About
The expert behind Noise Harvest is C J Biggs, who has 15 years’ experience in acoustics within large multi-disciplinary consultancy firms. He has completed the institute of acoustics diploma and has a masters in applied acoustics. During his employed years has won multiple awards and recognition.
What is?
Acoustics is often seen as a dark science, the following a small insight of some of the tasks and attempts to explain them.
A noise survey
These are used to establish what the existing acoustic climate is so that informed decisions and calculations can be made about the follow aspects of the building:
Glazing
Ventilation strategy
External walls
Roof construction
Noise levels of fixed items of plant
Without a noise survey there is a risk of over specifying items such as glazing costing the project money or resulting in poor acoustics.
Noise surveys can also be used to establish the impact of new industrial sources using BS 4142:2014 to assess them.
Prior to a noise survey and with the permission of the client an engineer may contact local authorities to agree a scope of works to try and prevent problems further in the process. They are carried out by using sound level meters which are set up at specific locations. These usually run for several hours or even days to log noise data. The engineer conducting the survey will also write notes and take photos to accompany the data.
Depending on the application of the data the relevant decibel will be used, as there are many different types of decibel and the descriptor will inform you of the different type. The typical descriptors from a survey are shown below:
Leq,t – is an average of the measured decibels over a time period
L90 – is a statical analysis of the sound level that is exceeded for 90% of the measurement period.
Lmax – this is the maximum sound level that has been measured and can be either fast or slow denoted with f or s in the subscript.
Calculation are made to the results, from distance correction and more complex calculations described in standards such as BS 8233:2014. These will then be used in the environmental report or specification.
Acoustic design
You will never notice good acoustic design, but you will notice bad acoustics.
Most buildings have design guidance or standards that they have to for follow acoustics the list below highlights the design areas, but they are not an exhaustive list:
Residential – Approved Document E (Doc E) & BS8233:2014
Schools – Building Bulletin 93
Healthcare Health technical memorandum 08-01 (HTM 08-01)
Industrial – BS 4241:2014
Within the above documents different targets are given, the most typical ones are listed below:
partition and floor performance often gives rise to a big confusion during the design and construction stage between the difference of on-site performance and laboratory measurements and why they are different. The following explains the difference between the two.
Rw– is a laboratory value and is usually between 5-7 dB higher than an on-site performance for light weight constructions and 3 dB higher for block work. This is because only a small part of the construction is tested within a large concrete wall and does not make allowances for flanking paths and on-site workmanship.
DnTw – Is the descriptor typically used for sound insulation testing. This does allow for onsite performance of the flanking paths and workmanship and therefore it is usually lower. The Term Dntw is not an exhaustive term for on-site performance but it is a common one.
Internal ambient noise levels (IANL)
This is the noise level of the space in question and includes noise break in from outside. This can be directly related from the noise levels measured from the environmental noise survey and the glazing and ventilation recommended from it.
Noise Rating Curve (NR)
If the building mechanically ventilated this is usually applicable to measure NR levels they are a method for rating the acceptability of indoor environments. They increase in steps of 5.
Reverberation time
This can sometimes be overlooked. Without the correct reverberation control the space can then be unusable if the room is too reverberant. It has a direct relation on speech intelligibility and will feed into the overall comfort of the space. A long reverberation time is a lively, whilst a short reverb is seen as a dead space.
Reverb is usually controlled with either a lay in ceiling, acoustic rafts or baffles or wall panels. Some space that have soft furnishings such as most residential dwellings will not require these as the furnishings will reduce the reverb.
The descriptor for reverb is usually either Tmf which are the middle frequencies (500, 1000 and 2000 Hz) or 500 Hz.
The acoustic design will usually be included in an acoustic specification with accompanying acoustic strategy drawings.
Sound insulation testing
These tests are carried out towards the end of the construction process and are used to demonstrate that the building meets its targets. They can also be used to establish the performance of separating elements if they are being retained in a refurbishment.
It looks a simple process but as with anything that looks simple there a lot of intricate details that an untrained eye will not see.
There are several different methods to sound insulation tests, but they should all end in the same single figure result.
The results of the testing will be in a separate report.