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, or 93.7, or some set volume. 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, or the meter may do that automatically for you.
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.
Off-site noise meter calibration
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.
Some manufacturers of meters really lean on customers to go for annual calibration but this is not necessary for a compliant noise assessment.
In the screenshot here, Cirrus are changing the way they support the meter by saying they will give a 15 year guarantee if you get your meter calibrated annually with them, but that is nothing to do with how often you need to calibrate it to do a compliant noise assessment. To be clear, you can buy a Cirrus noise meter and calibrate it every two years and be perfectly fine for a compliant noise assessment.
This trick they are using is neatly side-stepping the compliance side of it completely and instead using a sales technique to get customers spending money more regularly for a longer warranty. Given calibration is nothing like servicing the mechanics of a car this is stretching it a bit, but if you want the long warranty with Cirrus then that's the price you pay for it. Although annual servicing over 15 years ain't cheap! Oh, and I have a noise meter from 10 years ago and it is positively antiquated to a modern one, so do you really want to still be using one which is 15 years old, for which you've paid out a lot in excess calibration? Would you still use a mobile phone from 15 years ago, and over those years have paid a lot of extra cash out to the manufacturer for the privilege of still using it? No. Cirrus make GOOD meters and are a good choice, but my advice is to ignore this sales fluff.