Is there an Android app that can show fft spectrum...

Yes – there are Android apps that can show an FFT-based spectrum for pre-recorded audio files, not just live mic input.

Here are a couple that do exactly that:

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1. Aspect – Audio Files Spectrogram Analyzer (free)

• Purpose-built for analyzing local audio files rather than microphone input.
• Opens many formats: 3GP, FLAC, M4A/AAC, MP3, MP4, OGG, OPUS, WAV and more (with optional extra codecs like AIFF, APE, DSF, etc.). citeturn4view0
• Generates a detailed spectrogram (which is basically an FFT over time) and lets you zoom into time and frequency regions, change color palettes, and even play just the selected part of the spectrogram. citeturn4view0

If your goal is “drop in an MP3/WAV and see the spectrum,” this is probably the cleanest match.

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2. WaveEditor for Android – Audio Recorder & Editor

• Full audio editor/recorder that can import a ton of formats (WAV, MP3, FLAC, OGG, and many others). citeturn0search1turn2search9
• Includes “visual tools” such as FFT, oscilloscope, and spectrogram for the loaded file, so you can inspect the spectrum while editing. citeturn0search1turn2search15

This is overkill if you only want to look at spectra, but it’s great if you also want to cut, edit, or process the audio.

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A quick caveat about other spectrum apps

Most of the popular Android spectrum analyzers like Spectroid and various “Audio Spectrum Analyzer” apps are designed for real-time microphone input only – their descriptions specifically emphasize turning your phone’s microphone into a real-time analyzer and don’t mention loading files. citeturn0search5turn0search25turn3search7

You can always play a file through speakers and analyze it via mic, but that’s not what you asked for.

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Bonus: Browser-based options (works on Android too)

If you’re OK using a browser instead of a dedicated app, there are web tools you can open in Chrome on Android that accept audio uploads and do FFT/spectrogram analysis, for example:

• dCode Spectral Analysis – uploads an audio file (MP3, WAV, etc.) and shows a spectral breakdown. citeturn0search6turn3search17

These are handy if you just need occasional analysis and don’t want to install anything heavy.

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If you tell me what exactly you want to see (full spectrogram over time vs. a single FFT snapshot, specific frequency range, etc.), I can nudge you toward the best of these for your workflow.