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

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?