Case study · Wood processing - A wood mill above the exposure limit value
A real, anonymised assessment of a wood-processing plant — a hammer mill above the exposure limit value, and what the decibel scale really means.
Impact Acoustics · updated 10 Aug 2026 · 4 min read
The site
A wood-processing and animal-bedding plant in the West Midlands, running hammer mills, balers, a shredder and dust extraction. We measured fourteen task and area positions to Class 1 standard on an 8-hour basis.
What we found: wood processing at 95.4 dB(A)
Real measured daily personal noise exposure (LEP,d) by work area, from an anonymised recent assessment. Values above 85 dB(A) are Hearing Protection Zones.
Four hammer-mill and baler positions sat at or above the 85 dB(A) upper action value, led by an external hammer-mill position at 95.4 dB(A) — above even the 87 dB(A) exposure limit value that applies before hearing protection is taken into account. Six more areas fell between 80 and 85 dB(A). The loudest peak was 115.3 dB(C), below the 135 dB(C) action value.
The insight that pays for the survey
The headline figure is worth pausing on. Because noise energy doubles every 3 dB, the hammer mill at 95.4 dB(A) carries roughly ten times the sound energy of the 85 dB(A) action value — and an unprotected worker there reaches the daily limit in about 44 minutes. Crossing the exposure limit value before protection means reducing exposure isn’t optional or gradual — it has to happen. The assessment set the priority squarely on the hammer-mill line: engineering controls and maintenance first, with hearing protection managing the risk that remains rather than being the whole answer.
What the client got
- Mandatory Hearing Protection Zones across the four hammer-mill and baler areas over the upper action value, with signage and defined boundaries.
- A source-control and maintenance priority list for the hammer-mill line (enclosure and guarding integrity, extraction noise, worn parts), because one area is over the limit that applies before protection.
- Protectors matched to each area’s required attenuation and verified to bring the ear-level into the safe range, plus audiometry for the regularly-exposed staff.
That is the difference between a survey that just says “you’re over the limit” and one that tells you exactly where to act — and keeps it current, which is what the 4-Year Care Plan is for.
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