How Do We Measure SPL? A Guide to SPL Metrics

How Do We Measure SPL? A Guide to SPL Metrics

What is SPL?

Sound Pressure Level is a measure of the pressure fluctuations in air caused by a sound wave. It’s measured with a microphone, and can help to characterize the level of any source of sound or noise.

Before Measuring...

Before Smaart is ready to take SPL measurements, it must first be calibrated. SPL calibration requires referencing a known physical amplitude (or acoustic test signal) provided by a sound level calibrator. Typically, a calibrator will generate a 1 kHz tone at 94dB or 114dB, a known value by which Smaart can be set. When performing the calibration, Smaart will calculate the difference between the current measurement level and what the calibrator is set to, then apply it as an offset to your measurements. It is possible to enter an arbitrary calibration offset without an SPL calibrator, but where accuracy is required, an SPL Calibrator (with the appropriately sized acoustic coupler/adapter ring) must be used. 

An in-depth guide to calibrating Smaart for SPL measurements can be found here.

What are SPL metrics and why do they matter?

Various metrics allow us to characterize SPL measurement data in different ways, much like a photographer could use different camera lenses to give different views of the same subject. They are all valid, but some metrics are more useful than others, depending on why we are measuring.

Just tossing out a number like “95 dB” doesn’t communicate enough information to be meaningful – it’s “not a complete sentence” from a technical standpoint. We need to include enough context to clearly indicate which metric we’re referring to. Before any productive dialogue can occur, everyone needs to be on the same page – that means always clearly indicating the complete metric when discussing SPL values.

Which Metrics Should I Use?

Different metrics tell us different things about the data, so it’s important pick the right tool for the task at hand.

Sound exposure measurements (“Is this a safe level?”) are generally best approached with a 15-minute A-weighted average (LAeq 15) or by measuring directly as percent dose (Exposure N for safety, Exposure O for OSHA compliance). Typically, the A Weighting curve is a good statistical predictor of sound exposure that leads to Noise-Induced Hearing Loss. However, high levels of low frequency energy can also lead to hearing damage, and deserve special consideration when the C-weighted levels exceed the A-weighted by 20 dB or more (C-A > 20 dB).

Nuisance noise measurements (“Are we bothering the neighbors”) should accurately characterize the low frequency energy most likely to travel long distances and disturb others. Relatively short-term C Weighted and 63 Hz Octave Band measurements are good choices (SPL C Slow and SPL Slow 63 Hz).

Various jurisdictions specify different metrics and measurement methods as part of their noise ordinance policies, and some are more appropriate and effective than others. Luckily, there’s no need 

to choose, as Smaart offers the capability to measure and log any combination of metrics at once.

For a more in-depth look at SPL weighting curves, see this article.

What is Leq and why is it important?

The “Fast” and “Slow” metrics typically found on handheld meters decay far too quickly to make a meaningful statement about the level of the last verse, or song, or performance due to the dynamic nature of live events. Instead, a metric called Equivalent Continuous Sound Level (Leq) averages levels over a longer period of time.

What does “Class Compliant” mean? Do I need it?

Class Compliant means that the measurement rig – and all its components – have been individually lab-certified to meet all the requirements of the applicable standards. This doesn’t mean the measurement is any more accurate, but it does mean that the SPL log data collected by the system can rise to a legal standard of evidence. Systems like 10EaZy come pre-calibrated and tested from the factory, and are tamper proof, so you know the measurement data can be trusted.

Class Compliant hardware becomes important if you need to prove in court that your levels were not in violation of a noise ordinance, workplace noise regulation, or other legal requirement.