Earplugs for Commuters

Recently I have moved down to London for a placement role as an acoustic consultant over the next year (before I go back to uni and finish my degree). Since being here I have learnt many new things about my subject as well as been getting into the London life and that as a commuter in a working job. Because of this I have obviously been spending a fair amount of time on tubes and trains around the capital, and have been getting to know the tube map quite well. Anyway, it was as I was on one of these tube journeys the other day that I had a thought – should commuters wear ear plugs?

London Underground

It was as I was on the Jubilee line that I noticed high noise levels, so being the acoustician I am, I pulled out my iPhone and opened up the dB meter app. As I looked at the screen I was shocked to see levels around 90-100dB. If you are not aware of the scale of deciBels, this is about the same level as a klaxon or a nightclub. For workers, there are regulations set out to employers for daily/weekly exposures that employees can experience. These are 80dB as a lower limit and 85dB as an upper limit, whereby hearing protection should be provided for the lower limit and hearing protection is mandatory at the upper limit. Note, these aren’t maximum levels that can be experienced, but average levels over a whole working  day/week. Although saying this, if a commuter had an hour commute each day (to and from work) with an average level of 86dB during these periods, then they would have already reached the lower action limit of their daily noise dose! For occasional exceedances, there is a level of acceptability in this, however for continued exceedance, long term effects can occur.

The image below shows a noise map for Zones 1-3 on the tube produced by London Southbank University. These levels are averages, so theoretically higher levels are possible that what is shown.

Noise map of zones 1-3 of London underground, produced by London Soutbank University

Noise map of zones 1-3 of London underground, produced by London Soutbank University

With newer trains being introduced to the underground, these levels are being reduced to a more acceptable level (see the circle line above) however should we do something about it ourselves?

One option is for commuters to wear earplugs, however I don’t realistically see any chance that TfL may start handing out free earplugs or commuters wearing them any time soon! The alternative I see is simple and is already common to many commuters already – headphones. With headphones in our ears, even though we are adding extra noise, we are also reducing the background noise around us, thus reducing noise exposure (so long as we don’t have our music too loud – but that’s an issue for another day)!

From what I found by looking at various sources, I reckon that a standard set of headphones can reduce ambient noise by 20-30dB (depending on quality/type) with noise-cancelling headphones giving even more attenuation. Due to an acoustic phenomenon called sound masking, we can assume that anything that is more than 10dB less than the dominant level is not clearly audible, thus making the background noise from our commute seem no problem compared to our music. There will still be elements that we do notice however, mainly bassy rumbles and thuds, but this is because headphones do not produce low frequencies like this so well.

With the world around us getting noisier than ever, we need to protect our hearing as otherwise we will lose it later on in life. A daily commute is something that so many don’t even think to be a problem, when really it can be, but we’ve just learnt to live with it. What is your daily commute like, do you ever worry about it being to loud?

Notes

[1] London Southbank University Research

[2] Sennheiser Website

[3] Health and Safety Executive Wesite – http://www.hse.gov.uk/noise/index.htm

Advertisements

Plane Annoyance

There are some noises in life that are really annoying and others that we just learn to live with; aircraft and airport noise are often subject to a lot of debate as different people would put them into different categories. But with it being such a hot topic, how do the airports keep track of their noise emissions?

First of all, we must understand what causes aircraft noise. [1] There are 2 sources of noise from aircraft: air friction (as it passes over the wings or fuselage) and engine noise. These both occur in the direction of the planes travel (i.e. most sound travels in the direction of approach/descent) rather than travelling sideways from the runway.

There are 2 main methods that are used to keep track of airport noise, both with their advantages/disadvantages. The first method is to measure the Sound Level created by one aeroplane taking off, then to add levels accordingly depending on the number of flights that will occur. An average Sound level for the whole period of time can then be calculated, depending on the number of flights. This is often used in prediction of airport noise, as it is quick and easy. However it is not always accurate because each plane makes a slightly different level.

The second method is to measure what is known as an LAeq. This is where a fixed microphone is set up so the sound level can be measured over a prolonged period of time for many departures/arrivals. Because the sound level varies with time (i.e. increasing when planes take off) it is then averaged. From this, the constant equivalent level (hence the eq part of LAeq) that would have to be made to create the same overall noise level can be calculated. At Norwich International Airport (NWI), my local airport, 3 fixed microphones are placed on the airport perimeter, so that the noise levels are constantly monitored and do not exceed regulations.

If an LAeq measured over a time period of 1hour is taken and reads 75dB,* this does not mean that it is constantly below 75dB. This could however mean that it is 90dB for 2 minutes and 45dB for the rest of the hour, which would give an average equivalent noise level of 75dB for the hour. The graph below shows this example:

LAeq

One problem with airport noise is that some planes take off at night, which is a time where the noise is generally considered more of a nuisance. For this reason another measurement can be made using the LAeq, which takes this into account. The night day and evening measurements are done separately and then the evening one has 5dB added to it and the night one has 10dB added to it, because they are perceived as louder at night. These new values are then averaged again over the whole time period of a 24-hour day.

With these measurements, noise maps can be made, where ‘contour’ lines mark different dB levels. In 2005 NWI commissioned an external body to produce a noise map for its surrounding areas. [2] The values used for these maps depend on a day-to-day basis, but are an average over a period of time. Factors such as wind direction/speed and flight path can affect these, but measures are taken by airports to take this into account (e.g. flight paths over lesser populated areas).

The map below shows the LAeq noise contours of the airport over a 16-hour period [3]:

NWI LAeq Noise Contours

This next map is similar to the one above, but this time is based on measurements over a 24-hour period with the evening and night penalties added, so it is more representative of the general perception of the level [3]:

NWI Lden Noise Contours

You will see, as stated before, that the noise mainly travels in the direction of flight and the levels are greater towards the ends of the runways; this is because of the noise created by the engines and the friction on the wings in takeoff/landing. In order to get an idea of comparison, 65-70dB is roughly the level of a busy restaurant or a busy A-road, which is 20m away. The background level for a housing area would generally be expected to be up to around 50dB without noise from the airport.

A general rule of thumb for sound is that it will decrease at -6dB for every distance doubling (e.g. if you are at 5m and it is 85dB then at 10m it will be 79dB). This rule is true if there are no reflections, so in reality it is often not quite this much. This rule can be seen on the maps where the distance between the contours increases the further away from the runway.

For many, the airport can be seen as a nuisance and should be made to reduce noise; in ways this has happened as planes have become quieter, but despite this there has been an increase in the number of planes. For local authorities, noise complaints need to be assessed as whether the source (i.e. the airport) has many pros (e.g. local jobs). Also, the type of nuisance needs to be addressed as to whether it is interfering with the individual’s enjoyment of their land or whether it has affected many.

What are your views on your local airport; do you feel like the disadvantages of the noise outweigh the benefits of the airport?

Notes

*dB stands for decibels which is the units we use to measure sound.

Acknowledgements

Thank you to Norwich International Airport for kindly supplying me with the noise survey data in order to write this blog.

References

[1] What causes aircraft noise? – http://www.gatwickairport.com/business-community/aircraft-noise/air-traffic-noise-explained/causes-of-aircraft-noise/

[2] Norwich Airport Noise Policy – http://www.norwichairport.co.uk/content.asp?pid=52

[3] Department for Environment, Food and Rural Affairs Airport Noise Action Planning for NWI (Noise Contour Mapping) – http://www.norwichairport.co.uk/downloads/airport_datapack_2013_norwich_airport_egsh.pdf

Selective What?

So we’ve all heard of selective hearing, it’s the process whereby we listen in to things depending on whether we want to or not. I often remember being told by my mum that I only listened to things I wanted to hear and never when I was asked to do jobs around the house… (surprise surprise!) But is this actually a real thing?

Well in short yes, it is and is also known by some as the ‘Cocktail Party Effect‘ which is where we selectively pick out and listen in on the conversations we want to hear amongst a general background noise of many conversations. When I read about this, it reminded me of a story we were told at school in assembly once, which went something like this…

There were once two guys walking down the road together, trying to hear themselves and have a conversation over the noise of the cars going past. Then one of them stopped, turned to the other and said “Can you hear that grasshopper?”. “Of course I can’t! That’s crazy, you can’t hear that” replied the second man, “It’s far too noisy to hear something so quiet!” However, the first man knew he was right and replied “No, I did hear it and I bet you could hear it as well if you wanted, you just don’t want to”.

At this he pulled a small coin out of his pocket and dropped it onto the pavement below; as he did so several people around them stopped and looked down as if to check if it was them who had dropped their money. “See” said the first man, “They can all hear the coin, just like I can hear the grasshopper, the only difference is what we’re listening out for”.

This, although a very basic idea of what I’m talking about, does represent the theory quite well. The effect is something that is of particular interest to neuroscientists and also in the realm of psychoacoustics. It is linked to the brain’s capability to focus on particular information that it is interested in or can relate to (i.e. when you hear someone mention the name of the town you’re from). Studies have now been done in audio signal processing to create a program that can separate out different signals from a general noise (e.g. separating a piece of music from a signal that has talking over the top of it). This helps us to work  out how the brain works and also may have the potential to be used for security and data analysis in the further.

Coffee Shop

Because of this effect, people often find it hard to work when conversations are going on around them, which could be part of the reason why coffee shop noise is considered by many to be good for creativity. Background noise for many, is useful as it is so common now in our everyday life that a lack of it can often seem unnatural. With careful acoustic design through sound diffusion and by using large volume spaces (i.e. high ceilings) sound in these places can easily be dispersed giving a general background noise but where individual conversations are hard to pick out. Nowadays, as open plan offices are becoming more and more common, this knowledge is becoming more and more vital in order to create a creative and calm working atmosphere.

Where do you find it best to work, do you like the idea of open plan offices/coffee shops or do you work best with complete silence? Have a listen to Coffitivty and see if you find it easier to work to!

Stationary Falling

As it is currently half term, I have had several family members vising this week, meaning I have been out and about in Salford and Manchester, often visiting places I haven’t been before. One of these trips out was to Airkix, an indoor skydiving tunnel by the Trafford Centre, where I was lucky enough to be treated to a flight with my uncle and cousin.

It was such a surreal experience, falling in a vertical tunnel, yet not moving anywhere, whilst spectators sit around drinking tea and watching you. Despite my clear lack of skill, it is something I would definitely recommend if you ever get the chance.

Obviously, when doing activities like this there are many risks involved, hence why we had to fill in forms saying we were fit to partake. Apart from the obvious risks (dislocated shoulders, falling, or crashing into walls) there are other aspects that need to be taken into consideration; the main one of these, which you may have already guessed, is your hearing.

I don’t know if you’ve ever been on the phone on a stormy day, but it is often really hard work when all you can hear is the wind, making the conversation very hard. Now this is one thing, but imagine winds of up to 180mph (the speed that this tunnel can reach) and imagine not only how hard it would be to hear, but also the effect from pressure it would have on your ears. Thankfully, before anyone goes into the tunnel, they are given earplugs in order to protect hearing, but do these make any difference?

First of all I calculated the sound pressure of 100mph winds (about what it took to lift me) as around 1,200Pa, which when converted straight into decibels, is around 150dB. This initially does look extremely high (the human threshold of pain is around 120dB) even with the earplugs, which have around 30-40dB attenuation; however a few factors need to be taken into account first to understand how this is safe. This pressure is obviously needed to lift a person, but may not necessarily directly relate to sound pressure.

There may be a very high pressure at a stationary point in the tunnel, but the ears are covered by the helmet which are also facing outwards, not down, so most of the air goes rushing past. This leads to probably the main principle of acoustics, which is that you need something to vibrate in order to make sound. In the tunnel, if wind is rushing past at around 100mph, which may make a lot of noise due to the fans, the vibrating air particles are flying straight past you so fast that none of them are around enough to cause high Sound pressures. There is also quite a distance from fans to receiver and the way these fans work is shown on the Airkix website.

The duration of the flights are only 1 minute long (around 15 seconds longer than the free-fall when parachuting) which shouldn’t increase the Leq (the equivalent noise level if played constantly over a given time) excessively based on the whole duration of the visit to the centre. This is often why parachutists may wear earplugs for the noisy and longer plane journey up, but not for the flight down.

One final major factor could be the psychoacoustics of the noise, being the sound of wind, which is familiar meaning the flier feels less threatened and can be perceived as less. This could be linked to the frequency distribution of wind, which may be such that it does not affect the human ear so much. There are also likely psychoacoustic effects of giving out earplugs, whether they make much difference or not.

Image

What do you think? Have you ever been skydiving or experienced high winds and noticed any effects on your hearing?