When we calibrate noise measurement instruments, we are doing two things: ensuring the equipment is highly accurate and proving that your measurement data is valid.
While “calibration” has a very strict definition in the scientific world of metrology (the science of measurement), professionals in the acoustics industry typically use the term to describe two entirely different processes:
- Field Calibration (The Daily Check): Quickly testing your measurement system with a portable sound calibrator at the beginning and end of every measurement session.
- Laboratory Calibration (Periodic Verification): Sending your gear to an accredited lab for rigorous testing to ensure it complies with national and international standards (like ANSI and IEC).
Here is a breakdown of how both processes work and why they matter for your project.
1. Field Calibration: The Daily Check
In the US and international acoustics standards—such as ANSI/ASA S1.4 and ISO 1996-2—performing a field check is often referred to as “field calibration.”
This check ensures your sound level meter is performing exactly as intended before you start collecting data. Most regulatory frameworks and measurement protocols require a field calibration both before and after every measurement session. This creates a reliable “bookend” for your data, proving that the instrument’s sensitivity didn’t drift or change while you were out in the field.
Why this matters: Documenting your field calibration is vital if your data is ever used in local zoning disputes, OSHA compliance audits, or legal proceedings.
How to Perform a Field Calibration
The process is straightforward:
- You place a portable, specialized sound calibrator directly onto the microphone.
- The calibrator generates a highly stable, known sound pressure level (usually 94 dB or 114 dB) at a specific frequency (typically 1 kHz).
- If your sound level meter doesn’t read the exact output of the calibrator, you adjust the meter’s sensitivity until it matches.
Note: While modern sound level meters are incredibly stable, microphones are still highly delicate instruments that can easily be altered or damaged by humidity, temperature drops, or minor bumps. A quick field check catches these issues immediately.
2. Understanding Instrument Classes
Not all sound level meters—or calibrators—are created equal. Under standard US and international guidelines (ANSI/ASA S1.4 and IEC 61672), sound measurement tools are divided into two primary tiers based on their accuracy:
- Class 1 (Precision): Designed for lab use and precise field measurements. They have a wider frequency response and tighter tolerances, making them the standard for legal and environmental enforcement.
- Class 2 (General Purpose): Designed for general field use, such as basic OSHA workplace noise assessments or preliminary surveys.
Because a sound level meter is only as accurate as the tool checking it, acoustic calibrators are also divided into classes. This ensures a high-precision Class 1 meter isn’t compromised by a less accurate field calibrator.
3. Laboratory Calibration (Periodic Verification)
Field calibration is great for day-to-day confidence, but it doesn’t test the full health of your instrument. That is where Periodic Verification comes in.
Every one to two years, your sound level meters, microphones, and calibrators should be sent to an accredited calibration laboratory. The lab subjects your gear to an extensive battery of tests, checking its frequency response, linearity, and overall accuracy across its entire operating range.
Think of it this way: Field calibration is like checking your car’s oil before a road trip. Laboratory verification is the comprehensive factory service that ensures the engine is performing to exact specifications.
Scantek can provide traceable, reliable, and expert calibration services for most major brands of sound and vibration instrumentation. For more information, please visit our Cal Lab webpage.