I'm looking for an app that can produce a visual representation of a voice over time, i.e. a graph showing how the voice rises and falls over a sentence. All dictation devices give a visual representation of the voice as you speak. What I want is to record a sentence or a short piece of speaking, and then after show a graph showing the voice changes. I teach English to speakers of other languages and I want them to be able to 'see' their voice and analyse it after speaking. It only needs to be a simple graph and does not need to be too detailed.
It seems apps like Visible Sound do this, but as I need it for a lot of people to download I'm looking for a free version. I tried the free Voice Analyser and Graphipad, but both were terrible. Many speech therapy apps exist but are for specialists and understandably expensive.