The trademark ABBA sound consists of two main ingredients: keyboard-based wall-of-sound arrangements, and the soprano and mezzosoprano voices of Agnetha and Frida respectively. As Benny has pointed out, the records would have sounded completely different if the lead vocalists had been any others than Agnetha and Frida.
A strong melody was of course always at the core of ABBA's recordings. Experimentation with effects, often courtesy of their sound engineer Michael Tretow, added to the overall impression of state-of-the-art production values. The very earliest recordings from 1972 and 1973 were rather sparsely produced, however. The main exception - and the first indication of ABBA's future soundscape - was the song Ring Ring.
By 1975, the group had pretty much arrived at the multi-layered sound that would carry them through the peak years of their popularity on albums such as ABBA, Arrival, ABBA - The Album and Voulez-Vous. The arrival of the 1980s saw a return to the slightly more bare production of their early days - with Benny's synthesizers very much to the fore - although with an added maturity that put its mark on their last album, The Visitors, in 1981.
