An electrician may be able to adapt something like the Additional Vibration Pad for (Ei175 and Ei176) deaf alarms at:
to meet the requirement.
Otherwise you may have to ask a handyman, contractor or electrician to make the vibrating pad(s) that you want.
All that is required is a small electric motor with an unbalanced weight, cam or eccentric disc that is allowed to rotate, i.e. knock, against a sounding board (which may be the door itself - if it is made of wood or other suitable material).
The type of motor to which I refer is listed at:
Such motors are used in products such as the Vibrating Floor Pad at:
Alternatively a loudspeaker mounted on a sounding board that is connected to a low frequency oscillator should do the same job.
The more usual problem is to prevent vibration – not to create it!
Peter Hamlin [Tel: 0 (044) 1895 279158 Fax: 0 (044) 1895 279737]
North Thames Regional Environmental Control Equipment Service (NT RECES)
Alderbourne Rehabilitation Unit (Entrance 'C' - off Royal Lane)
The Hillingdon Hospitals (THH) NHS Foundation Trust
Pield Heath Road