Spectrum is a room frequency response app designed to be easy to use while packing the features that are useful for audio and HiFi enthusiasts. While it isn't going to replace an ECM8000, it is at least portable, free and can double as a sound meter.
View real-time amplitude and frequency data from the phone
Store last recorded amp/freq into the built-in database
Built-in 5kHz, 8kHz tones and 20-20kHz sweep
Fine control over graph update speed.
You will need to grant microphone access for this app to work. This app is completely offline so all data remains on your device.