Audible Alerts

I'd say you ought to simply poke around your facility, using your ears to find your "constant sound"...

Get a Db meter...

Unless the meters are in exactly the same place at the same time, which is physically impossible they won't pick up exactly the same pressure.
It's probably just a matter of tolerances and natural variation.If you try to calibrate them all against eachother you will be creating a money pit, and endless problems.
Take one as a reference and have it calibrated regularly, use the others as an indication or hide them. Accurate sound meaurement is a nightmare.
Don't waste your money trying to measure to unrealistic accuracy.
Del

"Accurate sound measurement is a nightmare"

How true! Unless you have a sound intensity probe AT THE SOURCE you are measuring energy bouncing off the floors/walls, equipment: everything in the room. Every time there's a reflection, the character of the sound will change.

LL

The sounds in your facility are a mixture of many sounds combined. Unless your sensors are side by side there is no reason to expect that they would give the same reading. Distance from the noise source, surfaces that reflect and compound the noise, and the frequency sampled will all affect the readings.

Calibrators emit a single frequency that is read by the sensor of the unit. Your manufacturer can tell you what the calibration frequency is.

I'd buy a cheap frequency generator, amplifier and speaker and, in a quiet room, put each unit in front of the speaker, in the same, exact location relative to the noise source and adjust them accordingly.

This won't tell you if you are out of calibration, but will allow you to get the same reading, from this source, at a distance and frequency at which you test.

I'd pay to have one calibrated, then use it as a baseline for adjusting the others.

GA for a good logical approach.

Thanks.

Crude, but effective. At least they will all have the same response. Hope OSHA isn't coming soon, if these are for employee protection.