Reducing train vibration in bedroom floor, to aid restful sleep.

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

1. To understand the building construction &, in turn, assess the practicability of potential remedial works.

2. 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 vibration testing.

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

• The dominant frequencies (Hz) of the vibrations propagated by the floor during underground train pass-bys, i.e. this informs the requisite natural resonant frequency of the remedial system.

• The vibration levels (m/s2) propagated by the floor during underground train pass-bys, across the x, y & z axes, i.e. to benchmark against British Standard 6841 & International Standard 2631-1: "Guide to Measurement & Evaluation of Human Exposure to Whole Body Vibration & Repeated Shock", as a way of determining the desirable performance criterion for remedial works.

Here, the 2nd floor master bedroom was the focus for remedial works. The floorplan below shows the elected vibration test positions.

As per the assessment of pre-works test data, the highest levels of vibration, from train pass-bys, were in the horizontal x & y axes, &, as such, it was decided that, in the first instance, the existing suspended timber joist floor should be structurally shored-up/stabilised. This process included: 

1. Introducing through-bolts to the existing coupled joists.

2. Tying the existing joists together, with an 18 mm plywood sub-deck.

3. Mechanically fixing & gluing 18 mm T&G cement particle boards, atop the joists (to serve as the supporting floor).

Following this phase of remedial works, &, as per subsequent vibration measurements, an additional isolating platform was installed atop the stabilised floor; designed to support the bed frame & mitigate vibrations in the vertical z-axis.

The isolating platform was specified with a natural resonant frequency of no greater than 8 Hz, in accordance with the principles of vibration control.

Following remedial works to the 2nd floor, master bedroom floor, pre-completion vibration mitigation tests were undertaken.

The floorplan below shows the elected vibration test positions.

Vibration levels, from passing underground trains, were remeasured at test positions Vb (stabilised floor) & Vc (isolating platform). The 'mean', 'max' & 'typical (peak)' m/s2 values, across all three axes, are itemised below, alongside the percentage change, compared, in terms of Vb, with Va (original pre-works floor) measurements, & Vc, compared with both Va & Vb measurements:

Vb m/s2 values, & percentage change compared with Va:

• X-axis: 0.003 (88% decrease in vibration) / 0.007 (90% decrease) / 0.002 (71% decrease)

• Y-axis: 0.003 (71% decrease) / 0.016 (85% decrease) / 0.001 (86% decrease)

• Z-axis: 0.014 (12.5% decrease) / 0.068 (84% increase) / 0.016 (60% increase)

Vc m/s2 values, & percentage change compared with Va:

• X-axis: 0.003 (88% decrease in vibration) / 0.007 (90% decrease) / 0.002 (71% decrease)

• Y-axis: 0.002 (90 % decrease) / 0.004 (96% decrease) / 0.001 (86% decrease)

• Z-axis: 0.004 (75% decrease) / 0.016 (57% decrease) / 0.002 (80% decrease)

Vc m/s2 values, & percentage change compared with Vb:

• X-axis: 0.003 (0%) / 0.007 (0%) / 0.002 (0%)

• Y-axis: 0.002 (33% decrease in vibration) / 0.004 (75% decrease) / 0.001 (0%)

• Z-axis: 0.004 (71% decrease) / 0.016 (76% decrease) / 0.002 (88% decrease)

It is noteworthy that, whilst stabilising works to the floor significantly reduced horizontal vibration, on both the x & y axes (due to increased lateral stiffness), it substantially increased vertical vibration; by 84% 'max' & 60% 'typical (peak)', compared with the original untreated floor. The isolating platform, however, corrected this; reducing it by 57% 'max' & 80% 'typical (peak)', compared with the untreated floor, & by 76% 'max' & 88% 'typical (peak)', compared with the stabilised floor.

Regarding the isolating platform, cross-checked against the British Standard 6841 & International Standard 2631-1: "Guide to Measurement & Evaluation of Human Exposure to Whole Body Vibration & Repeated Shock" (in relation to thresholds for the perception of vibration by seated persons), only the ‘max’ measurement on the vertical axis, at 0.016 m/s2, exceeds 0.015 m/s2, the level at which 50% of alert, fit persons can just detect a weighted vibration. The ‘mean’ measurement on the z-axis, however, registers substantially below that figure, at 0.004 m/s2, as does the ‘typical (peak)' measurement, at 0.002 m/s2 (at the dominant frequency of 12.5 – 16 Hz). Concerning this ‘typical (peak)’ measurement, notably, it aligns with background vibration (during no train pass-bys), which is a strong indication of low-impact, in terms of vibration perception.