For domestic use, the loudest doorbells have a maximum volume of 90dB, since any greater volume might startle people or cause disturbance to neighbours.Wireless models to meet the 90db level are the 200m versions of the Friedland Libra+ range, the D911S, and the additional chime model D917S and The DC9xxx series in the Honeywell range. These units have adjustable volume controls, and they also have a strobe light which can attract attention, when the unit is in the same room as the listener. This strobe can be switched off if not required. For the Byron range, the BY301W and the BY302B should be considered, and possibly the BY302T twin pack, since they all have a 90dB volume.
The loudest mechanical wired doorbells are the models 1200xx series the 7721 buzzers and London Striker Bells which again operates at 90db, but there are no options to change the volume. These units must be powered by the mains via an 8v, 1 amp transformer code 7770. The new Honeywell Wired Chime Model DW915SW will operate at 90dB (adjustable) and can be powered by either batteries or an external transformer
An alternative to a single loud doorbell is to distribute many wireless bells around the property, possibly in each room, and perhaps with each having a flashing light. Suggestions might be the D914ksTwin twin plug-in chime kit with additional D917s chime units.
It is also worth considering the pitch of the chime. Some people will detect a deeper tone such as the Low Frequency tone of the Jacob Jensen Wireless Chimes.