no-tech noise-cancelling headphones
The Project:
Noise-cancelling headphones work by generating an equal but opposite sound wave which “cancels” out the incoming ambient noise. This is called “active noise control” or ANC. Unfortunately, to turn on the ANC, you have to use battery power. Also, the white noise that is generated by the ANC gives me a headache, so I end up just relying on the padding of my headphones to block out unwanted noise. For this project, I attempted to create a pair of no-tech “passive noise control” headphones which require no power to reduce unwanted ambient noise.
Background:
I spend a lot of time working in studio, and sometimes there are specific sounds that one would like to block out. Many times, there are specific voices or conversations that are distracting, even if you turn up your music quite loud. Further, there are times when you don’t actually want to be listening to music, but still put on your headphones just to block out some of the distracting noise. Thus, some kind of noise-cancelling device that doesn’t require any extra energy to work would be perfect in these situations. [A bonus would be if this device were relatively discreet. Sometimes, it seems unnecessarily rude to put headphones on, when it’s obvious that you are doing so to block out the conversation of your neighbors.]
Research:
To begin with, I dug up my acoustics textbook, and did a little bit of brushing up on sound. Basically, the range of human hearing is between 20 to 20,000 Hz. The human ear is most sensitive to sound between 500 to 4,000 Hz (also the range generated by the human voice). Calculating the wavelength of this range of sound waves, we find that 500 Hz has a wavelength of 2 ft, whereas 4,000 Hz has a wavelength of about 3.4 in. So in order to simply block the sound waves generated by most human voices, there would need to be an object greater than 2 ft that would physically block the sound. Since earphones cannot comfortably be this large, I looked further at absorption, reflection and diffusion to see if there was another way to try to cancel out sound.
Prototypes:
I decided to try out three separate experiments to see if it was possible to create no-tech noise-cancelling headphones.
First, I figured that a solid material would literally just shut sound out, so I cast some ear pieces out of rockite.
Second, I decided to try out a microperforated sound-absorptive clear material technique that I have been wondering about for awhile. I came across this product last spring when I took a class in architectural acoustics (http://www.rpginc.com/products/clearsorbersheet/index.htm). Essentially, this sound absorptive material is a piece of clear plastic that has been perforated by many tiny holes. These work as Helmholtz resonators, where sound is absorbed as it passes through the holes because the perforations are comparable to the thickness of a boundary layer of air, and this viscous air layer absorbs sound. I took the approximate thickness of their material (1 to 1.5 mm thick plastic), as well as the perforation diameter (0.5 mm) and spacing (5 mm x 5 mm apart), and replicated the material using plexiglass and the lasercutter. 
Third, I combined the above microperforated material with a layer of more traditional absorptive material (fabric). This combination of a diffusive/reflective layer with an absorptive one behind is used in most typical architectural applications.
Results:
Unfortunately, none of these seem to be particularly effective at shutting out sound. In fact, my real head phones (or even my earmuffs!) are much better at simply blocking noise. A large part of this is due to the fact that my prototypes are not really designed to snugly fit against the head. Unlike head phones or earmuffs, there isn’t any soft, cushiony material to create a seal between the head phone and the area around the ear. Further, the mockup of the microperforated material probably is not working because the surface area exposed to soundwaves may not be large enough (architectural applications cover entire walls and rooms, whereas this experiment is only the size of a human ear). More investigation of the properties of sound and the mechanics of human hearing, as well as the creation of a better designed mockup is needed.
