How Do We Measure SPL?

Created by Jake Bedard, Modified on Thu, 06 Jun 2024 at 03:33 PM by Jake Bedard

How Do We Measure SPL?

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 – a rock concert, an orchestra, traffic noise, or even your neighbor mowing the lawn.

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. The SPL history timeline below shows ten minutes of SPL data from an event, and we can see how nine different metrics give 9 very different answers to the question “what is the SPL?”.

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.

The image below shows how a fifteen minute average (Leq 15) provides a more meaningful characterization of the level of the show than the SPL Slow metric for the same period of time. The longer averaging times also allow a mix engineer to use loud and quiet moments to achieve a dynamic, impactful mix while still keeping long-term levels compliant.

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.

In Review:

  • dBA - SPL reading using A-weighted curve, a statistical predictor of sound exposure that leads to Noise-Induced Hearing Loss
  • dBC - SPL reading using a C-weighted curve, which is similar to A-weighted but accounts for low frequencies
  • dBLeq10/dBLeq15 - "Equivalent Continuous Sound Level", an SPL reading derived from a 10/15-minute average. (Sometimes shown in "dBLAeq15" format to show A-weighting)

Was this article helpful?

That’s Great!

Thank you for your feedback

Sorry! We couldn't be helpful

Thank you for your feedback

Let us know how can we improve this article!

Select atleast one of the reasons
CAPTCHA verification is required.

Feedback sent

We appreciate your effort and will try to fix the article