The generic word 'sound' includes the words 'ultrasound' and 'infrasound'.
The distinction between 'sound' - as in what we can hear - and the other two is a bit arbitrary. 'Ultrasound' refers to sounds that have frequencies above the top limit of our hearing; 'infrasound' refers to sounds that have frequencies below our lower limit of hearing.
From the Greek: 'ultra' = above; 'infra' = below.
