The flash premiums of VADs are between 0.5 Hz and a pair of Hz. VADs may perhaps contain a synchronisation perform to do away with the possibility of a flash frequency that can bring about adverse effects, including inducing epileptic fits, when gadgets are in close proximity. Dr Zzeus Q&A: https://www.firesense.co.uk/type/conventional-fire-system/conventional-sounders-vids-vads/