Example Letter to Stop the Asphalt Plant, Cole Green Lane, Welwyn Garden City. @SpeakUpWGC

Noise Evidence

The Asphalt Plant Noise Problem: Real Measurements From Inside a Welwyn Garden City Home

Residents across Welwyn Garden City have long raised concerns about noise from the BP Mitchell Asphalt Plant. Until now, only modelled predictions supplied by the applicant (figures that estimate noise levels rather than measure them) were available.

Over three consecutive days in February 2026, we carried out indoor sound measurements from a home approximately 650 metres from the plant. The results show a clear, repeatable pattern:
the asphalt plant is generating intrusive low‑frequency noise that penetrates the building envelope and raises indoor noise levels by 10–15 dB(A) every time it operates.

This article presents the real‑world evidence.

Why These Measurements Matter
Noise assessments submitted by the applicant rely on modelling, which often assumes ideal conditions and perfect mitigation. In contrast, these measurements were taken:

Inside a real residential bedroom
With the window closed
Using digital sound measuring equipment
Across multiple days
At a distance where such noise should not be intrusive

The results show what residents actually experience!

Low‑frequency industrial noise is particularly important because:

It travels much further than higher‑frequency sound
It penetrates walls, glazing, and insulation with ease
It accumulates inside rooms, especially bedrooms
It is more noticeable in quiet morning and daytime periods
It is strongly linked to loss of amenity and sleep disturbance

This is why Environmental Health and planning policy treat low‑frequency noise as a serious concern.

What the Data Shows Across Three Days
Across all three days, the same pattern appears:

A quiet indoor baseline of 33–36 dB(A) when the plant is off
A sustained uplift to 42–48 dB(A) when the plant is running
Immediate rises and falls that match the plant’s start/stop times
A smooth plateau typical of low‑frequency industrial machinery
Noise clearly audible inside the home, despite closed windows

This is not a coincidence. It is a consistent industrial noise signature.

Day 1 – 25th February 2026
Two operating periods were recorded. In both cases, indoor noise levels rose sharply and remained elevated for the entire duration of plant activity.

Sound Measurement taken from a residential home 650 meters away from the BP Mitchell Asphalt Plant

This demonstrates a clear cause‑and‑effect relationship between plant operation and indoor noise intrusion.

Day 2 – 26th February 2026
The plant began operating at 07:30. Indoor noise levels jumped immediately and remained elevated until the plant stopped at 08:50.

This early‑morning disturbance is especially concerning because background levels are lower and residents are more sensitive at this time.

Day 3 – 27th February 2026

Two further operating periods produced the same uplift of 10–15 dB(A), with immediate rises and falls matching the plant’s activity.

This third day confirms the pattern is not weather‑related, not random, and not caused by local activity.

What This Means Under BS 4142
BS 4142 is the UK’s standard for assessing industrial noise. It compares:

Background sound level
Specific sound level from the industrial source
Any penalties for tonal, intermittent, or low‑frequency characteristics

Across all three days:

Background: 33–36 dB(A)
Specific level during operation: 42–48 dB(A)
Difference: +10 to +15 dB(A)
Likely penalties: +4 to +6 dB

This places the rating level difference in the region of +14 to +21 dB.

Under BS 4142:

+10 dB = likely adverse impact
+15 dB or more = likely significant adverse impact

This evidence falls firmly into the significant adverse impact category, the highest level of concern.

Why Indoor Measurements Are So Important

Indoor measurements reveal the actual lived experience of residents. They show:

Low‑frequency noise travelling unusually far
Penetration through walls and glazing
Impact on sensitive rooms such as bedrooms
Noise levels that would not normally be expected at 650 metres
A clear loss of residential amenity

This is the type of evidence Environmental Health officers rely on when assessing statutory nuisance and planning compliance.

What This Means for Birchall Garden Suburb

The findings also have serious implications for the proposed Birchall Garden Suburb (BGS) development. The new neighbourhood will place hundreds of homes far closer to the BP Mitchell Asphalt Plant than existing properties in Welwyn Garden City.

Some of the planned residential plots will be located as close as 150 metres from the plant; less than a quarter of the distance from the home where these measurements were taken.

This raises fundamental questions about the suitability and viability of placing new residential development so close to a heavy industrial operation that already affects homes much further away.

Conclusion

The three days of indoor sound measurements provide clear, repeatable, and compelling evidence that the BP Mitchell Asphalt Plant is generating intrusive low‑frequency noise that penetrates homes in Welwyn Garden City. The uplift of 10–15 dB(A) during operation represents a significant adverse impact under BS 4142 and demonstrates a real‑world problem that modelling alone cannot capture.

What these findings also reveal is why the applicant has relied so heavily on modelled noise predictions rather than submitting real operational measurements. Despite operating the asphalt plant daily, the applicant has not provided any genuine on‑site or residential monitoring. The real‑world data presented here shows a level of indoor intrusion that would be extremely difficult to justify within a planning application. It is therefore unsurprising that the applicant chose to present theoretical modelling instead of actual measurements taken while the plant is running.

This evidence shows that the lived experience of residents is very different from the picture painted by the applicant’s modelling. It highlights the urgent need for independent monitoring, proper scrutiny, and meaningful action to protect residential amenity.

All evidence gathered has now been formally passed to both Environmental Health and the Planning Officer for review, ensuring that the real‑world impact on residents is fully considered as part of the ongoing planning application of the asphalt plant’s operations.

Noise Evidence

The Asphalt Plant Noise Problem: Real Measurements From Inside a Welwyn Garden City Home

Inside a real residential bedroom

With the window closed

Using digital sound measuring equipment

Across multiple days

At a distance where such noise should not be intrusive

The results show what residents actually experience! Low‑frequency industrial noise is particularly important because:

It travels much further than higher‑frequency sound

It penetrates walls, glazing, and insulation with ease

It accumulates inside rooms, especially bedrooms

It is more noticeable in quiet morning and daytime periods

It is strongly linked to loss of amenity and sleep disturbance

This is why Environmental Health and planning policy treat low‑frequency noise as a serious concern. What the Data Shows Across Three Days Across all three days, the same pattern appears:

A quiet indoor baseline of 33–36 dB(A) when the plant is off

A sustained uplift to 42–48 dB(A) when the plant is running

Immediate rises and falls that match the plant’s start/stop times

A smooth plateau typical of low‑frequency industrial machinery

Noise clearly audible inside the home, despite closed windows

This is not a coincidence. It is a consistent industrial noise signature. Day 1 – 25th February 2026 Two operating periods were recorded. In both cases, indoor noise levels rose sharply and remained elevated for the entire duration of plant activity.

Day 2 – 26th February 2026 The plant began operating at 07:30. Indoor noise levels jumped immediately and remained elevated until the plant stopped at 08:50.