Reviving the Sound Level Indicator (SLI)

Our director Ian Campbell MSc, FIOA has spent his career working in the acoustic industry and has written a full technical report on reviving the sound level indicator. A shortened version can be read below, see the full report at the bottom on this post.

While modern standards like BS 61672 introduced high-precision engineering and precision instruments in 2002 , the older Grade 3 Sound Level Indicators (SLIs) are far from extinct. In fact, the market is currently flooded with models ranging from under £100 to over £500. However, because these devices have been absent from official quality control standards for over 25 years, users must be diligent about the uncertainty of the results they provide.

The Precision Gap

The primary risk with SLIs is specification creep, where manufacturers make vague claims about conforming to British Standards that can easily be misunderstood by beginners. Unlike professional Sound Level Meters (SLMs), SLIs rarely come with a sound calibrator. Without this reference tool to set the device to the international standard of 94 dB, any natural drift caused by the environment or time goes completely unnoticed.

Understanding High-Frequency Errors

A major technical hurdle involves how sound interacts with the device itself. To ensure a flat response in an open free-field, microphone designers actually engineer the microphone’s internal pressure response to fall off at higher frequencies. While this works perfectly in open air, it causes the SLI to significantly under-report decibel levels when checked with a standard closed-cavity calibrator.

Furthermore, the physical body of the SLI reflects sound waves, creating interference that further distorts measurements. While professional SLM manufacturers provide complex correction tables to account for these reflections, such data is rarely available for budget SLIs.

A Sustainable Calibration Path

To bridge this gap economically, owners should consider an initial free-field calibration in an anechoic chamber. This one-time test captures the combined effects of the microphone and case reflections, providing the necessary data to calculate corrections. Once this baseline is established, future re-calibrations can be performed using lower-cost pressure methods while maintaining technical integrity.

Ultimately, while SLIs lack the stability and dynamic range of professional meters, they remain a valuable entry point for noise measurement if their limitations are managed through proper, recorded calibration.

Full report: