Timing Belt Decibel Measure

Firstly, never heard of SLA light, is it a sonic sound tool?
The engine is not running and the workshop must be quiet. The belt is then 'strum', as you would a guitar string. both sides of the belt.
Imagine tuning a guitar, as you tighten the string, the note changes.
I have always just used the 5mm vs 6mm allen key method as a base on older single cam engines, only using sonic methods for later, 'high end' or customer requests.
Both methods with video's explained here.
scroll down to watch video's.

I use a guitar tuning app on my laptop plus a separate microphone to capture the sound. I've found it to be very easy to adjust the belts on my S4RS to nearly exactly the factory spec. Just takes a little fiddling to get the proper technique down but once you can replicate the frequency you'll know you're doing it right.

I've used the app from Gates a few times: Carbon-Drive
But like @ronski, I've mostly relied on the 5mm hex key trick.
Brad the bike boy has some other great videos too: here

I've used phone apps before and found them a bit hit and miss, the Gates one being the best of a bad bunch IMO.
I now use the program written by JP of JPDiag/DucatiDiag fame, a cheap lavalier microphone on my laptop with this works great. It's the program at the bottom of this page called Courroies (belts in English): Download page

After reading all your posts I think I will stay with the 5mm hex key method. Cheers