Distance beyond which one does not have to attend congregational prayer in the mosque

If we assume that the muezzin is giving the call to prayer from the roof-top of the mosque (without the aid of amplifiers), that he has a good strong voice, that the one who hears him has good hearing, that the wind is calm and that there are no barriers or noise that will prevent the sound from reaching people, then the person who does not hear this call because of being far away is not obliged to attend the congregational prayer in the mosque. Imaam Ahmad estimated the distance at which the voice of the muezzin would be inaudible in such circumstances as being more than a farsakh (one farsakh is approximately five kilometers). And Allaah knows best.