I recently purchased the QC 35 headphones and was immediately disappointed with the noise cancellation technology. I have downloaded the app, updated the firmware and have gone through various means of troubleshooting. I do notice there is a small difference when I use the app to turn on and off the noise cancellation. When the noise cancellation is "off" I notice there is a muffled sound from the external enviroment. When in "low" or "high" cancellation I notice the muffled noise decreases, however even on "low" and "high" I can still hear distant fans, AC and even the television (on low) in the background. I also notice no difference between the "low" and "high" noise cancelling options. I have played around with these in various environments and have not found it to optimally work. I have a friend with a pair of Bose headphones that work perfectly. I do not wear glasses and I have tiny ears that allow the headphone coushins to fully seal each ear. I am considering returing these if I cannot solve the issue soon. Please let me know if you have anything I could try to fix this before I return them. These headphones were purchased for noise cancellation with air travel in mind and I read many reviews that said these were the best option of all the competitors.