Kennington

Controlling underground train rumble, in Victorian terraced house.

Mid-terrace townhouse disturbed by Kennington loop; 15 meters below ground.

Location

SE17 London

Tube Line

Northern Line

Circumstances & Project Brief

The project involved a three-story, mid-terrace Victorian townhouse, located on a residential road in Kennington, South London.

Northern Line London Underground trains run between Kennington & Oval stations, nearby, with the nearest section of track approximately 100 metres to the west. Underground trains also make use of the Kennington loop, which passes directly beneath the property, at a depth of c.15 metres.

The client, as part of a larger refurbishment project, including a side return extension, wished to attenuate groundborne noise & vibration from passing underground trains, throughout the ground floor, where the disturbance was greatest.

Technical Statistics

Before Works

+11 dB / +13 dB / +15 dB

Difference between mean LAFmax (dB) of train pass-bys & mean LAeq (dB) of background noise (Living Room / Dining Room / Kitchen)

37 dB / 38 dB / 38 dB

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

53 dB (50 Hz)

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

After Works

+3 dB / +3 dB / +5 dB

Difference between mean LAFmax (dB) of train pass-bys & mean LAeq (dB) of background noise (Living Room (now kitchen) / Dining Room / Kitchen (now living room))

27 dB / 30 dB / 30 dB

Mean LAFmax (dB) of pass-bys. (Living Room (now kitchen) / Dining Room / Kitchen (now living room))

36 dB (50 Hz)

Mean Leq (dB) at dominant frequency of train pass-bys. (Living Room (now kitchen)

Groundborne Noise & Vibration Guidelines.

Results

8 dB / 10 dB / 10 dB

Reduction in difference between mean LAFmax (dB) of train pass-bys & mean LAeq (dB) of background noise (Living Room (now kitchen) / Dining Room / Kitchen (now living room))

10 dB / 8 dB / 8 dB

Reduction in mean LAFmax (dB) of train pass-bys. (Living Room (now kitchen) / Dining Room / Kitchen (now living room))

87%

Reduction in highest measured vibration level – RMS Acceleration (mm/s2). (Kitchen (now living room))

17 dB (50 Hz)

Reduction in mean Leq (dB) at dominant frequency of train pass-bys. (Living Room (now kitchen))

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 ground floor living room, dining room, kitchen & proposed side return extension, 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 (13 samples in 25 mins)	28 (23 - 32)	36 (33 - 40)
N2	Train pass-bys (13 samples in 29 mins)	28 (24 - 33)	38 (32 - 48)
N3	Train pass-bys (10 samples in 19 mins)	29 (25 - 35)	37 (34 - 42)
N4	Train pass-bys (11 samples in 18 mins)	31 (28 - 35)	39 (32 - 52)
N5	Train pass-bys (10 samples in 17 mins)	29 (27 - 34)	35 (31 - 40)
N6	Train pass-bys (6 samples in 9 mins)	27 (25 - 32)	38 (34 - 47)

N1: Frequency Characteristics of Groundborne Noise

N3: Frequency Characteristics of Groundborne Noise

N6: Frequency Characteristics of Groundborne Noise

Assessment of Pre-Works Test Data

In terms of noise, the highest levels were measured at test position N4 (dining room), with an average, from 11 train pass-bys, of 31 dB, LAeq & 39 dB, LAFmax, followed by test positions N2 (living room), N3 (dining room) & N6 (kitchen), with an average of 28, 29 & 27 dB, LAeq & 38, 37 & 38 dB, LAFmax, respectively.

All four positions satisfied TfL’s "Noise & Vibration Asset Design Guidance", i.e. at sub 40 dB, LAFmax, the noise generated in these spaces “should not be considered significant”, by their standards. Nevertheless, they exceeded their more stringent requirement of 35 dB, LAFmax, by 4, 3, 2 & 3 dB, respectively.

Concerning the WHO’s daytime guidance for living spaces, this was comfortably met, i.e.“…it is desirable that the daytime LAeq, 16h internal noise level should not exceed 35 dB within living spaces, or 40 dB, LAeq, 16h in dining rooms.”

However, the mean LAFmax (dB) noise levels of train pass-bys, notably exceeded the mean LAeq (dB) background noise levels, at all six test positions, i.e. by 8, 14, 14, 12, 13 & 15 dB, respectively; hence the perceived noise nuisance, i.e. as per BS 4142:

“A difference of around +5 dB is likely to be an indication of an adverse impact, depending on the context."
"A difference of around +10 dB is likely to be an indication of a significant adverse impact, depending on the context."

In terms of vibration, from train pass-bys, the highest 'max' levels were measured at test position Vj (kitchen tiles), in the vertical ‘z’ axis, at 45.90 mm/s2, followed by test position Vd (dining room floorboards), in the vertical ‘z’ axis, at 42.10 mm/s2. The third highest levels were measured at test position Vc (living room substrate), in the horizontal 'y' axis, at 33.88 mm/s2. The fourth highest levels were measured at test position Va (living room floorboards), in the vertical ‘z’ axis, at 32.80 mm/s2.

Critically, test positions Va, Vk & Vj both had a peak in vibration levels at the 50 Hz frequency band, corresponding with the 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 living room, dining room & kitchen, &, as such, it was decided these surfaces should be the focus for remedial works.

It was recommended these works also include the provision of acoustic bearing pads (see technical drawings below), to isolate the structural steel columns (required to from the side return extension), from the supporting foundations, i.e. if left untreated, these columns could transmit groundborne vibration into the building shell; setting into motion connecting surfaces & increasing reradiated noise, generally.

The corrective system, for the floors, was specified with a natural resonant frequency of no greater than 16 Hz, in accordance with the principles of vibration control.

Pre-Completion Noise & Vibration Testing

Following remedial works to the living room (now kitchen), dining room & kitchen (now living room, including side return extension), 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 (kitchen (previously living room)), N3 (dining room) & N6 (living room, including side return extension (previously kitchen)), with the following LAeq & LAFmax values recorded from 8, 14 & 7 train pass-bys, respectively:

N1 = 26 dB, LAeq & 27 dB, LAFmax
N3 = 29 dB, LAeq & 30 dB, LAFmax
N6 = 27 dB, LAeq & 30 dB, LAFmax

Regarding the LAFmax (dB) noise levels, all three positions bettered TfL's "Noise & Vibration Asset Design Guidance" more stringent requirement of 35 dB, LAFmax, by 8, 5 & 5, dB, respectively, whilst the average LAeq,T (dB) noise levels, also bettered the WHO's daytime guidance for living spaces, by 9, 11 & 8 dB, respectively.

Concerning the difference between the mean LAFmax (dB) noise levels of train pass-bys & the mean LAeq (dB) of background noise levels, the following, significant, reductions were recorded:

N1: pre-works = +8 dB / post-works = +3 dB
N3: pre-works = +14 dB / post-works = +3 dB
N6: pre-works = +15 dB / post-works = +5 dB

Crucially, as per BS 4142, "The lower the rating level is relative to the measured background sound level, the less likely it is that the specific sound source will have an adverse impact or a significant adverse impact."

Finally, at 50 Hz, where noise from passing trains peaked, pre-works, at 53 dB, Leq (test position N1), there was a 17 dB reduction, down to 36 dB, Leq.

In terms of vibration, from passing trains, levels were remeasured at test positions Va (kitchen floor (previously living room)), Vd (dining room floor) & Vj (living room floor, including side return extension (previously kitchen)), with the mm/s2 (max) values itemised below, alongside the percentage reduction, compared with pre-works measurements:

Va = 5.90 mm/s2 (vertical 'z' axis) / 87% reduction
Vd = 6.62 mm/s2 (vertical 'z' axis) / 84% reduction
Vj = 3.88 mm/s2 (vertical 'z' axis) / 88% reduction

"The impact has been dramatic. For the most part, we no longer think about the trains as we're not aware of them. In one part of the "new" space, we can sometimes hear a rumble, but the intensity is much reduced and not a problem for us. Mostly, it is so very quiet you cannot believe you are in inner London. The fact is that we are really enjoying our "new" space." - Client feedback

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 (8 samples in 30 mins, 17 secs)	26* (24 - 28)	29 (27)** (27 - 30)
N3	Train pass-bys (14 samples in 50 mins, 17 secs)	29* (27 - 30)	32 (30)** (32- 34)
N6	Train pass-bys (7 samples in 25 mins, 17 secs)	27* (26 - 28)	31 (30)** (30 - 32)

*Underground train measurements were < 10 dB above background noise &, as such, the mean LAeq (dB) measured levels were increased, somewhat, by other sources, e.g. passing pedestrians, powers tool usage (in neighbouring gardens), kitchen refrigerator, etc...

**The value in brackets is the mean LAFmax (dB) level of train pass-bys, corrected to account for background noise interference.

N1: Frequency Characteristics of Groundborne Noise

N6: Frequency Characteristics of Groundborne Noise