"As expected, the iPhone 5's response is ruler-flat."
"iPhone 5 Frequency Response, 200 kΩ load, 1 V RMS output, expanded scale. Not bad; down only 0.01 dB at 20 Hz and -0.08 dB at 20 kHz, flatter than most labs can measure."
"The near-zero (4.5 Ω) output impedance of the iPhone 5 is much better (lower) than most dedicated headphone amplifiers, even most exotic audiophile amps, and therefore its response is not significantly affected by the headphones in use. Most dedicated amplifiers have about ten times the output impedance of the iPhone 5, and allow about a dB or two of false bass boost with the Ultrasone Edition 8. With the iPhone 5, the sound is accurate and well controlled."
"iPhone 5 Harmonic Distortion Spectral Content, 1 kHz, 200 kΩ load, 1 V RMS.
"To my surprise, the harmonic content is mostly second-order, with higher harmonics at least about 15 dB down from the second harmonic. I'm more used to seeing this sort of signature from tube amplifiers!"