The project involved a three-storey, mid-terrace Victorian house, located in Clapham, South London. Northern Line underground trains run between Clapham South & Balham stations nearby, with the nearest section of track approximately 290 meters away.
Having lived at the property since 1994, the client had not been adversely impacted by tube noise, until 2018, when it became significantly louder, notably, in the ground floor living room & adjacent hallway. As such, these spaces were the focus for remedial works.
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.
|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|
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 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.
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.
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.
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.
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.
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 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 ground floor living room & adjacent hallway were the focus for remedial works. The floorplan below shows the elected noise & vibration test positions.
|Test Position||Noise Source||LAeq (dB)|
(12 samples in 1 hr 15 mins)
(31 - 34)
(37 - 41)
|N1||No train pass-bys|
(4 samples / total of 2 mins 26 secs)
(24 - 28)
(34 - 40)
(16 samples in 1 hr 15 mins)
(34 - 38)
(41 - 44)
|N2||No train pass-bys|
(4 samples / total of 2 mins 5 secs)
(20 - 23)
(30 - 32)
At test position N1 (front living room), mean groundborne noise levels were significantly lower than N2 (rear living room), i.e. 32 dB, LAeq & 39 dB, LAFmax (compared with 37 dB, LAeq & 43 dB, LAFmax). As such, TfL would conclude that noise levels here “should not be considered to be significant.”
In terms of noise, the highest levels were measured at test position N2 (rear living room), with an average, from 16 train pass-bys, of 37 dB, LAeq & 43 dB, LAFmax. Notably, the former was 2 dB higher than the WHO’s daytime guidance for living spaces, whilst the latter was 3 dB higher than TfL’s "Noise & Vibration Asset Design Guidance", i.e. the noise generated in this area was “significant”, by their standards.
In terms of vibration, from train pass-bys, the highest levels were also measured at test position N2, in the vertical ‘z’ axis, from the substrate, at 0.054 m/s2 (max), followed by the floorboards, directly above, also in the ‘z’ axis, where they measured only slightly lower, at 0.042 m/s2 (max).
Critically, each of the test positions had a peak in vibration levels in the 125 Hz & 160 Hz frequency bands, corresponding with the peaks in noise; supporting the inferred correlation between these two signals.
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 rear living room &, as such, it was decided this surface should be the focus for remedial works.
The remedial system, for the floor, was specified with a natural resonant frequency of no greater than 62 Hz, in accordance with the principles of vibration control.
Following remedial works to the rear living room, pre-completion sound insulation tests were undertaken. To ensure measurements were not distorted by the untreated front living room, a temporary dividing wall was installed.
Subject to the results, the client would decide whether to extend the remedial system to the front living room.
The floorplan below shows the elected noise and vibration test positions.
In terms of noise, levels were remeasured at test position N2, with an average, from 14 train pass-bys, of 28 dB, LAeq & 37 dB, LAFmax; a 9 dB & 6 dB reduction, respectively, on the levels measured pre-works. Notably, regarding the average LAeq,T (dB) noise levels, this represents a 7 dB improvement on the WHO’s daytime guidance for living spaces of 35 dB, LAeq. Concerning the LAFmax (dB) noise levels, TfL would conclude that they "should not be considered to be significant"; measuring 2 dB above their most stringent design target.
At 125 Hz, where noise from passing trains peaked, pre-works, at 53 dB, Leq, there was a 10 dB reduction, whilst, across the dominant frequency bands (between 20 Hz & 160 Hz), post-works measurements showed a markedly 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 N2, in the vertical axis, at 0.002 m/s2 (max), compared with 0.054 m/s2 (max) from the substrate, pre-works; representing a 96% reduction.
Based on these results, the client decided to extend the remedial system to the front living room.
|Test Position||Noise Source||LAeq (dB)|
|N2 (a), (b) & (c)||Train pass-bys|
(14 samples in 50 mins 17 secs)
(28 - 33)
(36 - 44)
|N2 (a), (b), & (c)||No train pass-bys|
(5 samples / total of 2 mins 36 secs)
(21 - 26)
(36 - 40)
If Mute Tube® can be of assistance, then please do not hesitate to get in touch; we look forward to discussing your project with you.