Stockwell

Remediating tube noise & vibration, for short-let apartment.

Proposed Airbnb investment, hindered by Northern Line noise & vibration.

Location

SW9 London

Tube Line

Northern Line

Circumstances & Project Brief

The project involved a four-storey, semi-detached Victorian townhouse, located on the main A3 road, linking Kennington & Clapham. The Northern Line is approximately 14 meters below ground level, at this location.

Having purchased the property in 2021, the client, as part of a larger refurbishment, chose to convert the lower ground floor into a self-contained, short-let apartment. However, to make it more commercially viable, groundborne noise & vibration from underground trains needed to be adequately attenuated.

Technical Statistics

Before Works

36 dB / 38 dB

Mean LAeq (dB) of train pass-bys. (Living Room / Hallway)

43 dB / 46 dB

Mean LAFmax (dB) of train pass-bys. (Living Room / Hallway)

58 dB (80 - 125 Hz)

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

After Works

31 dB / 32 dB

Mean LAeq (dB) of train pass-bys. (Living Room / Hallway)

38 dB / 39 dB

Mean LAFmax (dB) of train pass-bys. (Living Room / Hallway)

49 dB (80 - 125 Hz)

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

Groundborne Noise & Vibration Guidelines.

Results

5 dB / 6 dB

Reduction in mean LAeq (dB) of train pass-bys. (Living Room / Hallway)

5 dB / 7 dB

Reduction in mean LAFmax (dB) of train pass-bys. (Living Room / Hallway)

91%

Reduction in highest measured vibration level – RMS Acceleration (m/s2). (Living Room)

9 dB (80 - 125 Hz)

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

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® designs 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.

On-Site Feasibility Survey

The first step involved an on-site 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, most instructive, when done 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 building's internal surfaces, responsible for 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 World Health Organization & Transport for London guidelines, respectively, as a way of determining the desirable performance criterion for remedial works.

In this case, the lower ground floor bedroom, living room & adjacent hallway were 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 (28 samples in 50 mins 15 secs)	36 (31 - 39)	43 (39 - 47)
N2	Train pass-bys (38 samples in 1 hour 35 mins 18 secs)	36 (33 - 39)	43 (39 - 48)
N3	Train pass-bys (14 samples in 20 mins 15 secs)	38 (34 - 40)	46 (41 - 49)

N1: Frequency Characteristics of Groundborne Noise

N2: Frequency Characteristics of Groundborne Noise

N3: Frequency Characteristics of Groundborne Noise

Assessment of Pre-Works Test Data

In terms of noise, the highest levels were measured at test position N3 (hallway), with an average, from 14 train pass-bys, of 38 dB, LAeq & 46 dB, LAFmax, followed by test positions N1 (bedroom) & N2 (living room), both with an average of 36 dB, LAeq & 43 dB, LAFmax. All three positions exceeded TfL’s "Noise & Vibration Asset Design Guidance" (by 6 dB & 3 dB, respectively), i.e. the noise generated in these spaces was “significant”, by their standards. Concerning N1 & N2, the former exceeded the WHO’s night-time guidance for bedrooms by 6 dB, whilst the latter exceeded their daytime guidance for living spaces by 1 dB.

In terms of vibration, from train pass-bys, the highest 'max' levels were measured at test position Va (bedroom substrate), in the horizontal ‘x’ axis, at 0.153 m/s2, followed by test position Ve (living room substrate), in the vertical ‘z’ axis, at 0.148 m/s2. The third highest levels were measured, again, at test position Va, in the horizontal 'y' axis, at 0.138 m/s2. The fourth & fifth highest levels were measured at test positions Vf (structural internal wall, between bedroom & living room) & Vd (bedroom chimney breast), at 0.056 m/s2 (horizontal 'y' axis) & 0.037 m/s2 (horizontal 'x' axis), respectively. Note: vibration levels were not measured from the hallway substrate.

Critically, test positions Va & Ve both had a peak in vibration levels at the 80 Hz frequency band, corresponding with one of the two peaks in noise at those locations; supporting the inferred correlation between these two signals.

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 floor in the bedroom & living room &, as such, it was decided these surfaces, along with the directly adjacent hallway floor, should be the focus for remedial works.

It was recommended these works also include the structural internal wall, between the bedroom & living room, & the bedroom chimney breast, based on the following factors:

Relatively high vibration levels measured from these surfaces.
Vibration levels measured from the bedroom floor unusually ascendant in the horizontal 'x' & 'y' axes (rather than the more typical vertical 'z' axis), i.e. propagation of vibration, from the floor to the walls, is more likely to occur under these circumstances & may, in part, explain factor 1.

For budgetary reasons, however, it was decided these surfaces would be treated retrospectively, subject to the efficacy of remedial works to the floors.

The corrective system, for the floors, was specified with a natural resonant frequency of no greater than 30 Hz, in accordance with the principles of vibration control.

Pre-Completion Noise & Vibration Testing

Following remedial works to the bedroom, living room & hallway, 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 positions N1 (bedroom), N2 (living room) & N3 (hallway), with the following LAeq & LAFmax values recorded from 13, 25 & 12 train pass-bys, respectively:

N1 = 35 dB, LAeq & 44 dB, LAFmax
N2 = 31 dB, LAeq & 38 dB, LAFmax
N3 = 32 dB, LAeq & 39 dB, LAFmax

Regarding the average LAeq,T (dB) noise levels for test position N1, this conforms to the WHO's daytime guidance for bedrooms, whilst the levels measured at test position N2 represents a 4 dB improvement on their daytime guidance for living spaces. Concerning the LAFmax (dB) noise levels, TfL would conclude that they "should not be considered to be significant" at test positions N2 & N3.

At 80 - 125 Hz, where noise from passing trains peaked, pre-works, at 58 dB, Leq (test position N3), there was a 9 dB reduction.

In terms of vibration, from train pass-bys, levels were remeasured at test positions Va (bedroom substrate), Ve (living room substrate, marked "Vc" for pre-completion testing) & Vf (structural internal wall, between bedroom & living room, marked "Vb" for pre-completion testing), with the m/s2 (max) values itemised below, alongside the percentage reduction, compared with pre-works measurements:

Va = 0.022 m/s2 (horizontal 'x' axis) / 86% reduction
Ve (Vc) = 0.013 m/s2 (vertical 'z' axis) / 91% reduction
Vf (Vb) = 0.026 m/s2 (horizontal 'y' axis) / 54% reduction

Understanding Discrepancies in Post-Works Performance.

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 (13 samples in 30 mins 12 secs)	35 (32 - 39)	44 (38 - 47)
N2	Train pass-bys (25 samples in 1 hour 21 secs)	31 (25 - 34)	38 (30 - 42)
N3	Train pass-bys (12 samples in 15 mins 20 secs)	32 (26 - 36)	39 (33 - 44)

N1: Frequency Characteristics of Groundborne Noise

N2: Frequency Characteristics of Groundborne Noise

N3: Frequency Characteristics of Groundborne Noise