Scope of Report

To measure levels of noise and vibration at ... Road and to provide initial comments regarding remediation works for redevelopment.

1. INTRODUCTION

1.1 ... is a disused church hall in .... The client intends to redevelop the building into three 'luxury' apartments.

1.2 The site is next to a railway. The client wishes to take noise and vibration into account during the redevelopment so that interior noise and vibration levels are commensurate with the desired 'luxury' environment.

2. THE SITE

2.1 Indicative floor plans of the building are shown in Figure 1.

2.2 The site is just to the south of ... overground station and is adjacent to a junction where lines join the mainline from the east.

2.3 The railway is to the rear of the building atop a steep embankment. The railway is a mainline from ... to ... and ... via ..., but also carries through trains arriving at from other southern locations and the ....

2.4 The Express is worthy of a separate note. The train is heavier and, as it does not stop at, is usually travelling faster than the other passenger trains using the line. The Express passes, on average, eight times an hour and is possibly the most significant train event for this site.

3. MEASUREMENT PROCEDURE

3.1 Measurements were made from 0800 on Thursday 22^nd June 00. The weather was dry and blustery.

3.2 The measurement locations are shown in Figure 1.

3.3 Vibration measurements were made using a PC fitted with a National Instruments DAQ PCMCIA card, running LabVIEW software and a four channel data acquisition program written by Civil Engineering Dynamics. The sample rate was 2560 samp/sec.

3.4 Four Bruel & Kjaer 4378 accelerometers and 2646 line drives were used. The signals from the transducers were conditioned by a Kemo power supply / filter unit. The Kemo unit employed an anti-alias filter set to 625 Hz.

3.5 Samples were taken of a number of train pass-bys. Each sample was 16 seconds long.

3.6 The transducers were fixed to metal right-angle brackets using magnets. The brackets enable the transducers to be mounted in one of three orthogonal orientations.

3.7 Measurements were made twice; first with all transducers orientated vertically, then with all the transducers orientated radially from the railway track.

3.8 Noise samples were taken using a Bruel & Kjaer 2260 Investigator Digital Sound Level Meter. This unit measures all acoustic parameters simultaneously.

4. RESULTS

4.1 Vibration

4.1.1 Vibration time histories were post processed using Matlab software.

4.1.2 The maximum 1 second running r.m.s. acceleration value for each train pass-by was calculated. A frequency response was calculated for each time history using the Matlab Signal Processing Toolbox.

4.1.3 Figure 2 shows an example of a vibration time history of a train pass-by, and the associated frequency response.

4.1.4 The maximum 1 second running r.m.s. acceleration value and the dominant low frequency were plotted on charts of relative human response to building vibration as shown in BS 6472: 'Guide to Evaluation of human vibration in buildings (1 Hz to 80 Hz)'. The results are shown in Figures 3 and 4.

4.1.5 The curves in these charts suggest suitable vibration levels for various environments. Curve 1 is said to be the threshold of human perception. The curves indicate that human are less sensitive to higher frequencies of vibration.

4.1.6 These curves can only serve as a guide. BS 6472 states: 'Within residential areas people exhibit wide variations of vibration tolerance. Specific values are dependent upon social and cultural factors, psychological attitudes and the expected degree of intrusion.'

4.1.7 BS 6472 suggests that, for continuous vibration, satisfactory levels of vibration are below curves 2 or 4 during the day and below curve 1.4 at night. Trains are a combination of continuous and transient vibration, so use of this criteria must be seen only a guide. We might expect human reaction to a train source to be worse than for a continuous source of the same magnitude and similar dominant frequency.

4.2 Noise

4.2.1 A total of 28 noise samples were taken over the four measurement locations (see Figure 1).

4.2.2 The time for a train pass-by was between 10 and 24 seconds. The distribution of sample times is shown in Figure 5(a).

4.2.3 A summary of the noise results is shown in Table A.

Table A: Noise Results Summary

Location Sound Exposure Level, L_AE (dBA)

Average Maximum Minimum

2 (inside rear) 66.5 69.9 61.5

3 (outside middle) 85.6 88.4 78.9

4 (inside front) 60.2 63.7 54.4

5 (inside middle) 69.1 73.6 60.1

4.2.4 Figure 5(b) shows 1/3^rd octave band measurements of the noise samples. It is misleading to show all of these plots on the same axes as they are L_eq,T measurements with differing record lengths (T). They are included in this way to give broad indications of the possible frequency characteristics of train pass-by noise at various locations. The chart also demonstrates some of the variation in noise characteristics possible with seemingly similar trains.

5. CONCLUSIONS

5.1 Vibration

5.1.1 With reference to Figure 3 and BS 6472, it would be possible to suggest that the vibration currently existing in the building is satisfactory for residential use.

5.1.2 However, the term 'luxury' is open to significant interpretation. One interpretation would be that the trains should not be perceptible. On this site, this would only be achievable if the building were base isolated. This means demolishing the existing building and starting again from scratch, which is not intended for this project.

5.1.3 The recommendations of this report seek to describe measures that could be implemented to reduce the vibration due to trains to the lowest reasonably achievable levels, bearing in mind the structural limitations of the building. Attention is paid to the possible frequency response of floors to try and avoid magnification of vibration at particularly sensitive frequencies or a frequency shift to a more sensitive frequency range.

5.1.4 Many trains will be perceptible to some people. Some trains may be considered a nuisance by some people. Again, we refer to BS 6472: 'Within residential areas people exhibit wide variations of vibration tolerance. Specific values are dependent upon social and cultural factors, psychological attitudes and the expected degree of intrusion.'

5.2 Noise

5.2.1 The sound insulation properties of the current fabric of the building are insufficient.

5.2.2 We expect that the roof and the windows to be the main weaknesses in the acoustic integrity of the building fabric.

5.2.3 However, the amount to which you can increase the sound insulation properties of a building by adding extra elements is limited. There are also practical considerations that limit the extent to which modifications can be made.

5.2.4 The recommendations of this report seek to describe measures that could be implemented to reduce the noise of trains to the lowest reasonably achievable levels, bearing in mind the structural limitations of the building.

5.2.5 Some trains will be perceptible, to a lesser or greater degree, to some people. There are wide variations in expectation and response to intrusive noises between individuals. If the recommendations are applied, then we would expect there to be a low probability of adverse comment, assuming that no-one would expect to live in such close proximity to a busy mainline, and never hear a train.

6. RECOMMENDATIONS

6.1 Windows

6.1.1 All windows should be deep secondary sashes. We suggest something like 10 mm outer glass, 200 mm air gap (lined with acoustic window liner), 6 mm glass. The important features to achieve are a deep air gap, and different thickness' of glass.

6.1.2 Ideally, from a noise insulation point of view, the units should be fully sealed. However, we appreciate the clients desire to provide the option of opening the windows for direct ventilation as a choice for the occupants to make. We also note the desire to use wooden framed glazing units.

6.1.3 The important features to achieve are air-tight seals throughout the whole glazing unit. This is why extruded aluminium is more commonly used for this type of window, although we appreciate the aesthetic limitations of these units.

6.1.4 We suggest that the client considers forms of artificial ventilation to the habitable rooms. Suitable glazing units will be air-tight as to not provide even the smallest amount of ventilation, and we suggest that the occupants should have an option to preserve their acoustic environment on hot weather by using air-conditioning.

6.2 Doors

6.2.1 Where possible, there should be two doors between any habitable room and the outside of the building.

6.3 Party Walls

6.3.1 The airborne sound insulation properties of a party wall are described using the weighted standardised level difference (D_nT,w), measured in dB, as defined by BS 5821.

6.3.2 The Building Regulations Approved Document E suggest that an individual wall should have a D_nT,w of at least 49 dB. It is widely accepted that this is the minimum acceptable value, and does not ensure a particularly high level of privacy in some situations.

6.3.3 We suggest that the client should aim for above average sound insulation properties. BS 8233 'British Standard Code of practice for Sound insulation and noise reduction for buildings' suggests four types of party wall construction for achieving higher levels of sound insulation. These are reproduced in Figure 6.

6.3.4 We suggest that option (d) is probably most likely to achieve the higher range of sound insulation.

6.4 Roof

6.4.1 We note the clients desire to retain the internal view of the existing ceiling. Therefore, it is not possible to provide the required sound insulation in the ceiling of the flats.

6.4.2 We suggest that a suitable way to achieve the high sound insulation required is to construct a cavity roof structure. A suggested cross-section is shown in Figure 7. Ideally, the two sections of roof should be supported on separate joists. It is not possible to achieve separate joists in this case, but we do not expect the sound insulation of the roof to be noticeably affected.

6.5 Floors

6.5.1 The aim of the floor design is to try to avoid magnifying the vibration as it passes through the building.

6.5.2 Suspended ground floors should be avoided, as the advantage conveyed by ground bearing ground floors can be significant.

6.5.3 The important features of the floors (from the point of view of reducing human vibration) are high mass and high stiffness. The high mass means that the floor will vibrate with a lower amplitude than a similar floor of lower mass, and when it does vibrate, it is more likely to be at a higher frequency than a less stiff floor. Remember that humans are less perceptive to higher frequency vibration.

6.5.4 Following discussion with the client regarding the practicality of various floor constructions, the recommended floor constructions are as follows:

Ground Floors

Thick ground bearing concrete where possible.

OR

Where ground bearing not possible, thick concrete topping on pre-cast reinforced concrete planks with the spans reduced as small as possible using sleeper walls and/or short columns.

First Floors

It is not possible to use any other floor construction than timber floors on timber joists for the first floors. However, mass and stiffness can be increased to a certain extent by using larger than necessary beams, and by making as many of the walls below load bearing walls, as this will break up the spans.

6.5.5 It is expected that trains will be perceptible (to a lesser or greater extent, depending upon the individual) on these floors, but we do not expect the vibration to be any worse than the measurements made at location 2 (suspended wooden floor at rear of building), which are not severe.

7. REFERENCES

BS 6472:1992 'Guide to Evaluation of human exposure to vibration in buildings (1 Hz to 80 Hz).'

BS 8233:1987 'British Standard Code of practice for Sound insulation and reduction for buildings.'

The Building Regulations 1992. Approved Document E - Resistance to the passage of sound.

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