[Mansour] was disappointed to find out that his Bose QC15 headphones had a dead right channel. These headphones have active noise cancelling, which uses a microphone to capture ambient noise and digital signal processing to insert an out of phase signal. Since they’re quite expensive, [Mansour] was determined to resurrect them.
First, he determined that the right speaker had died, so he found a replacement on eBay. These were designed for a different set of headphones, but matched the impedance of the original Bose part. After replacing the driver, it seemed that the repair was a failure. The sound cancelling wasn’t working, and a the playback was high-pitched. As a last attempt, he potted the speaker with glue, to match the original construction. Much to his surprise, this worked.
The problem was that the new driver didn’t have sufficient sound isolation from the microphone, which is meant to pick up passive noise. This feedback likely caused issues with the noise cancelling DSP. A little glue meant a $20 fix for a $400 pair of headphones.