Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Numerous approaches have been tried, including perceptually motivated methods [1,2, 3,4], parametric signal model-based methods [5,6], classification-based methods [7] and parametric spectrum model-based methods [8,9,10,11].
Category: