Sustained listening to sound pressure levels (SPLs) above 85 dB has been shown to cause permanent hearing loss. There is a great iOS app from Studio Six Digital called “Mobile Tools” that includes RTA, FFT, and SPL analyzers. And there are similar apps for Android devices. There is also the free SPL app from NIOSH (National Institute for Occupational Safety and Health). Search NIOSH SPL. Add the inexpensive Dayton iMM-6 calibrated microphone and you’ll have a reliable audio measurement system. The microphone that is already in your device is probably close to ideal, so the accessory mic is not mandatory. Just add ~3 dB to what you see in the app to account for variations in readings.
Of all the recommendations that have been offered here on 10 Audio since we started in 1997, suggesting to you that you listen at “safe” loudness levels is easily the most important and relevant advice. The best sounding audio component on the planet is worthless if you can’t hear it! Fortunately, music can be very satisfying when played with peak levels at or below 85 dB. Hear long and prosper!