In order [?] travel, sound [?] have a medium. Sound does not [?] travel in air, but also in solids and liquids. In fact, solids and liquids [?] sound with greater velocity [?] air. Sounds are produced by vibrations. For example, the sounds of human [?] are produced by vibrations of our [?] cords and the sounds of a guitar are produced [?] the vibrations of the strings. The [?] the frequency of the vibration is, the higher the pitch of the sound is and [?] versa. The frequency of the vibration is [?] in hertz. One hertz is [?] to one vibration per second. The [?] cords of humans normally produce sounds [?] range from 120 to 500 hertz. [?], the human ear can [?] sounds from 20 to 20,000 hertz. Frequencies lower or higher than these two values are [?] infrasound and ultrasound. Sound cannot travel [?] a vacuum. It needs a medium to travel in and the [?] of the sound becomes weaker as the distance [?] the source of the sound and the listener becomes greater. [?] regard to the speed at which sound travels, this [?] on the density of the medium through [?] it is travelling. For example, in air sound travels [?] approximately 330 metres per second, [?] in water the velocity is about 1,500 metres per second and in iron the velocity [?] to around 5,000 metres per second.