King's Cross St. Pancras

Mitigating tube noise & vibration in iconic apartment block.

Plasterboard wall-linings in luxury apartment, amplifies vibration from underground trains.

Location

N1 London

Tube Line

Northern, Piccadilly & Victoria Lines

Circumstances & Project Brief

The project involved a two-storey (4th & 5th floor), penthouse apartment, located in the iconic, grade I listed, St. Pancras Chambers. The Northern, Piccadilly & Victoria Lines are below ground level, at this location.

The client wished to mitigate subsequent structureborne noise & vibration from underground train pass-bys, specifically, inside one of the bedrooms, to create a quiet space, isolated from the rest of the apartment, for work, rest & relaxation.

Technical Statistics

Before Works

29 dB

Mean LAeq (dB) of train pass-bys.

33 dB

Mean LAFmax (dB) of train pass-bys.

44 dB (50 - 63 Hz)

Leq (dB) at dominant frequency of train pass-bys.

After Works

28 dB

Mean LAeq (dB) of train pass-bys.

29 dB

Mean LAFmax (dB) of train pass-bys.

38 dB (50 - 63 Hz)

Leq (dB) at dominant frequency of train pass-bys.

Groundborne Noise & Vibration Guidelines.

Results

1 dB

Reduction in mean LAeq (dB) of train pass-bys

4 dB

Reduction in mean LAFmax (dB) of train pass-bys

55%

Reduction in highest measured vibration level – RMS Acceleration (mm/s2)

6 dB (50 - 63 Hz)

Reduction in Leq (dB) at dominant frequency of train pass-bys

As a guide to how a change in sound level might be perceived subjectively, the table below sets out descriptions of subjective impression & commonly used adjectives, according to various bands of sound level change.

Guide to Noise Level Changes

Band of Change in Sound Level (dB)	Subjective Impression	Descriptive Adjective
0 to 2	Imperceivable change in loudness	Marginal
3 to 4	Perceivable change in loudness	Noticeable
5 to 9	Up to a doubling or halving of loudness	Significant
10 to 15	At least a doubling or halving of loudness	Substantial
16 to 20	Up to a quadrupling or quartering of loudness	Substantial
21 or more	More than a quadrupling or quartering of loudness	Very Substantial

Our Process

On-Site Feasibility Survey:

The on-site feasibility survey is an opportunity for Mute Tube® to understand the building construction, assess the practicability of potential remedial works & help the client understand the implications of those works, before they commit to pre-works noise & vibration testing.

Pre-Works Noise & Vibration Testing:

Pre-works noise & vibration testing is arguably the most critical process of all.

Here, Mute Tube® measures & maps the building's internal surfaces, responsible for propagating noise producing vibrations from passing underground trains & establishes the dominant frequency bands of those vibrations.

Assessment of Pre-Works Test Data:

The building's dominant vibration pathways, established during pre-works testing, are the internal surfaces most in need of remedial treatment; these may be the floor, walls &/or ceiling.

The dominant frequency bands of those vibrations, informs the remedial system's requisite natural resonant frequency.

Design & Installation:

Mute Tube® develops the remedial system, as per the aforementioned assessment, often in collaboration with an architect &/or structural engineer.

Typically, installations form part of larger building contracts; as such, Mute Tube® works closely with the main contractor, to ensure supporting works facilitate a successful outcome.

Pre-Completion Noise & Vibration Testing:

Following installation, Mute Tube® conducts pre-completion noise & vibration testing, to gauge the efficacy of remedial works.

Typically, this is done once the building is ready for habitation, such that any residual noise & vibration levels measured, reflect what the end-user's acoustical experience will be.

Assessment of Pre-Completion Test Data:

Mute Tube® benchmarks pre-completion test data against Transport for London’s "Noise & Vibration Asset Design Guidance" & the World Health Organization’s general internal noise guidance for dwellings.

"Crossrail Information Paper D10", is also referenced, where appropriate.

Feasibility Survey

The first step involved a feasibility survey, which is fundamental to every Mute Tube® project, for the following reasons:

To understand the building construction &, in turn, assess the practicability of potential remedial works.
To help the client understand the implications of potential remedial works, which is, typically, easier to do in-person.

The client was then able to make an informed decision, to progress to the next step: pre-works noise & vibration testing.

Pre-Works Noise & Vibration Testing

Pre-works noise & vibration testing is designed to establish the following, critical information:

The internal surfaces most notably propagating noise producing vibrations, from passing underground trains & the dominant frequency bands of those vibrations, i.e. the former informs the surfaces most in need of remedial attention & the latter informs the requisite natural resonant frequency of the remedial system.
The average LAeq,T (dB) & LAFmax (dB) noise levels, from passing underground trains, i.e. to benchmark against WHO & TfL guidelines, respectively, as a way of determining the requisite performance criterion for remedial works.

In this case, the 4th floor bedroom was the focus for remedial works. The floorplan below shows the elected noise & vibration test positions.

N: Summary of Measured Noise Levels

V: Summary of Measured Vibration Levels

Test Position	Noise Source	LAeq (dB) Mean (Range)	LAFmax (dB) Mean (Range)
N1	Train pass-bys (33 samples in 1 hr 09 mins)	29 (27 - 30)	33 (30 - 35)
N1	No train pass-bys	27	-

Mean structureborne noise levels were measured at 29 dB, LAeq. As such, they satisfy the WHO's internal noise guidance for dwellings, which recommends that day & nighttime noise in bedrooms should not exceed 35 dB & 30 dB, LAeq, respectively.

Test Position N1: Frequency Characteristics of Structureborne Noise

Assesment of Pre-Works Test Data

In terms of noise, the levels measured at test position N1, averaged, from 33 train pass-bys, at 29 dB, LAeq & 33 dB, LAFmax. Notably, the former was 6 dB lower than the WHO’s daytime guidance for bedrooms, whilst the latter was 2 dB lower than TfL’s more stringent standards, set out in their "Noise & Vibration Asset Design Guidance".

In terms of vibration, from train pass-bys, the highest levels were measured at test positions Vb & Vf, in the horizontal ‘y’ axis (perpendicular to wall), from the plasterboard wall-linings, at 16.2 mm/s2 (max) & 13.4 mm/s2 (max), respectively. Notably, at the same test positions, from the supporting masonry wall (directly behind), i.e. Va & Ve, vibration levels, on the same 'y' axis, measured 2.23 mm/s2 (max) & 1.68 mm/s2 (max); demonstrating an 87% acceleration in vibration, caused by the plasterboard wall-linings.

Design & Installation

As per the assessment of pre-works test data, the highest levels of noise producing vibration, from train pass-bys, were measured from the plasterboard wall-linings &, as such, it was decided these surfaces should be the focus for remedial works.

Because of the building's timber joist construction, the floor was not compatible with the requisite remedial system; both in terms of subsequent height & mass. Fortuitously, the mean vibration levels measured at position Vg (floor), in the vertical 'z' axis (where they were highest), were 40% less than those measured at positions Vb, Vd & Vf (walls), in the horizontal 'y' axis.

Pre-Completion Noise & Vibration Testing

Following remedial works to the bedroom, pre-completion sound insulation tests were undertaken.

The floorplan below shows the elected noise and vibration test positions.

Assessment of Pre-Completion Test Data

In terms of noise, levels were remeasured at test position N2, with an average, from 15 train pass-bys, of 28 dB, LAeq & 29 dB, LAFmax; a 1 dB & 4 dB reduction, respectively, on the levels measured pre-works.

Notably, regarding the average LAeq,T (dB) noise levels, this represents a 2 dB improvement on the World Health Organization’s nighttime guidance for bedrooms of 30 dB, LAeq. Concerning the LAFmax (dB) noise levels, these surpass, by 6 dB, Transport for London’s most stringent design target of 35 dB, LAFmax.

At the 50 - 63 Hz frequency range, where noise from passing trains peaked, pre-works, at 44 dB, Leq, there was a 6 dB reduction, whilst, across the dominant frequency bands (between 31.5 Hz & 200 Hz), post-works measurements showed a notably closer relationship between train pass-bys & typical background noise levels (see graph below). Generally, the extent to which the former exceeds the latter, is an indication of adverse impact.

In terms of vibration, from train pass-bys, levels were remeasured at test position Va (position Vd, pre-works), in the horizontal 'y' axis, at 4.48 mm/s2 (max), compared with 10.00 mm/s2, pre-works, which equates to a 55% reduction.

Summary of Measured Noise Levels in Bedroom

Test Position	Noise Source	LAeq (dB) Mean (Range)	LAFmax (dB) Mean (Range)
N2	Train pass-bys (15 samples in 34 mins)	28 (27 - 28)	29 (28 - 32)
N2	No train pass-bys	27	-