SPL Measurement Guide

SPL Measurement Guide

Frequent Questions, Confusions, and Next Steps

 
This page offers some background on SPL concepts and measurement, addresses a few common misunderstandings, and will help you figure out next steps.

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.

Why not just “use your ears?”

SPL is an objective measure of the strength of sound waves in air – whereas human loudness perception is subjective and complex. It’s possible to feel completely comfortable while being exposed to dangerous sound levels, and it’s also possible to feel like it’s “too loud” even when it’s completely safe.

Use your ears for loudness perception. Use a meter for SPL.

If SPL isn’t a measure of “how loud,” why measure it?

We measure SPL when we need an objective answer to an objective question. Three common reasons to measure SPL are:

  1. A tool for the mix engineer. Is this the same level as it was last time? Are we giving the audience a consistent experience every night? A “quality control” measurement.
  2. Sound exposure. Is this sound level safe? Are we exposing people at this concert to dangerous sound levels? Are we in violation of sound exposure regulations or recommendations such as OSHA or NIOSH?
  3. Nuisance noise. Are we bothering the neighbors? Are we in violation of local or regional noise ordinances?

 

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.

Meet the metrics in this video:

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.

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.

Can’t I just use an inexpensive handheld meter?

Most handheld sound level meters (SLMs) are fine for relative measurements – making sure you’re hitting the same level every night. Just keep the placement consistent and the battery fresh. However, if the number on the screen needs to be meaningful, the odds are against you, and many meters that claim to be within +/- 2 dB accuracy on their packaging are actually not (and they can drift significantly over time). More importantly, most handheld SLM can only measure the SPL Fast and Slow metrics, not the Leq metrics that are important for event SPL measurement.

It is possible to take accurate SPL and Leq measurements with professional-grade handheld instruments such as the NTI XL2. As with all calibrated solutions, they tend to be far more costly than the “bargain bin” handheld sound meters, and should come with calibration documentation.

Can’t I just use my smartphone?

Smartphones can be great for low-level relative measurements, but there are accuracy issues with internal microphones (and the apps themselves) and most phone mics are overloaded at concert levels. Like handheld meters, smartphones and tablets can exhibit a response that varies quite significantly based on how you hold the device. Although mobile apps are fun and handy, they are generally not an appropriate solution for concert SPL measurement.

What equipment is recommended for SPL measurement?

Unlike most general measurement applications, SPL measurement at concert levels can be quite demanding on the hardware. Many measurement microphones cannot physically handle the high Peak levels encountered in a typically concert-level measurement situation. Even occasionally overloads can be problematic, as an overload event will result in all averaging buffers being flushed and invalidation of large periods of log data.

We recommend a measurement microphone with a Max SPL rating of at least 135 dB SPL, preferably 140 dB SPL. This may seem excessive, but it is not. A typical musical program material will have instantaneous Peak levels 30 to 35 dB higher than average A-weighted levels. For example, if a show hovers around 99 dB SPL A Slow, you can expect Peak values in the mid 130’s.

In addition, the microphone should have relatively low sensitivity. At high SPL, some sensitive microphones produce very high voltages that can overload the connected preamp and converter. As a rule of thumb, multiply the mic’s sensitivity in mV/Pa by 200 to determine its voltage output at 140 dB SPL, and then make sure your IO device can accommodate that voltage.

For example, an Earthworks M30 has a sensitivity of about 34 mV/PA. At 140 dB SPL, it will pass a voltage of 6.8Vrms (+19 dBu) to the interface, and many preamps will be overloaded.

By contrast, an iSemCon EMX-7150 has a sensitivity of 6 mV/PA. At 140 dB SPL, it will pass a voltage of 1.2Vrms (+4 dBu) to the interface, which most preamps can comfortably accommodate at minimum gain.

In addition, a sound level calibrator is necessary for accurate SPL measurement. The calibrator produces a known acoustic pressure at the mic, which allows the software to associate between that acoustic level and the resulting digital value.

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.

 

 

Next Steps

A Smaart SPL rig consists of the following components:

  • Smaart SPL or Smaart v8 software. Both versions of the software offer the full SPL featureset. Smaart v8 also contains a full suite of FFT-based measurement and analysis capabilities, whereas Smaart SPL is a standalone SPL measurement tool with a purpose-built interface.
  • A computer to run the software. Smaart SPL is not a particularly system intensive application and should run fine on most modern machines. If in doubt, we recommend reviewing the recommended system requirements and downloading the demo to make sure.
  • A measurement microphone. As explained above, a low-sensitivity measurement mic rated to at least 140 dB SPL is recommended for concert-level measurement. We have found that the iSemCon EMX-7150 is very well suited for SPL measurement.
  • An audio interface to connect the mic to the computer. The microphone input needs to be able to accommodate the voltage produced by the mic at show levels. The Focusrite Scarlett series is a good, affordable choice.
  • A sound level calibrator. An accurate calibration is critical to generating meaningful SPL data, particularly when measuring for sound exposure. Even relatively small errors can result in exposure measurements with unusably large uncertainty. (At just 2 dB error, the exposure time uncertainty is as long as the event.) The calibrator should be Class 2 or better and include certification documentation. We recommend the iSemCon SC-1 calibrator. In some cases, it is more affordable to purchase a pre-calibrated hardware bundle, although this may not be a technically practical solution for some applications.

If you have questions about how to proceed, or have a unique SPL measurement application, please feel free to contact us. You may also wish to review the Smaart SPL Quick Start Guide or video to learn more about Smaart SPL operation and features.

 

Further Reading

Understanding and Managing Sound Exposure and Noise Pollution at Outdoor Events (AES Technical Committee on Acoustics and Sound Reinforcement Technical Document)

NIOSH – Noise and Hearing Loss Prevention: Guidance and Regulations

NIOSH – Occupational Noise Exposure: Criterion Document

WHO – Hearing Loss Due To Recreational Exposure to Loud Sounds

WHO – Safe Listening Devices and Standards