Here is the method
1) Install Spectroid app on your Android phone from Google playstore
2) Ensure there is relative silence while doing this test, we want to keep the noise floor down.
3) Prepare to run frequency sweep from youtube
This is a sweep from 25Hz to 150Hz. You may use any other way of sweep if you want to go lower than 25Hz. Pause the video at start.
4) Bring your android phone mic as close to the port as possible keep 1 cm of distance but dont touch the speaker or sub with your phone. The idea is that the mic should mostly catch the audio from port only and other sounds should appear as noise coming from far.
5) Start the app
6) Run the sweep from youtube, when it completes, notice the peak in response in bass in the app, that's your tuning frequency
7) Repeat above to be sure.
Note:- Android phone mic is not a good way of measurement but for this purpose, it works
Below is my speaker's 60Hz tuning

Thanks and Regards,
Diyaudio
1) Install Spectroid app on your Android phone from Google playstore
2) Ensure there is relative silence while doing this test, we want to keep the noise floor down.
3) Prepare to run frequency sweep from youtube
4) Bring your android phone mic as close to the port as possible keep 1 cm of distance but dont touch the speaker or sub with your phone. The idea is that the mic should mostly catch the audio from port only and other sounds should appear as noise coming from far.
5) Start the app
6) Run the sweep from youtube, when it completes, notice the peak in response in bass in the app, that's your tuning frequency
7) Repeat above to be sure.
Note:- Android phone mic is not a good way of measurement but for this purpose, it works
Below is my speaker's 60Hz tuning

Thanks and Regards,
Diyaudio