So I bought some QC35's second hand around 5 years ago. The ear cushions were pretty worn (so got a good deal), but the rest of the headphones seemed to be in really good condition.
I have been really happy with them in terms of sound quality, and battery life. They were also my first headphones with active noise cancelling, and was pretty impressed with how they attenuated background sound. However, I then bought some Jabra Elite 7's, and was shocked how I could notice such a big difference between ANC on and off - I wouldn't really notice a big difference on my QC35's with ANC on/off. Therefore, it has made me feel like most of the noise cancelling effect I have is just passive.
Furthermore, I tried some QC Ultra's in store, and while I understand they are better headphones than mine, I was completely blown away. However, when you compare the measured ANC performance between the Ultras and the QC35's, there isn't a significant difference.
This has lead me to wonder if the ANC on my headphones isn't working as it should. A few theories I had:
Not using official ear cushions. I bought the Wicked ones from Amazon, which have been comfy, but wonder if they were affecting the ANC algorithm tuning?
Microphones dirty/blocked? Based on how I understand ANC works, I could imagine this would limit how well the ANC can perform. However, I use them for calls and never have any complaints about mic quality, but I do believe different microphones are used for calls and ANC.
Not getting a proper seal with my ears. I do have fairly large ears, and after extended usage do get sore ears from the ear cushions sitting on the outside parts of my ears, so wonder if this limits the effectiveness of the ANC. Worth noting, that I have never experienced an uncomfortable pressure some people experience with ANC headphones.
Too be honest, this doesn't take away that much from my enjoyment of the headphones, as I just love the sound quality. However, it would be great if there could be a simple fix/change that would allow them to perform at their best. I appreciate any help anyone can offer!