Decibel meter app

Just as an example, one sequence to calibrate both the online sound meter and headphones for hearing testing is as follows (watch video).

Step 1: Use an acoustic calibrator and calibrate an iPhone sound meter using the free NIOSH SLM app made by the CDC.

Step 2: Using the calibrated NIOSH app as a guide, calibrate the online sound meter above.

Step 3: Calibrate the headphones by changing the volume manually using the NIOSH app as a guide while playing the calibration audio files.

Another method is to:

Step 1: Calibrate the headphones with a professional sound meter.

Step 2: Calibrate the online sound meter with a professional sound meter.

And yet another method is to (assuming you own a recent iPhone):

Step 1: Install the "Hearing" program from within the Control Center Settings.

Step 2: Calibrate compatible headphones/air pods using the "Hearing" program accessible via the Control Center.