Plant noise from commercial kitchens: assessment and fixes

Commercial kitchen plant noise is refused on the numbers, and the numbers come from a plant schedule most applicants have not written yet. The assessment method is BS 4142:2014+A1:2019, which compares the rating level of your plant at the nearest noise sensitive facade with the background sound level there. If the rating level exceeds background by around 10 dB the standard indicates a significant adverse impact. Around 5 dB indicates an adverse impact. At or below background, the effect is likely to be low. Everything else in the case is detail hanging off that comparison.

The good news for a restaurant or takeaway operator is that plant noise is an engineering problem with engineering answers. Unlike a layout objection, it can be fixed after the design is fixed.

Identifying every source in the plant schedule

Start by listing every piece of fixed mechanical equipment that will run, with its location and its operating hours. The list is longer than most people expect.

Kitchen extract fan and its discharge terminal. Make-up air supply fan. Refrigeration condensers for the cold room, the under-counter units and any display cabinet, each of which cycles independently. Air conditioning condensers for the dining area and, separately, for the kitchen. Any dumbwaiter or lift plant. Cellar cooling for a bar. Glass-wash and dishwasher extract where it discharges externally. And the grease filtration or ozone unit if one is fitted in the duct run.

Mark each one on a plan with its height above ground and the surfaces around it, because a condenser in a brick lightwell behaves very differently from the same unit on an open flat roof. Then note which items run at night. Refrigeration runs all night. Extract usually does not.

Sound power data and where to get it

An assessment needs sound power level, written as Lw in decibels, in octave or third-octave bands, for each item. Sound pressure at three metres is not the same thing and cannot be substituted without assumptions the acoustician will have to flag.

Get the data from the manufacturer's technical sheet rather than a sales brochure, and get it for the exact model and the exact speed setting proposed. A fan quoted at one speed and installed at another is the commonest reason a commissioning test fails. Where the data is not available, the acoustician can measure an identical installation elsewhere or use a published generic value, and either approach has to be stated as an assumption in the report.

One practical step saves weeks. Ask the mechanical and electrical designer to lock the plant selection before the Noise Impact Assessment is commissioned. Assessing a provisional schedule and then changing the fan means assessing it twice.

Predicting levels at the nearest window

The prediction is a propagation calculation from each source to each receptor, corrected for distance, directivity, screening and reflections, then combined. For a single source radiating over a reflecting surface, the illustrative starting point is the sound pressure level equalling the sound power level minus 20 times the logarithm of the distance in metres, minus 11 decibels. Real cases add barrier attenuation, duct breakout and reflection from facing walls, which is why the arithmetic belongs to an acoustician.

The receptor matters as much as the source. Identify the nearest noise sensitive facade, meaning a window or a private garden serving a dwelling, and take the height right. A flat above a neighbouring shop puts a bedroom window four metres up and five metres away from the roof where you wanted to put the condensers.

Background sound level is measured, not assumed. The relevant value is the LA90 over the assessment period at the receptor, and the period that decides the case is almost always the quietest one during which the plant runs. For a restaurant with refrigeration running through the night, that is the small hours, when background in a residential street may drop far lower than the daytime figure everyone quotes.

Attenuators, enclosures and anti-vibration mounts

Mitigation works in a fixed order of effectiveness, and the lowest cost interventions come last rather than first.

Anti-vibration is the item most often left out and most often the source of the complaint that arrives anyway. A condenser hard-mounted to a flat roof over a bedroom transmits through the structure, and no amount of airborne attenuation touches it. Specify mounts by static deflection matched to the unit mass and the disturbing frequency, and put flexible connections in every pipe and duct crossing the mount line.

Barriers are the measure clients believe in most and deliver least. Before designing one in, read what an acoustic barrier actually achieves, because a 2m fence beside a rooftop unit that a first floor window looks down onto does almost nothing.

Night time operation and the character corrections

BS 4142 does not stop at the measured level. The rating level is the specific sound level plus corrections for acoustic character, and those corrections can add substantially to a plant that measures acceptably.

Tonality attracts a correction of up to 6 dB, and fans produce tones at the blade pass frequency, so a fan that sings is penalised twice: once for the level and once for the character. Impulsivity attracts up to 9 dB and is usually irrelevant for kitchen plant. Intermittency attracts 3 dB and is directly relevant to refrigeration, because a condenser cycling on and off through the night is more noticeable than a steady source at the same level. Another 3 dB is available for other distinguishing characteristics.

So a compressor at background plus 4 dB with a tone and a cycling pattern can rate at background plus 11 dB, which reads as the top of the scale. Design for the corrections rather than discovering them.

Night hours are where the corrections and the low background meet. Where refrigeration must run overnight, consider a lower-speed night setting, relocating condensers away from the residential elevation, or a fully enclosed plant room with attenuated ventilation. The related issue of night deliveries and servicing noise at commercial sites usually arrives in the same officer report.

Commissioning tests after installation

Expect a condition requiring a post-installation verification test, and welcome it, because it is the mechanism that closes the file.

1. Agree the test method, receptor locations and the assessment periods with environmental health before installation, in writing.
2. Test with the plant operating in its noisiest normal configuration, not on a mild afternoon with half the units idle.
3. Measure background at the same receptor during the same period, since the comparison is a comparison.
4. Report the rating level with the character corrections applied and shown separately.
5. Where the result exceeds the condition limit, implement the contingency measure already identified in the assessment rather than starting a new design.
6. Submit the report to discharge the condition and keep a copy, because a later complaint will be judged against it.

Naming a contingency measure in the original assessment is the single most useful thing an acoustician can do for an operator. It turns a failed test into a purchase order instead of a planning problem.

Conditions that survive a change of equipment

Ask for a performance condition, not an equipment condition. A condition naming a specific fan model locks the kitchen to that model for the life of the permission, and replacing it in five years technically breaches the condition.

The wording that works sets a rating level at a defined receptor: the rating level of noise from all fixed plant, determined in accordance with BS 4142:2014+A1:2019, shall not exceed the background sound level at the nearest noise sensitive facade during any assessment period. Some authorities require background minus 5 dB. Either version lets the operator change equipment freely provided the limit is met.

Where the extract also raises odour, the Ventilation and Extraction Statement usually carries both issues, and Defra guidance on the control of odour and noise from commercial kitchen exhaust systems is the reference environmental health officers use. Remember too that a planning permission is no defence to a statutory nuisance action, as statutory nuisance after planning permission is granted explains.

Enter your postcode on PlanScope to see which reports a catering scheme is likely to need, then have the plant noise assessment prepared or reviewed by a suitably qualified acoustician.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.