How do the headphones that block out the background noise on aeroplanes work? I know that it must be something to do with wave inversion and addition but exactly how. What is the electronic device that performs the function? Is it a diode?
Presumably this technology could be applied to a room. If you treated the window as the principle source of sound, could you not set up a microphone, wave inverter and speaker to cancel out the noise from outside?
The key to echo detection is standing waves. They get amplitude boosts that cause feedback. A simple delay mechanism prevents the waves from being close enough together to be perceived as a single wave.
you’re onto the correct answer. I don’t know about the particular electronic design & components. basically, the background noise is sampled, inverted, & pumped back out, cancelling the noise.
To my knowledge, ANC (Active Noise Cancellation) can’t generally be done with a room because the cancellation effect is very localized. You have to point the inverted source just right & have the listener in just the right place for the cancellation to work. (kinda like finding the ‘sweet spot’ for your stereo). That’s why it’s a common technology in headphones, but not in speakers.
tangent: one navy application of this ‘sweet spot’ several years ago was on a vessel with the bunks right next to the engine room. They needed noise cancellation not just to permit sleeping, but to prevent hearing damage! There were small ANC speakers above the sailors bunks. As long as they slept on their backs with their heads right in the middle of the pillow, they were fine…
Is it a diode?
No, It is probably a microphone perhaps two or more and more computing power that it took to put men on the moon in 69. Which probably means that there are a huge amount of diodes and other electronic thingies.
Has anybody used these? How well do they work?
I can’t tell you the details of the electronic workings on these, except to say that whatever those components are they’re quite compact. The entire set-up is under 16 ounces, and frequently lighter than headsets without this technology. I think most of them run off either a 9 volt battery or can be plugged into an airplane’s power.
Yes, they work very well. You can stand next to a screaming jet engine, speak in a normal tone of voice to someone next you wearing a simillar headset, and be heard clearly. Kind of weird to be able to feel a sound and not be able to hear it, but it really does “edit out” the noise from your personal environment.
One caveat is that you do need to adjust the fitting with some care, the earpieces do need to be snug. Generally start at $1000 a set and go up from there. Oh well, that will definitely be on the wish list far into the future.
I have a pair of these headphones from Bose and can testify that they work very well on aircraft and that you do indeed have to take some care which way your head is pointing etc. One peculiarity is that in a quiet room they cancell out sound which you weren’t even aware was there. Weird.
Oh cool! I work at Bose headquarters and we invented those. They are called Quiet Comfort Headsets and previous posters are correct in how they work. I am just about to buy myself a pair with my employee discount. They do work very well. If anyone needs more detailed information, drop me an e-mail and I can connect you with one of the engineers that developed the Quiet Comfort headsets.
I’ve had a set for a few years that are not made by Bose. I forget which catalog I bought them from. As moonshine said it’s wild to sit in a quiet room and turn them on and have it get even quieter!
I sometimes use mine when cutting the grass with a power mower, but bought them for use on aircraft.
Here’s a web site with circuit diagrams and instruction for building your own set.
Outrageous they are not doing any fancy digital signal processing. How 20th century. (from the circuitry in GaryM’s link)
Of course you have to tweak the settings by hand to get the cancelation which I imagine you don’t have to do with the bose ones. So maybe I have some wiggle room there.