Extracted form Apple's Store In-Ear Headphone's overview:
- Frequency response: 5Hz to 21kHz
- Impedance (at 100Hz): 23 ohms
- Sensitivity (at 100Hz): 109 dB SPL/mW
- Drivers: Custom two-way balanced armature (woofer and tweeter in each earpiece)
If you are using the regular Earphones, although I couldn't find any official technical specs, it's dB output would be lower than the In-Ear's, something like 103 dB.
And for the safe decibels, just remember that:
- A typical conversation occurs at 60 dB – not loud enough to cause damage.
- A bulldozer that is idling (note that this is idling, not actively bulldozing) is loud enough at 85 dB that it can cause permanent damage after only 1 work day (8 hours).
- When listening to a personal music system with stock earphones at a maximum volume, the sound generated can reach a level of over 100
dBA, loud enough to begin causing permanent damage after just 15
minutes per day!
- A clap of thunder from a nearby storm (120 dB) or a gunshot (140-190 dB, depending on weapon), can both cause immediate