[auxym]

REW (free, but not FOSS): https://www.roomeqwizard.com/

A calibrated measurement microphone can be bought for 79$: https://www.minidsp.com/products/acoustic-measurement/umik-1 or https://cross-spectrum.com/measurement/calibrated_umik.html

[kristofferR]

Housecurve is the same, let's you use the iPhone microphone.

https://apps.apple.com/us/app/housecurve/id1470695018