Calibration of noise meters
Here's a good question - how often do you need to have your noise meter calibrated to make sure it is working correctly? There are two types of check you need to do.
On-site calibration check
Before starting any batch of noise measurements you have to do a calibration check to verify the meter is working correctly on that day, and then make any small sensitivity adjustments to ensure it all working as it should.
This is done using a small hand-held tone-generating jobby, usually done at 94dB. You pop it on the end of the noise meter and let the meter measure the noise. If it is a little over or below then use the meter's settings to adjust the sensitivity until it is reading accurately.
Why do this every time? Noise meters are very sensitive so for example changes in temperature or humidity can affect the reading by tiny amounts.
Noise meters should be professionally calibrated every two years. Yes, you read that correctly, every two years and not annually. Here is the extract from Appendix 1 of L108:
So if someone tries to flog you annual calibration, unless you have reason to believe your meter is dodgy from the pre-measurement verification checks, ignore them, every two years is fine.