Extech SL510 Class 2 Sound Level Meter ±1 dB: In-Depth Guide to A- and C-Weighting, Slow/Fast Response, and Why 85 dB Is the Swedish Work Environment Authority’s Threshold for Mandatory Hearing Protection

Published 8 July 2026 at 17:26

Sound measurement is one of the most misunderstood disciplines in modern industry. The decibel value on a display may seem self-explanatory — but it is an abstraction that hides at least five separate processing choices that the operator must understand for the measurement to have physical and regulatory significance. A ±1 dB instrument like the Extech SL510 meets Class 2 standards, but what the standards actually require — and when Class 2 is sufficient versus when it is not — is rarely obvious to the operator.

What decibels actually are

Decibels are a LOGARITHMIC unit — not linear. Sound Pressure Level (SPL) is defined as:

SPL (dB) = 20 × log₁₀(p / p₀)

Where p is the measured sound pressure in pascals (Pa), and p₀ = 20 μPa (reference = the threshold of human hearing). Practical consequences:

• 0 dB SPL = 20 μPa (hearing threshold — a completely silent room).
• 20 dB = 10× higher pressure than the reference (a whisper).
• 40 dB = 100× (library silence).
• 60 dB = 1,000× (normal conversation).
• 80 dB = 10,000× (traffic noise).
• 100 dB = 100,000× (chainsaw).
• 120 dB = 1,000,000× (jet engine).
• 140 dB = 10,000,000× (pain threshold, acute hearing damage).

Each 10 dB increase equals 10× more sound pressure, which is perceived by human hearing as approximately “twice as loud.” This discrepancy between physical measurement and human perception is at the core of what sound measurement aims to address.

A-weighting: matching human hearing

Human hearing is not equally sensitive to all frequencies. We hear 1–4 kHz very well (children’s voices, speech consonants) but poorly at 20 Hz (bass rumble) or 15 kHz (high overtones). A-weighting is a frequency filter curve that modifies the microphone’s raw signal so that the measurement mimics what the ear actually “hears.”

In practice: a 100 dB raw signal at 63 Hz may become 74 dB A-weighted (low frequencies are attenuated). At 1 kHz, the same 100 dB raw signal remains 100 dB A-weighted (reference frequency). At 12 kHz, 100 dB raw becomes 94 dB A-weighted.

ALL occupational health and safety legislation in the EU and Sweden uses A-weighted dB (written as “dB(A)” or “dBA”). The Extech SL510 measures A-weighted levels by default.

C-weighting: minimal filtering

C-weighting is a much broader curve that does not attenuate low-frequency sound. It is used when measuring total sound pressure — for example, explosions, shock waves, or low-frequency vibrations in machinery. The difference between dB(A) and dB(C) for the same sound can reveal whether the sound is low-frequency dominant:

• dB(A) – dB(C) ≈ 0: The sound is balanced.
• dB(A) – dB(C) < -5: The sound is low-frequency dominant (e.g., motors, compressors).

Fast and slow response: RMS time constant

Sound is physically a pressure variation that changes 1,000 to 100,000 times per second. The meter calculates SPL as an average value (RMS = Root Mean Square) over a time window:

• Fast (F) — 125 ms time constant: Captures short sound peaks. Used to detect pulsed sounds or short-duration impact noise (e.g., hammer blows).
• Slow (S) — 1000 ms time constant: Provides a stable display that averages over 1 second. Standard for occupational exposure measurements where average exposure is the target.

The ISO standard for occupational exposure measurement is typically “Slow.” “Fast” is used for machine diagnostics and short-duration sounds.

Class 1 vs. Class 2: what the standard actually requires

IEC 61672-2013 defines two accuracy classes:

• Class 1: ±0.7 dB accuracy, guaranteed frequency range of 20 Hz – 20 kHz. Free-field calibrated. Legally significant for formal occupational health documentation.
• Class 2: ±1.0 dB accuracy, narrower frequency range (typically 30 Hz – 8 kHz). Pressure calibrated. Legally significant for general occupational checks and preliminary assessments, but NOT for formal legal documents.

The Extech SL510 is Class 2 with ±1 dB — sufficient to confirm whether a workspace exceeds the 85 dB limit, but not sufficient for legal documentation by the Swedish Work Environment Authority in disputes regarding hearing damage compensation.

The Work Environment Authority’s 85 dB limit: what it means

AFS 2005:16 (Swedish Work Environment Authority) states:

• Below 80 dB(A): No obligation to provide hearing protection.
• 80–85 dB(A): The employer MUST OFFER hearing protection; employees choose whether to use it.
• 85–135 dB(A): Hearing protection is MANDATORY. Signage is required. Annual medical checks are required.
• Above 135 dB(A): Short-term, extreme exposure. Face shield + heavy-duty hearing protection required.

Note: the limit is TIME-weighted over 8 hours (L_EX,8h). Short-term 100 dB exposure may be allowed if the average over 8 hours remains below 85. This requires continuous logging, not single measurements. The SL510 is for spot checks, not dosimetry.

Calibration and wind protection

The microphone is a condenser capsule (electret) with a sensitive diaphragm. Practical consequences:

• Wind protection: Without protection, measurements are affected by air movement (even the operator’s breathing). A foam windscreen over the microphone reduces wind noise by 5–15 dB.
• Calibration interval: Annual verification against a pistonphone reference (94 or 114 dB calibration source). Instrument drift can be 0.2–0.5 dB per year.
• Reference orientation: The microphone should point toward the sound source, not be held in the hand pointing at the floor.
• Reflective surfaces: Proximity to hard walls, ceilings, or furniture can increase the reading by 3–6 dB. The standard requires a distance of ≥1 meter from all reflective surfaces.

Typical measurement in practice

To measure occupational noise in a workshop:

1. Calibrate the instrument in the morning (using a pistonphone or reference source).
2. Set to A-weighting + slow response.
3. Position the microphone ~30 cm from the operator’s ear (head height, arm extended).
4. Allow work to proceed normally.
5. Note the average value over at least 30 seconds.
6. Repeat at different workstations.
7. Document in a measurement protocol.

If values near 80–85 dB are measured at any station, dosimetry over the entire workday is the next step (the SL510 is not sufficient for this — it requires a mounted logging dosimeter).

Where the SL510 makes a real difference

Preliminary occupational screening: companies looking for potentially problematic noise levels before ordering formal dosimetry. Education: school labs and vocational training demonstrations of the dB concept. Maintenance diagnostics: machine bearing wear produces characteristic sound frequencies that change over time. HVAC balancing: noise levels in ventilation ducts at given flow rates. Residential analysis: noise mapping in homes for comfort.

2,500 SEK for a Class 2 instrument that meets IEC 61672-2013 is a fair price. Class 1 instruments (Rion, Brüel & Kjær) start at 15,000 SEK and reach 80,000 SEK — necessary for legal documentation but excessive for daily maintenance use.

Read more: Extech SL510 sound level meter in the shop →