Receptor identification in a noise impact assessment

Receptor identification decides the outcome of a noise assessment before a single measurement is taken. Get the noise assessment receptor identification wrong and every subsequent number is answering the wrong question, which is why environmental health officers read that section first and why they send assessments back when it is thin.

For a restaurant, gym, takeaway or small industrial unit, the receptors are the people who will complain. Identifying them accurately is not a courtesy to the neighbours. It is the difference between a permission that survives and a condition you cannot meet.

Defining a noise sensitive receptor

A noise sensitive receptor is a location where people are exposed to sound in a way that matters for their health or amenity. Dwellings are the obvious case, and the assessment usually takes the nearest facade or the nearest private garden.

Beyond housing, the category covers hospitals and care homes, schools and nurseries during their operating hours, places of worship, hotels and other overnight accommodation, and offices where speech intelligibility matters. Outdoor amenity space attached to a dwelling counts, and so does a balcony.

Some receptors are sensitive only at certain times. A school is a daytime receptor. A hotel is a night time one. A place of worship may be sensitive at specific hours on specific days, which is exactly the sort of detail a generic assessment misses and a local objector does not.

Write the sensitivity period against each receptor in the schedule. It changes which background period governs, and it changes the hours a condition should cover.

Choosing measurement positions and heights

The position has to represent the receptor, not the site boundary. A boundary measurement is convenient and is rarely what the assessment needs.

Measure free field where you can, at 1.2 m to 1.5 m above ground for a ground floor or garden assessment. Where the relevant receptor is an upper floor window, the assessment has to represent that height, either by measuring there with the occupier's agreement or by measuring at an equivalent exposure and justifying the transfer with a calculation.

Height changes the answer in two directions. An upper window often has a higher background from road traffic because it sees more of the road, and it also often has a clearer line of sight to a roof mounted plant item, so both the background and the specific sound rise. Assuming the ground floor case is the worst is a mistake that gets found at appeal.

Where a facade measurement cannot be avoided, state the correction applied for the reflected component and show it in the calculation rather than burying it in an assumption.

Existing, consented and allocated receptors

Existing dwellings are the start. They are not the end.

A dwelling with an extant planning permission that has not been built is a receptor the authority will expect to see assessed, because granting your permission on the basis that it does not exist creates a conflict the council will have to resolve later. The same applies to a site allocated for housing in the adopted local plan, and to a site with prior approval for conversion to residential.

Search the register for residential permissions within the study area and record the reference, the description, the decision date and whether development has started. Check the adopted plan's allocations. Then state, for each, whether it has been assessed and why.

This is the agent of change question in reverse. National policy places responsibility on the new development to mitigate its effects on existing uses, and equally requires new noise sensitive development to take account of existing businesses. A commercial applicant who ignores a consented housing scheme next door is inviting a condition drafted to protect a building that does not exist yet, on assumptions nobody has tested.

Facade and free field corrections

Two conventions circulate and mixing them corrupts the assessment.

A free field measurement is taken away from reflecting surfaces, at least 3.5 m from any facade. A facade measurement is taken close to a building and includes a reflected component, conventionally around 3 dB higher for a hard flat facade at normal incidence.

The criteria you are assessing against have their own convention, and internal criteria derived from the building standard for sound insulation are usually approached from a free field or facade incident level depending on the source. State which convention each figure uses, every time a number appears.

Where a glazing specification is being derived from the external level, the direction of the correction changes the required sound reduction index, and an error of 3 dB in the wrong direction is the difference between standard double glazing and an acoustic unit.

Photographic records of each receptor

Photograph every receptor from the source and every source from the receptor. Include the microphone position in at least one frame with something for scale.

A short appendix with a numbered plan, a photograph per receptor and a one line description does more to make an assessment credible than another page of methodology. It also lets a reviewer confirm the line of sight, the presence of intervening screening, and whether the garden that was assessed is the garden that exists.

Record the date. Fences come down, extensions go up, and a receptor photographed two years ago may have a new conservatory between it and your plant.

Justifying omissions

Every assessment leaves receptors out. Say which and why.

Acceptable reasons are that the receptor is screened by intervening built form and is further from the source than an assessed receptor in the same direction, that it is further away with the same or better screening, or that it is not noise sensitive. Unacceptable reasons are that access could not be obtained, or that the occupier did not respond, because neither removes the receptor from the assessment.

Where access is refused, assess at an equivalent position and say so. That is a stated assumption a reviewer can test, which is what the process needs.

Set the omissions out in the same schedule as the assessed receptors, with a column for the reason. A separate list buried in an appendix invites the officer to conclude that something has been quietly dropped.

Where environmental health officers disagree

Three disagreements come up repeatedly, and all three cost less to resolve before submission than after it.

The first is which receptor governs. The officer may hold that a first floor bedroom rather than a garden is the critical position, which changes the background and the criterion together. The second is whether a consented or allocated site should be treated as a receptor, where officers are usually firmer than applicants expect. The third is the sensitivity period, particularly for early morning deliveries and late night operation, where the relevant background is often the lowest of the day. Our note on night deliveries and servicing noise covers that case in more detail.

Agree the receptor schedule with the environmental health officer at pre-application stage, in writing. A Noise Impact Assessment built on an agreed receptor list is assessed on its conclusions. One built on a list the officer disputes is assessed on its foundations, and rarely survives.

Where extraction plant is involved the schedule should match the equipment described in the Ventilation and Extraction Statement, and where an acoustic barrier is proposed the claimed performance needs the scrutiny set out in our note on what an acoustic barrier actually achieves.

Councils expect a suitably qualified author for noise work and professional review before submission is strongly advised.

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