For domestic use, the loudest doorbells have a maximum volume of 90dB, since any greater volume might startle people or cause disturbance to neighbours. The only wireless models to meet the 90db level are the 200m versions of the Friedland Libra+ range, the D911S, D914S, D917b, D917c and the additional chime model D917S. These units have a rotating dial behind the front panel to adjust the volume, 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 wired doorbell is the model 1200B, which again operates at 90db, but there is no option to change the volume. This unit must be powered by the mains via an 8v, 1 amp transformer code 7770.
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 D917kt twin plug-in chime kit with additional D917s chime units.
It is also worth considering the pitch of the chime. Some people will hear the sound from a bell, say the SX-15iw, better than other standard chimes, or will detect a deeper tone such as the Low Frequency tone of the Jacob Jensen Wireless Chimes.