BS 4142 Explained

BS 4142 is the British Standard used to judge whether noise from plant — a heat pump, an air conditioning condenser, an extract fan, a chiller — is likely to cause complaint at nearby homes. If a planning officer has asked for a noise assessment for plant, this is almost always the standard they mean.

If you already know that is what you need, our Plant & Machinery Noise Assessment starts at $445 and is delivered in 3 working days.

What BS 4142 actually does

It compares two numbers:

• The rating level of your plant — how loud it is at the neighbour, adjusted upwards if it has features that make it more annoying
• The background sound level — how loud it already is there without your plant running

The difference between them drives the conclusion. Roughly: around +10 dB or more indicates a significant adverse impact; around +5 dB indicates an adverse impact; and a rating level at or below background is an indication of low impact. These are indications, not pass marks — the standard is explicit that context matters and that judgement is required.

The bit that catches people out: acoustic feature corrections

Plant noise is not judged on loudness alone. The standard adds a penalty for character:

• Tonality — a distinct hum or whine. Up to +6 dB
• Impulsivity — bangs, clatters, sudden onsets. Up to +9 dB
• Intermittency — switching on and off in a way that draws attention. +3 dB
• Other distinctive characteristics — +3 dB

A fan that is only marginally above background can fail once a tonal correction is applied. This is where most rejected assessments go wrong.

Why background sound level is the hard part

Background is the LA90 — the level exceeded for 90% of the time — measured over a representative period at the neighbour. A strict BS 4142 assessment needs it measured, which means someone with a calibrated instrument on site, usually overnight, because night-time background is lowest and therefore the hardest case.

We are desk-based, so we do not measure it. That gives you two honest options:

• You supply measured data — from your own acoustician or a previous survey. We then produce a full assessment.
• You take a screening assessment — we use published or estimated background levels, and the report says clearly that this is what it is. Useful for early design and for many domestic heat pump applications; not a substitute where the authority has explicitly demanded measured data.

We tell you which you need before you order.

Domestic heat pumps are usually a different test

For a single air source heat pump at a house under permitted development, the usual route in England is the MCS 020 calculation, which asks whether the unit produces no more than 42 dB(A) at the assessment position at a neighbouring dwelling. That is a simpler, more prescriptive test than BS 4142, and it is what our $445 tier covers.

What a good BS 4142 report contains

1. Plant schedule with octave-band sound power levels from the manufacturer datasheet
2. Identification of the nearest noise-sensitive receptors and distances
3. Background sound level, with its source and limitations stated
4. Specific sound level calculated with distance, directivity, screening and reflections
5. Acoustic feature corrections, with the reasoning for each
6. Rating level and the excess over background
7. Context assessment as the standard requires
8. Mitigation where needed, with the attenuation each measure delivers

Why assessments get rejected

• No measured background where the authority asked for one
• Feature corrections ignored, particularly tonality on fans and heat pumps
• Sound pressure at 1 m quoted as if it were sound power
• Night-time operation not assessed when the plant runs at night
• The assessed plant is not the plant installed

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