point might be how marine life responds to a sudden reintroduction of the human cacophony after an unexpected period of rest.
Binaural hearing allows for localizing the source of the sound, suppressing noise, example to better understand speech. To localize sound there is an important aspect of auditory perception that allows us to adjust to the room, namely spatial hearing. There are two processes in localizing sounds in humans, monaural cues and different cues.
Monaural cues are how each ear translates the captured sound signal. Monaural cues are the result of a convolution of sound sources with head-related transfer function (HRTF) impulses. Head-Related Transfer Function (HRTFs) is a form of transformation of sound wave propagation from the source to the ear or Head-Related Impulse Response (HRIR). HRTF is also defined as a form of modification of a sound from a certain direction that reaches the ear. This transformation involves diffraction and reflection from the anatomy of the ear. HRTF also depends on the location of the sound source relative to the listener so that it can determine the sound source.
Difference cues are how the difference between two ears translates to sound signals. These differences cues contain information on International Time Difference (ITD) and Interaural Level Difference (ILD). ITD is the difference in the arrival time of the left and right ear sound waves while ILD is the difference in pressure level between the left and right ears. Based on Duplex Theory, ITD values are used for localizing sounds at low frequencies, which is below 1.5 kHz while ILD is used for localizing sounds at high frequencies, which is above 1.5 kHz. Environmental sounds are in the range of low frequency and high frequency so that the human auditory system uses ITD and ILD.
The basic principles in ITD are illustrated in Figure 1
When the sound source is sound waves with low frequency, the propagation of sound waves will reach both ears without decreasing the sound pressure level. This is because the wavelength of sound is smaller than the dimensions of the head. However, there is a time difference received between the two ears. Therefore, sound waves at low frequencies are related to ITD.
The basic principles of ILD are illustrated in Figure 2. The ILD value is influenced by the size of the head and for sources that are very close to the head. When the sound source is in the high-frequency range where the wavelength of the sound is smaller than the dimensions of the head, the sound will reach the ears closer to the sound source. When will reach the other ear, the sound will be held up or there is a failure of propagation of sound waves for a while, this phenomenon is called an acoustic shadow. The sound that finally reaches the other ear will experience a decrease in the level of sound pressure caused by the phenomenon of acoustic shadow.
T. Potisk, “Head-Related Transfer Function,” 2015.
X. Zhong and B. Xie, “Head-Related Transfer Functions and Virtual Auditory Display,” Soundscape Semiot. – Localis. Categ., 2014
W. György, “HRTFs in Human Localization : Measurement , Spectral Evaluation and Practical Use in Virtual Audio Environment,” 2002.
K. Carlsson, “Objective Localisation Measures in Ambisonic Surround- sound,” 2004.