Listen Up! From the NHCA Experts…

photo courtesy NHCA

Measurement Basics: Don’t Push the Limits

By: Donald Finan, Contributor

Your sound level meter may be trying to tell you something: It has limits, as do we all. Depending on how far you push someone’s limits, you may receive a very clear response. For sound level meters, the response might not be quite as clear and often can be easy to miss—resulting in inaccurate measurement.

The sound level meter’s limits involve measurement ranges used to accurately measure low, medium and high sound levels. While some meters automatically “select” an appropriate range, others require the range to be set manually. An accurate measurement requires an appropriate measurement range.

So, how are the limits pushed? Well, that depends entirely on the sound. In general, measurement of low-amplitude sound requires a low measurement range, while high-amplitude sound requires a high measurement range. Here’s where things can be confusing, though. Measurement ranges will overlap. A “low” measurement range might be 30-80dB SPL (decibel sound pressure level), while a “high” range might cover 80-140dB (Figure 1). Making things worse, there might be an intermediate range from 50-120dB—overlapping both the low and high ranges!

Figure 1: Sound with increasing amplitude applied to three measurement ranges.

As a rule of thumb, choose a measurement range where the sound level is as close to the middle of the range as possible. If the range upper limit is exceeded (i.e., the range is too low for the sound’s level), the meter may display “over-range” (or a related symbol), indicating a “clipped” signal measurement that underestimates the sound’s true amplitude.

Figure 2: Time-varying sound with levels from 64 to 76dB SPL
applied to three measurement ranges.

Similarly, a range that is too high can result in overestimation of the actual level (Figure 2). While exceeding the range limits may produce an out-of-range warning, the meter might still display a consequently inaccurate measurement. So, keep an eye out for the over-range or under-range warnings, and don’t push those limits!

Donald Finan is a University of Northern Colorado Speech Scientist/Professor and National Hearing Conservation Association (NHCA) Expert. Visit NHCA at: https://www.hearingconservation.org/.