I like that phenomenon but i have no idea why some headphones have it and some don't, i had AudioQuest Nighthawks and they had very dark background, the things just popped out of nowhere.
I bought t50rp mod because i read that they have that to.
Can someone explain to me what exactly is that and the technical reason some headphones to have it and some dont?
That's a function of both a black background (and low ambient noise where you listen, or high isolation) as well as proper imaging. The angled driver mounts on the Nighthawk helps the latter. Black background is more of hwo a headphone interacts with an amplifier, keeping the noise low or that you have a really low noise amplifier - something like Meier, VIolectric, O2, etc - to begin with.
Having a black background means, to use an audiophile cliche, that there's nothing between you and the music. An extreme example of the opposite is when the noise is really there, like interference on radio. It makes it easier to hear all that's in the recording when there's no noise, whether it's electronic or environmental, which is why people with dedicated speaker set ups acoustically isolate the room and use high quality amps, even if they're spending a ton of cash on a relatively low power but still gigantic Class A amplifier like a First Watt.
What about the HD650's? They have an intimate as opposed to expansive soundstage like the HD 600's, but I'm not sure if that counts against imaging. Maybe they have good imaging too because I can still hear the location of instruments in the HD 600's.
There's also the HD 800(s), which certainly aren't dark but I've heard have excellent soundstage and imaging.
The "Describing Sound, A Glossary" describes dark as:
Dark - A tonal balance that tilts downwards with increasing frequency. Opposite of bright. Weak high frequencies.
Dark tone is rolled off treble. Black background is totally inaudible noise, ie, even if you strain to hear any interference, there's nothing, even with no music playing.
It's just weird because it changes with different headphones for him :/
It does because the impedance and sensitivity of a headphone can affect the amplifier's noise floor. Look up how many people have issues with the SE53x and amplification. You'd think a high sensitivity IEM won't ever need an amp, but no. Plug it into a random smartphone and you get noise. Power quantity isn't a problem but you get to a point where power quality becomes absolutely essential.
I found that a less sensitive headphone tends to have a silent background.
That depends. A low sensitivity headphone can result in higher noise too, depends on the interaction with the amp really. A high sensitivity headphone doesn't need the amp to get cranked up to get loud, so it doesn't start piling on noise. Use a headphone that has a low enough impedance and sensitivity, and noise will increase as you crank it up. Some amps or amp circuits/chips have an inherently higher noise floor, so plug a headphone with high enough sensitivity and you'll get noise. Just look up how many people have to use an amp, and not just any readily available and affordable Fiio, to drive a high sensitivity SE53x.
When I had the Fostex TH-X00 and I plugged it into my modded speaker amp it had a slight background hiss and showed channel imbalance at lower volumes. Any other lower sensitivity headphone I tried was silent.
Speaker amplifiers tend to have a higher noise floor than purpose built headphone amps because they have to make a compromise between power output level and noise. You just don't hear it easily with speakers unless you're standing right next to the speaker, but it's easier to hear with headphones that are sitting right by your ear canals. Of course, there's a reason why some people would blow a lot of money on a First Watt, and why despite having anywhere between 15watts to 25watts per channel, it sounds about as dynamic as practically most other more powerful amps out there on anything but the lowest sensitivity speakers. A more common example is how a 25 to 35wpc NAD can wipe the floor with receivers rated for 4x the power (or more), but if you look at the specs, the NAD has "0.001% THD, 110dB noise, both channels driven" or something, but the receiver has "1% THD, 95dB noise, one channel driven." Note that noise ratings when expressed in plain dB basically means you have to get to that level of output (not from the speakers, there's a different measurement on the amp, ie, like how receivers explicitly show volume level in dB, despite not knowing how efficient the speakers are) before noise starts kicking in.
