How to Measure Background Noise Level per BS 8233

What You Need

• A device with a microphone (phone, laptop, or tablet)
• A calibration reference if available (94 dB calibrator recommended for Class 2 accuracy)
• The room unoccupied and HVAC running at normal operating conditions
• SonaVyx SPL Meter open at /tools/spl-meter

Step-by-Step Procedure

1. Prepare the room. Close all doors and windows. Ensure HVAC is running at its normal daytime setting. Remove any temporary noise sources (phones, computers with fans). The goal is to capture only the steady-state background noise that occupants will experience.
2. Calibrate your microphone. If you have a 94 dB calibrator, use SonaVyx's calibration function under Settings. Place the calibrator over the microphone and tap "Calibrate." Without a calibrator, SonaVyx applies a default phone microphone correction curve, but results should be treated as approximate (±3 dB).
3. Position the microphone. Place the device at the measurement position — typically 1.2 m above floor level (seated ear height) and at least 1 m from any reflecting surface. For BS 8233 compliance, measure at the centre of the room or at the most sensitive receiver position.
4. Select measurement parameters. Set weighting to A-weighting (LAeq is the primary metric for BS 8233). Set time weighting to Slow. Enable statistical percentiles (L10, L50, L90). L90 represents the noise level exceeded 90% of the time — this is your background noise level.
5. Measure for at least 5 minutes. BS 8233 recommends a minimum measurement duration of 5 minutes for steady-state noise. For intermittent noise sources, extend to 15-30 minutes. Tap Start and leave the device undisturbed. SonaVyx logs continuously and calculates running Leq and percentiles.
6. Record octave band data. Switch to octave band view (1/1 octave) to capture the spectrum. This is needed for NC and NR curve comparison. SonaVyx automatically calculates the NC rating from octave band levels.
7. Compare against BS 8233 criteria. Check your LAeq result against the BS 8233 Table 4 criteria: bedrooms 30 dB LAeq, living rooms 35 dB LAeq, offices 35-40 dB LAeq, classrooms 35 dB LAeq, hospitals 35-40 dB LAeq. If your L90 exceeds these values, the HVAC or other building services need attention.
8. Export your data. Use SonaVyx's CSV export to save timestamped SPL data, octave band levels, and statistical percentiles. This provides the audit trail needed for a BS 8233 compliance report.

Key Parameters

Parameter	What It Means	Target
LAeq	A-weighted equivalent continuous level	Per BS 8233 Table 4
L90	Level exceeded 90% of time (background)	Typically 3-6 dB below LAeq
NC Rating	Noise Criteria curve tangent	NC-25 studios, NC-30 offices
NR Rating	Noise Rating (European equivalent)	Per project specification

Common Mistakes

• Measuring too short. A 30-second reading misses HVAC cycling. Always measure at least 5 minutes, ideally 15 minutes for a representative sample.
• Forgetting to set A-weighting. BS 8233 criteria are in dBA. Measuring with Z-weighting will overstate low-frequency HVAC rumble and give artificially high readings.
• Phone microphone placement. Phone mics have directional characteristics. Point the microphone upward, away from the phone body, and avoid covering it with your hand or a case.
• Not recording octave bands. The overall dBA number can pass while individual octave bands exceed the NC curve, indicating tonal issues that will annoy occupants.
• Measuring with people present. Background noise measurement must capture building services noise only. Even one person breathing adds measurable noise above NC-25.

When to Use L90 vs LAeq

LAeq captures total energy including intermittent events. L90 filters out transient sounds and represents the underlying steady noise floor. For HVAC noise assessment, L90 is often more representative. BS 8233 primarily references LAeq but notes that L90 may be more appropriate when intermittent noise sources are present. Report both values in your assessment.

Tool Bridge

Open the SonaVyx SPL Meter to start measuring. Enable octave band view and statistical percentiles for a complete BS 8233 assessment. Export CSV for your compliance report.