AudioTools
AudioTools is a suite of professional-grade audio and acoustic analysis apps. Included in the price of this app is a great collection of professional tools.
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Decibel X
Decibel X is one of very few noise meter apps with highly reliable, pre-calibrated measurements and supports dB468, dBA, dBC. It turns your mobile device into a professional sound level meter, precisely measures the sound pressure level (SPL) all around you.
sigrok
The sigrok project aims at creating a portable, cross-platform, Free/Libre/Open-Source signal analysis software suite that supports various device types (e.g. logic analyzers, oscilloscopes, and many more).