Sound Level Meter Calibration: Why Calibrate?

Best SLM Calibration Practices for Measurement Integrity

When we talk about calibrating measurement devices, what are we really talking about? People often mean one of two different things when they say they have calibrated an instrument, especially in the case of sound level measurement. Regardless of your definition, calibration should be an integral part of your process in order to reduce cost, document the health of your instrument, and minimize error risk.

Certain instruments–microphones, for example–are so sensitive to environmental conditions that they should be field validated before and after each measurement, or as often as is reasonably possible. Field validation is usually done as a single point calibration to confirm that the device sensitivity is correct. For microphones, this involves inserting the microphone into an acoustic calibrator to set the system sensitivity, typically at 94 dB or 114 dB and 1 kHz. This type of validation checks the entire measurement chain and allows for comparison of data at different times and environmental conditions. During this check, it is important to allow the microphone and calibrator to stabilize in the environmental conditions for the time stated in the calibrator manual.

LD-CAL-plus-Factory-Calibration-Program-IconFactory calibration, on the other hand, involves a complete, multi-point test of the instrument. Results from a high-quality certification lab are traceable and tied to the ISO 17025 standard for calibration facilities. Some calibration labs, such as Larson Davis’ CAL+ Program, offer additional services, including firmware upgrades, warranty extension, or factory repair. Although ANSI/ISO/IEC 17025:2000 makes clear that it is the responsibility of the customer to determine the appropriate factory calibration interval under the requirements of the customer’s own quality system, most experts agree that yearly factory calibration is a best practice.

Do you really need to calibrate?

When considering how often to calibrate, you should ask yourself: How important are your measurements? What is the cost of failure?

For example, if the test is for demonstration purposes in a university measurements course, the cost of retaking the data may be next to nothing. In fact, many professors design courses and instructions counting on students to make mistakes. Measurements may be taken a number of times. Learning from mistakes does a wonderful job of deepening our understanding and reinforcing proper techniques.

In a more standard test, where there is easy access to the noise source or multiple locations being measured, the cost of a single bad measurement may not be that high. In consumer product testing, the stakes are higher. Falsely stating operational noise levels can create legal problems for a manufacturer.

When measurements are presented as legal evidence, the first question asked will often be, “When was your equipment last calibrated?”

Further, in the case of a “one shot” test, the measurement has to be right. For instance, sound meters are often used to measure the community impact of airport or construction noise. The testing window can be limited if a noise consultant is coming onsite for the day to make the measurement or if the event itself is short in nature. There may be just one chance at getting the right measurement. Redundant measurement devices may be used. Channels are checked, double checked, calibrated, and re-verified.

Finally, there is an entirely different level of scrutiny for tests made for legal purposes. Legal metrology is the generally accepted method for providing quality legal evidence that measured values are correct within acceptable limits. When measurements are presented as legal evidence, the first question asked will often be, “When was your equipment last calibrated?” In the case of sound level meters in particular, the combination of field validation and factory calibration, preferably at an ISO 17025 accredited facility, are critical components for legal metrology.

Regardless of your "cost" of failure, a few things are always common... like the need for credible ISO 17025 accredited, responsive, and timely calibrations. Some organizations choose to build the capabilities in-house, while others outsource to local metrology houses or vendors. Whichever your specific choice, be sure you know, trust and audit their calibration expertise. A lot can be riding on that calibration.