Describing the HD 505 sound with a frequency response curve
Headphone frequency response graphs look very different from those for loudspeakers, which are ideally very flat. Here you can see a big rise on the right (treble) side and then it falls off a cliff. But this is in fact very close to what’s considered ideal for headphones. It’s called the Harman Curve. This industry benchmark is based on extensive psychoacoustic research into listener preferences.
For instance, it turns out that headphones negate the function of the outer ear, which otherwise amplifies sounds in the 3 kHz region. So the ideal response for headphones is much higher there. Resonances in the inner ear also make sounds between 7 and 10 kHz louder, which is why the graph dips abruptly around 8-9 kHz. We can see here that the HD 505 builds on the already excellent characteristics of the HD 599, extending even further into subbass territory while still offering strong, analytic mids and brilliant treble without sibilance or harshness.