Aggregate Plant: Tonal Penalty, Averaging Period, and a Signed Report
An aggregate and ready-mix plant holds a permit condition reading: 'No operation shall cause a sound level exceeding 55 dBA Leq(1 hour) at any residential property line. A 5 dB adjustment shall be applied where the source exhibits a prominent tonal or impulsive characteristic.' Responding to a complaint, an officer measures at the nearest residential property line, 45 m from an unsilenced blower bank. Three 20-minute samples taken between 12:10 and 13:30 read 53.8, 54.4 and 54.1 dBA. One-third-octave analysis shows the 250 Hz band 8 dB above the adjacent bands, and truck backup alarms sounded repeatedly. The officer also records 44 dBA at 06:00 the next morning with the plant idle. In the draft report the three samples are averaged arithmetically to 54.1 dBA, compared directly with 55 dBA, and the plant is found in compliance. The plant manager has already signed the report as accurate and the case file is awaiting closure.
1. Determine whether the condition was met. Show how the samples should be combined, whether the tonal adjustment applies, and whether background should be subtracted.
Read the worked answer
Samples must be combined as an energy average, not arithmetically:
L_eq = 10·log10[(10^5.38 + 10^5.44 + 10^5.41)/3] ≈ 54.1 dBA.
That the two methods nearly coincide here is coincidence, not method.
Tonal adjustment. A 250 Hz band 8 dB above adjacent one-third-octave bands is a prominent tone under any common criterion (usually ≥5 dB in a one-third-octave band). The condition says the adjustment 'shall be applied', so 54.1 + 5 = 59.1 dBA. Backup alarms are impulsive and may also qualify, but the condition supplies one adjustment, not two; do not stack them.
Background. With a total of 54.1 dBA and 44 dBA background, the source alone is 10·log10(10^5.41 − 10^4.40) ≈ 53.7 dBA, or 58.7 dBA after the penalty. The 06:00 idle background is almost certainly quieter than midday background, so subtracting it overstates the source contribution - and the conservative choice still yields a violation.
Conclusion. Exceedance of roughly 3.6–4.1 dB, comfortably outside ordinary measurement uncertainty. Not compliant.
2. Identify at least four weaknesses in the measurement design and specify a defensible re-measurement.
Read the worked answer
- Wrong metric. Three 20-minute samples spanning 12:10–13:30 cover two clock hours and do not constitute a 1-hour Leq.
- Background not contemporaneous - 06:00, plant idle, different traffic and activity.
- Tonal assessment applied to the total level, not to source-only data.
- Single position. The condition applies along the whole property line; the worst-case point must be identified.
- No weather record (wind speed and direction, precipitation, temperature), which governs validity.
- No instrument identification, class, calibration certificate reference, or before/after calibrator readings.
- No event log for the backup alarms.
- The plant manager's signature is not an acoustical QA review.
Re-measurement plan: Class 1 integrating meter with one-third-octave capability; tripod-mounted microphone 1.2–1.5 m above ground and ≥3.5 m from reflecting surfaces; windscreen fitted; wind under 5 m/s and no precipitation; one continuous 60-minute sample during representative production; concurrent plant-idle background the same day; one-third-octave spectrum of the source, with other plant equipment controlled if possible; timed event log; calibration check before and after with a traceable calibrator; photographed and mapped measurement point.
3. The report is signed and the plant may soon be idle. Can the file be closed, and what must happen to the record?
Read the worked answer
The file cannot be closed as compliance. As written, the data record an exceedance (54.1 dBA plus a mandatory tonal adjustment), the metric measured is not the metric permitted, and the background is unrepresentative. If the plant is now idle, the officer still holds complaint records, tone documentation and source identification. That may support an administrative notice or a negotiated correction, but it is weak evidence for a contested hearing on a 1-hour Leq violation.
The record must not be silently altered. Because the manager has signed it, issue a dated, signed addendum that identifies each error, withdraws the compliance conclusion pending re-measurement, and cross-references the original pages. Correcting a signed report without an audit trail would taint every future case from this program and would be far more damaging in testimony than the original error.