What Do I Need to Measure SPL?

What Do I Need to Measure SPL?

Microphone Selection

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

For this reason, we do not recommend the RTA-420 for SPL measurement. This mic has a max SPL of approximately 110dB (1 kHz @ 1% THD), below the recommended 135-140 dB SPL. The store page for the RTA-420 includes this disclaimer:

"The RTA-420 has a relatively low maximum SPL, so it is not ideal for critical or concert-level SPL measurement applications, however, it is still capable of generating accurate data at modest levels. Where critical SPL measurement is an important factor, consider purchasing at least 

one microphone that is better suited for SPL measurement and reserve the RTA-420 

for your spatial frequency response measurements."

Additionally, an SPL measurement 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. It is vital to ensure 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. For this reason, we have included a disclaimer on the store pages for all of our Earthworks measurement microphones:

"When measuring SPL at concert levels the relatively high sensitivity of Earthworks 

measurement microphones can overload most input device preamps. 

If SPL measurement is critical, we recommend that you confirm that your 

preamp's max input level meets or exceeds +19 dBu to prevent overload."

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, Smaart v9 Suite, or Smaart v8 software. Each version of the software offers the full SPL feature set. Smaart v8 and v9 Suite 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 youtube video to learn more about Smaart SPL operation and features.