Projects
Example Reports
NOISE SURVEYS ...
Scope of Report
To comment on a proposed change of use of a barn to private function venue with regards to the noise impact on neighbours.
1. INTRODUCTION
1.1 The owner of ... Estate has proposed conversion of a barn to a private function venue. We understand the anticipated clientele would be weddings, conferences and the like.
1.2 The local authority, ... District Council, have requested that the proposal be accompanied by an acoustic report that addresses the possible impact of noise adversely affecting local residents, with indications of mitigation measures.
1.3 Civil Engineering Dynamics suggested the following criteria, that was agreed verbally by the local authority:
- Noise levels from the barn should be at least 10 dB below the LA90 level of background noise measured at the nearest affected property. Background levels should be established from measurements on two occasions, between 20:00 and 00:00, one being either a Friday or a Saturday.
- It is clarified that the suggested criteria for acceptance should be based upon comparison of overall LAeq (1hr) levels emanating from the barn with LA90 (1hr) background measurements. It does not apply to Octave or 1/3rd Octave measurements.
- Background levels will have an element of variation, and noise from the barn will contain tonal characteristics and peaks whereby the suggested criteria will not ensure the sound from the venue is totally inaudible.
2. THE SITE
2.1 ... Estate is just north of ..., on the ..., north east. ...
2.2 Figure 1 shows a plan of the site.
2.3 One location chosen for background measurements was near the cottages to the north of the barn. This is location N1 as indicated in Figure 1. The ground slopes down towards the cottages approximately 7m over approximately 125m. Photographs Ph1 and Ph2 show views of the cottages from the site.
2.4 A second location chosen for background measurements was near the cottages to the west of the barn, 'West Lodge'. This is location N2 as indicated in Figure 1. The ground slopes down towards the cottages approximately 1m over approximately 175m. There is a rise in ground level between the barn and the road junction of approximately 0.5m. A two storey building, used as offices, partially obstructs the barn from West Lodge. Ph3 and Ph4 show views of West Lodge - Ph3 from the side of the barn, Ph4 from the junction.
2.5 Ph5 to Ph7 show views of the barn.
2.6 The barn is currently redundant. The doors are poorly fitting and Ph6 shows an example of this.
2.7 The roof appears to be natural slate over bituminous felt.
2.8 The northern wall is approximately 2m high and made of approximately 300mm of brickwork.
2.9 Ph8 shows the clients proposed north elevation for the barn. Note the 'barn-doors' are replaced by glazing units and shutters.
3. MEASUREMENT PROCEDURE
3.1 All noise measurements were undertaken using a Bruel and Kjaer 2260 Investigator Sound Level Meter. The microphone was taped to a tripod approximately 1.5m above the ground. The sound level meter logged hourly values of LA90 and third octave measurements of hourly LL90. Calibration was verified using a Bruel and Kjaer 4231 acoustic calibrator.
3.2 The first set of background measurements were undertaken from 20:00 to 00:00 on Tuesday 11th December 01. The measurement location, N1, was at the cross-roads of the farm tracks to the south of the cottages, as shown in Figure 1. The weather was initially misty (see Ph4), clearing by 21:00, very cold, and dead calm throughout the measurement period.
3.3 The second set of background measurements were undertaken from 20:00 to 00:00 on Friday 14th December 01. The measurement location, N2, was at the eastern end of the garden of West Lodge, as shown in Figure 1. The weather was clear, dry, cold, with a light breeze from the north-east, gusting to around 3 m/s throughout the measurement period.
3.4 Measurements of the sound attenuation properties of the fabric of the barn were undertaken using a Bruel and Kjaer 2260 Investigator Sound Level Meter and a JBL Eon powered loudspeaker.
3.5 Fairly high levels (80 - 90 dB) of Pink Noise were generated in the barn using the loudspeaker. The 2260 has an in-built Pink Noise signal generator. 5-second Leq measurements were made inside and outside the barn, with the sound level meter and the loudspeaker in a variety of locations.
3.6 Indicative loudspeaker and sound level meter positions are shown in Figure 2. The exterior measurement locations are approximately 10m from the façade of the barn. This location was chosen as it is assumed that the roof is a weaker part of the fabric of the building than the walls, so that a measurement at a closer proximity would feature a greater attenuation by the wall that would not be expected to feature to such an extent in the far field.
3.7 Additional 5-second Leq measurements of background noise were made at the exterior locations, which would confirm that the noise from the barn in this test gave a clear and measurable reading that was not influenced by the background.
4. RESULTS
4.1 The results of the background noise measurements are shown in Table A.
Table A: Background Noise Measurements
LA90 (dBA) N1 N2 11th December 01 14th December 01 20:00 - 21:00 33.6 35.6 21:00 - 22:00 31.1 33.3 22:00 - 23:00 30.5 31.7 23:00 - 00:00 28.8 31.0
4.2 Figure 3 shows sample third octave spectra of background measurements.
4.3 Table B shows a summary of the results of the building sound attenuation measurements.
4.4 Figure 4 shows a chart of the third octave spectrum of the difference between internal and external noise levels, where it is evident that the noise attenuation performance of the building is weaker at lower frequencies.
Table B: Building Sound Attenuation Results
Leq,5-sec (dB) L1 - L2 (dB)
Average of Internal Noise Level Measurements (L1) Average of External Noise Level Measurements (L2) Average of Background Noise Level Measurements 1/3rd Octave Centre Frequency (Hz) 100 94.3 70.6 46.4 23.7 125 95.2 71.4 42.7 23.8 160 94.8 69.1 51.4 25.7 200 90.4 64.2 33.7 26.2 250 89.1 63.4 42.3 25.7 315 90 64.3 39.3 25.7 400 89.1 60.6 38 28.5 500 89.3 60.8 38.6 28.5 630 89.3 58.6 34.8 30.7 800 86 56.8 33 29.2 1k 84.4 53.6 32.5 30.8 1.25k 82.4 51 29.4 31.4 1.6k 85.1 52 27.5 33.1 2k 87.1 54.7 26 32.4 2.5k 88 56.4 27.5 31.6 3.15k 86.3 54.5 24.5 31.8
5. SUMMARY
5.1 Two surveys undertaken indicate that background noise levels at the closest properties to the barn between 20:00 and 00:00 are very low, with LA90 around 30 dBA.
5.2 'The Calculation of Road Traffic Noise' (Ref. 6.1) suggests a method for predicting the attenuation of noise with distance. Using CRTN, the expected attenuation over the 125m from the barn to the cottages to the north is around 15 dB.
5.3 The attenuation to West Lodge can be expected to be higher due to its' further distance from the barn, partial screening by the offices, and as the barn is side-on to West Lodge it would be expected to have a lower radiation efficiency in that direction. Therefore, if the suggested noise criteria are met at the cottages, we would expect the criteria to also be satisfied at West Lodge.
5.4 The sound attenuation of the barn in its' current state is relatively poor, particularly at low frequencies. This is largely due to the large gaps around the doors, although the current roof should not be expected to provide particularly great attenuation. A broadband attenuation of 25 dB is assumed, based on site measurements.
5.5 With the barn in its current condition, we would expect the maximum allowable noise level in the barn, without breaching suggested noise criteria, is around 60 dBA (LAeq,1hour). This is commensurate with continuous subdued conversation.
5.6 The sound attenuation properties of the barn will improve following refurbishment. As much as 35 - 40 dB attenuation could be achieved if the following features are included, subject to detail design and good quality workmanship:
- Barn door openings in northern façade replaced with 'fixed' secondary glazing formed of two sets of good quality sealed acoustic double glazing (e.g. Pilkington Audioscreen). The cavity should be at least 200mm and be fitted with acoustic absorbing reveals.
- Barn door openings in southern façade replaced with good quality acoustic double glazing (e.g. Pilkington Audioscreen).
- The gaps in-between roof joists to be filled with 50mm thick Rockwool 'noise' insulation and covered by two layers of 12.5mm plasterboard arranged with overlapping joints. If the depths of the joists permit, a cavity should be left between the Rockwool and the felt. The structure will need to be evaluated to see if it will accommodate the additional load, or determine any strengthening that may be required.
- Attention paid to all aspects of design and construction to ensure good sealing of the fabric of the building.
- Mechanical ventilation and comfort cooling provided, with sound attenuating ducts and louvers to the southern façade.
5.7 We expect such modifications to permit noise levels of around 70 - 75 dBA (LAeq,1hour), without breaching suggested noise criteria, thus permitting the barns' use for conferences and weddings provided that music is limited to light background music that would not be expected to interfere with conversation without raised voices. Noise levels from disco's and live bands are far in excess of 75 dBA (LAeq,1hour) and are therefore unacceptable under the noise control detailed in 5.6.
5.8 Disco's and live bands could be expected to generate sound levels around 100 - 110 dBA (LAeq,1hour). The sound attenuation required by the fabric of the barn for such an event would need to be 65 dB at the very least, with close attention paid to the low frequency performance. Such attenuation's are difficult to achieve with a single skin construction, and require very heavyweight construction which in this case would necessitate major alterations to the barn structure, which would be impractical.
5.9 If high attenuation for disco's and live bands is required then a 'room within a room' construction must be considered. This is where a second room is built within the existing one. The internal room is supported entirely from it's base so that it is totally separate from the existing structure and a cavity exists between the two. The internal room is either built from wood or brickwork, depending upon the magnitude and the frequency response of the attenuation required. The cavity width also affects performance, but 200mm may be typical. The internal room would be in addition to the modifications of 5.6.
5.10 A room within a room type construction may be an onerous task if applied to the main hall of the barn, but could be a consideration for a side room treated in such a way and used as a 'disco room'. In addition we recommend that it is installed with it's own sound reinforcement system and noise limiter, set by agreement with the Local Authority. Music from the disco area could be relayed to the main hall but at a lower level. The main hall and its' use would then be as in 5.6 and 5.7.
5.11 Even with a room within a room solution to the 'disco area', it may ultimately prove necessary to prevent certain types of live bands from performing at this venue, as they can have a greater potential to generate higher noise levels.
5.12 This report addresses the cottages of local residents off site, but not the owners' residence on the south side of the barn, which has implications to its' future independent use.
5.13 BS 5228 (Ref. 6.2) provides some information on how meteorological conditions can affect the way that sound travels, and can result in higher than expected noise levels due to focusing of the sound.
5.14 Consideration will need to be given to controlling noise from vehicular movements, with strategic location of car parking and/or providing screening to this area. This report has considered the issue of noise emanating from inside of the barn only, as was indicated by the Local Authority when they set out their concerns.
6. CONCLUSIONS
6.1 Following refurbishment, we expect the barn to be suitable for weddings and conference's provided that the refurbishment includes the improvements to the roof and barn door areas as described in this report (5.6 and 5.7), and provided that music is limited to light background music that would not be expected to interfere with conversation without raised voices.
7. REFERENCES
7.1 'The Calculation of Road Traffic Noise', Department of Transport, HMSO 1988.
7.2 BS 5228 : Part 1 : 1997, 'Noise control on construction and open sites'.
Glossary of Terms Used:
dB - decibel. Unit used to express noise levels. The decibel scale is a linear numbering scale used to define a logarithmic amplitude scale, thereby compressing a wide range of amplitudes to a small set of numbers. A change in noise level of 3 dB is usually said to be just noticeable, a change of 5 dB clearly noticeable, 10 dB twice, or half, as loud.
dB(A) - The human ear hears some frequencies better than others. The 'A'-Weighting Curve is a filter applied to a noise measurement to represent the frequency non-linearity of the human ear. Noise measurements made with an 'A'-Weighting filter have the unit dB(A). The 'A'-Weighting Curve is shown in Fig G1.
LAeq - The 'A'-Weighted Equivalent Continuous Noise Level. The noise level may significantly vary over a measurement period (as Fig G2). The LAeq is the level that a steady, non-varying noise would be to have the same energy as the actual time varying noise level over the same measurement period. It is usual to state the measurement duration with the value of the measurement (e.g. LAeq,1hour = 68 dB(A))
LA90 - The 'A'-Weighted noise level that is exceeded 90% of the time. Commonly used as parameter for background noise as it is less contaminated by occasional peaks that are not characteristic of the background noise.
LL90 - The linear (i.e. without 'A'-Weighting filter) equivalent of LA90.