content: It's not that closed-back headphones have "no" soundstage, but that theirs tend to be considerably smaller due to how they have to be constructed in order to provide isolation. For an over-ear headphone to isolate you from your environment, it needs to create an airtight seal. When you put on your closed-back headphones, they create a room of air that is completely sealed off. No air can enter, no air can leak out. This is necessary because of air being the primary medium through which the sound waves travel in this scenario. As you have seen in the video, open-back cans can sometimes even be seen through. This has three effects:  Closed-back headphones use more material (because they have no holes) and are therefore often heavy in comparison. My K240 MKII is ridiculously light and isn't even completely open (semi-open). My K550 on the other hand... Due to the fact that there is a confined pocket of air when you use closed-back headphones, they are much more prone to give you hot, sweaty ears as the air heats up but cannot be exchanged for fresh air. The confined pocket of air also affects how the cans sound. If the air cannot leave, but is pushed towards your ear, it builds far more pressure than an open back headphone could. This pressure can make for some extremely powerful bass, but also dampens everything.  The first two effects are why open-back headphones tend to be more comfortable. The third effect is what causes them to have a smaller soundstage.  To better understand it, take the word "soundstage" literally. A smaller soundstage means that the band you're listening to, plays on a smaller stage. This means that they have to stand closer to each other because there isn't enough space. This logically makes it harder for you to tell the individual instruments apart. A large soundstage means that there's plenty of space between the different instruments, which makes it easier to tell where they're coming from. You can test this by yourself quite easily. Take a roll of toilet paper and your phone. Play a song (not too loud so it doesn't distort) and place the roll so that the speaker of your phone is inside. Now place your ear on the other end. If you manage to create a more or less airtight seal on both sides, you'll hear how the sound signature of your smartphone's speakers change.  Notice how it sounds deeper, dampened and generally less clear. I hope this clears that up.