PA Tuning and SPL Measurement

What is SPL?

Sound Pressure Level is a measurement of the pressure changes in a sound wave in relation to a standard reference. This is measured in decibels (dB) and is often A-weighted (dB-A) to represent loudness to human hearing. This is a logarithmic scale, meaning small increments could be double the loudness.

Why measure SPL?

Measuring SPL is a very useful reading to have, as it gives a factual reference of the loudness in the room. This is essential in areas with sound limits to ensure the limit is not pushed. Having an accurate SPL measurement can also remove bias, whether someone thinks it is too loud or too quiet, and whether the sound operator is tired or has lots of energy. It also makes each show more consistent in loudness and can help to protect hearing.

How to measure SPL?

From my own research, I already knew a little about SPL but this is my first time actually putting together a measurement rig and using it myself. Measuring SPL involves having a calibrated measurement microphone connected to a computer running software such as Smaart or OpenSoundMeter. I did some independent research including this video underneath.

Creating a Measurement Rig

What would I need?

• A measurement mic

• A mic calibrator

• A laptop with OpenSoundMeter

• An audio interface for laptop I/O

I did some research into affordable measurement microphones and came across the Behringer ECM8000, a value microphone at only around £18 while still giving accurate measurements. Typically this wouldn't be suitable for large scale arena productions but for my application this is perfect. I of course needed a reference to ensure my measurements are accurate, as they can change by simply increasing the gain on the mic. To achieve this I purchased an affordable mic calibrator with the option of 94dB as this is the reference that OpenSoundMeter uses, the software I will be using to get measurements. I will also be using my MacBook and Scarlett interface that I already own, along with some balanced TRS-XLR cables to output pink noise from OpenSoundMeter to the PA for tuning.

Final Product

Overall, I found having a constant measurement of the room to be very helpful. It helped me to make mixing decisions and know when it was too loud or too quiet. Most of the show, I aimed for around 85-90dB. At the loudest, it pushed 95dB, and I found that audience applause is very loud, meaning that particularly in the finale I had to push levels higher to get them over the audience.