Night Tube And Noise!!!

Night Tube and Noise!!!

After months of delays, London’s long-awaited Night Tube services launches on 19 August.

Transport for London (TfL) first unveiled plans for a 24-hour Tube service in September 2014.

The first lines to open will be the Victoria and Central lines, with the Victoria Line running through the entire route while the Central line trains will run between Ealing Broadway and Loughton / Hainault​.

Central Line trains are expected to run every 10 minutes between White City and Leytonstone, and approximately every 20 minutes between Ealing Broadway and White City, and Leytonstone and Loughton/ Hainault.

Meanwhile, the Victoria Line will run every 10 minutes.

Other lines to be added later in the autumn include:

  • Jubilee line: trains will run on the entire line.

  • Northern line: trains will run on the entire line except on the Mill Hill East and Bank branches.

“We also plan to expand the night time service to parts of the Metropolitan, Circle, District, and Hammersmith & City lines once our modernisation programmes are complete” TfL has said.

The idea is undoubtedly great for the economy but did anyone consider the effects of train movement induced tactile vibration and re-radiated noise affecting the thousands of residents near these underground tunnels?

Currently, thousands of Londoners are affected by underground train induced re-radiated noise. In the vast majority of cases, tactile vibration due to underground train movements does not adversely affect residents but may affect sensitive equipment housed in laboratories or medical facilities (or similar).

Currently no British Standards exists which recommends a method by which to assess intermittent re-radiated noise, such as that induced by underground trains which differs from airborne noise (e.g. noise entering through windows) in that it is low frequency and cannot be reduced by closing windows or going into a back room. It is also perceptible at night by transmission from the floor through the bed.

Whilst there is no widely accepted method of evaluation of re-radiated noise, there is some consensus that for levels at and above 50 dB LAsmax (the maximum noise level, measured using a slow response (1 second) and A-weighted) during the daytime, there is likely to be a significant adverse reaction. For residential situations the LAsmax noise levels for which there is likely to be very little adverse comment can be taken as around 30 dB LAsmax during the daytime and around 25 dB LAsmax during night time.

Re-radiated noise is of greatest concern when it is the dominant noise, and also when the source cannot be seen, as in the case of trains in a tunnel. Criteria for re-radiated noise are therefore generally intended to apply to sources such as underground trains.

Historically, 40 dB LAsmax has been regarded as the threshold of complaints for existing dwellings over established railways. In new build developments the threshold can be expected to be lower. Some London boroughs require 35 dB LAsmax, and while Crossrail has a design target of 40 dB LAsmax, it is required to seek to achieve 35 dB LAsmax. The Northern Line Extension is targeting 32 dB LAsmax. Therefore the underground lines which are part of the Night Tube programme may result in train induced re-radiated noise in nearby apartments of around 40 dB LAsmax and this may even reach 45 dB LAsmax at worst cases.

Night Noise Guidelines for Europe, published in 2009 by the World Health Organization states that based on research “noise-induced motility is a sign of arousal, and frequent (micro-) arousal and accompanying sleep fragmentation can affect mood and functioning next day and lead to a lower rating of the sleep quality. Therefore, motility is relevant for adverse health effects, but more than a few intervals with noise-induced motility are needed for inducing such effects. Although additional, more sophisticated analyses could be performed to refine this estimate, we propose LAmax = 32 dB(A) as the currently best estimate of the threshold for motility induced by transportation noise. The threshold found for EEG awakening was LAmax = 35 dB(A), that is, only a little higher than the 32 dB(A) found for noise-induced awakenings”.

It should be noted that the difference between LAfmax and LAsmax for typical London underground train pass bys is around 3 dB (LAsmax being the lower figure).

Previous guidance from the World Health Organization did recommend that night-time LAfmax events in bedrooms should not exceed 45 dB LAfmax around 10-15 times.

Therefore the current proposals by Transport for London may give rise to adverse health effects and quality of life due to the associated noise impact during the night-time. This has the potential to affect thousands of residents since recent research has found that the target level used in the past is now considered unacceptable (i.e. Crossrail, Northern Line Extension).

The recent status of London as the “capital of the planet” with the associated huge influx of wealthy individuals in central London has raised another potential grave issue associated with the Night Tube proposals. Will wealthy individuals spend millions of pounds on their London apartments and houses only when they know that they will be able to hear frequent train movements in every room during the night-time? What is a suitable target level for “high end” properties? Probably something in the region of 15 dB LAsmax.

Is Transport for London going to provide measures to reduce the current train induced impact? The options are really limited and probably prohibitingly expensive. If a 20-30dB reduction is sought, a floating slab track would be required. It is fair to assume that this measure is unfeasible retrospectively (for instance Crossrail has or will have sections of tracks build over a floating slab track when passing below certain residential areas and below the recording studios in Soho).

The common (and most cost effective) mitigation measure to attenuate London underground train induced vibration and re-radiated noise is to vibration isolate the building using elastomeric or spring bearings. As this is extremely costly for existing buildings it is only a viable option for new build developments.

Elastomeric bearings on a building overlooking Oxford Street

Vibration isolation of a building structure is a relatively complex process which fundamentally involves the introduction of resilient bearings between the substructure and superstructure.

Elastomeric bearings typically operate at a natural frequency of 7-12 Hz and can provide a re-radiated noise attenuation of around 10-15 dB.

Spring bearings typically operate at a natural frequency of around 3-5 Hz and can provide a re-radiated noise attenuation of around 20-25 dB.

Laboratory testing of structural bearings

The performance of the bearings is highly dependent on the dominant frequencies of incident vibration.

An exceptional high degree of attention to detail during the design and construction stage is required in order to ensure that flanking transmission and bridging doesn’t reduce the attenuation provided by low frequency spring bearings.

Apparently London property prices increase substantially when located near a tube entrance. I wonder if the Night tube proposals will actually reduce or even reverse this trend.

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