Katy Perry says her fourth album will be inspired by Joni Mitchell.

The 28-year-old singer is getting ready to release her third studio album 'PRISM' and she thinks a natural progression for her will be leave behind the cheeky pop and dance tunes she's famous for.

Instead, the musician - who did release one self-titled LP under her real name Katy Hudson before becoming Katy Perry - plans to make an acoustic record in the style of 'Big Yellow Taxi' songwriter Joni.

Speaking to Billboard, Katy said: ''I'll probably turn into more of a Joni Mitchell. As I inch towards my 30s, I think my fourth record will be more of an acoustic guitar album.''

Although 'PRISM' will be similar in style to her previous releases, Katy insists all the tracks are about her life, with 'By The Grace of God' chronicling the suicidal thoughts she had after her marriage to Russell Brand ended in December 2011.

She said: ''I can only write autobiographically. I put all the evidence in the music. I tell my fans if they want to know the real truth about stuff, just listen to the songs.''

Speaking about 'By The Grace of God' - which contains the lyrics ''Lay on the bathroom floor/Couldn't take it anymore / By the grace of God / I looked in the mirror and decided to stay / Wasn't gonna let love take me out / I thank my sister for keeping my head above the water'' - she revealed: ''That song is evident of how tough it really was at a certain point. I asked myself, 'Do I want to endure? Should I continue living?'

''All the songs are real-life moments. I can only write autobiographically. I put all the evidence in the music. I tell my fans if they want to know the real truth about stuff, just listen to the songs.''

Meanwhile, Katy has shared new 'PRISM' track 'Walking on Air' online.

The song is reminiscent of a 90s-style house track and contains lyrics such as ''There's disco paradise, even heaven is jealous of our love / Yes, we make angels cry, raining down on earth from up above.''

'PRISM' is to be released on October 18.