Digital Noise-Cancellation Headphones Senior Project Proposal Korey Adams EE Class of 2016 1
Summary This proposal is for the design of a pair of digital noise-cancelling headphones. These headphones will sample the ambient noises around the user, and play back a shifted version of the sampled sounds, so that as the original sound gets to the ear, the shifted sound will play through the headphones, effectively cancelling both sounds out. The user will also be able to record a tone, and then have the headphones allow what was recorded to pass through, while still cancelling everything else out. So for sounds such as a fire alarm that you would not want to be cancelled, the user would have the option to allow the main frequency of the alarm to pass through, while everything else would be silenced. Benefits I think these headphones would be a great benefit to those who work in factories next to noisy machines, or even at home when using appliances such as lawnmowers or leaf blowers. According to the National Institute on Deafness, noises around 75 db can cause permanent ear damage and hearing loss after long exposure. [3] According to a study from the Temple University Department of Civil and Environmental Engineering, leaf blowers and lawnmowers emit sound at around 90 db when you re right next to them, while factories, such as steel mills, emit sound on average of about 100 db. [4] Because of this, the Occupational Safety and Health Administration (OSHA) has required employers to provide hearing protection devices (HPD s) to their employees who are exposed to noises of at least 85 db for 8 hours a day. [5] The HPD s provided are normally just earmuffs or earplugs, and very rarely noise-cancelling headphones. One flaw to this is that if a fire alarm went off while you were wearing the HPD s, the fire alarm would be muffled along with all the other sounds, which could be dangerous. These headphones would allow the user to pick a frequency that the fire alarm sounds at to pass through unaltered. By allowing the main frequency that the fire alarm uses to pass through the headphones, you would still be able hear the alarm while cancelling out all the other noises, making for a safer work environment. These headphones would not only be useful for the work environment, but they d also be useful for home use. Even though it takes quite a while of being exposed to loud sounds for them to cause permanent damage, being exposed to them for a short time can still cause some hearing problems. [4] So this means these headphones would also help with everyday noisy tasks such as mowing the lawn. Also, similar to the work environment, If you were just listening to music in your home, and cancelling out all the other ambient noises, if your home fire alarm were to go off with these headphones you would be able to hear it. These noise-cancelling headphones differ from what s already on the market in that most noisecancelling headphones available now do not allow you to pick certain tones to get through the cancellation, and only allow you to cancel out the ambient noise. Most noise-cancelling headphones on the market today are also fairly expensive, with most of them going for around $300. [6] These will cost considerably less than that to make them more reasonable to use in a work environment. 2
Description My objective for this project is to create a pair of working, noise-cancelling headphones. The main feature, other than having the headphones be able to cancel out ambient sounds, is to allow the user to be able to record a certain tone, and to have this tone be able to get through the cancelation stage, so that I does not get cancelled out. One future feature that I would like to implement if I get more time is to have the user be able to choose if they want to filter out human voices or not. This would be done by having the headphones be able to test if the frequency of what was received is in the range of the frequencies that humans normally speak at, and if they are equal, not cancel out those frequencies. Another future feature that would be cool to implement is to have these headphones also be wireless, but because of the time constraint, this feature might have to be implemented in future updates to this project. Prioritized list of features: 1) Design noise-cancelling headphones 2) Allow the user to select a single tone to skip cancellation 3) Have headphones be able to allow human voices to skip cancellation 4) Make the headphones wireless Constraints The main constraint for this project is that it has to be completely finished in 8 months time. Because of this, I will probably not be able to implement all of the features that I wanted to, so I will have to leave some out, such as making the headphones wireless, or having them allow human voices to pass through unfiltered. The next constraint is that the headphones should be able to cancel out sounds that are at least 100 db. In order for these to help with fixing hearing damage, they would at least have to cancel out sounds that are 75 db, but with most mill s having noises that go up to 100 db, in order for these to be used in a work environment they would need to work at that noise level without having the speaker blown out. Another constraint is that it can t be too expensive to produce. Most noise-cancelling headphones on the market currently are priced at around $300. In order for these to be able to be used in a work environment by many employees, the price would have to be reasonable so as to not cost too much for the company. My final constraint is that all the circuitry needs to be small enough that it can fit on something that can go on the users head. All headphones fit nicely on your head, so it wouldn t be reasonable to have to connect the headphones to giant circuit or microcontroller that isn t very 3
portable. All of the components also need to be light enough so that wearing these headphones do not cause any strain on the user s head or neck. Development plan In order to develop and implement this project, I will need three main components: a microphone, microcontroller, and a pair of speakers. I will need to have a small microphone that will be able to sample the ambient sounds around the user constantly while the headphones are on. This microphone will be connected to a microcontroller, which will control all of the noisecancellation and decision making of which frequencies the user wants to cancel. The way noise-cancellation works is that when two signals are exactly the same, except for one of the signals is a kind of anti signal, that is 180 degrees out of phase with the other. [2] If you were to combine these signals together, they would cancel each other out, making it so you wouldn t be able to hear either signal as shown in figure 1 below. Figure 1: Noise-cancellation In order for the microcontroller to implement this noise-cancellation, it will first convert the analog ambient noise signal into a digital signal that can then be tested and changed if needed. The microcontroller will also be able to receive a signal from the user, and store the value of the main frequency component of that signal into memory. It will then test if the ambient sound has frequencies equal to the value that is stored in the memory, and if it is, the microcontroller will allow those frequencies through unchanged. If the signal doesn t have frequencies equal to what was saved in memory, then the microcontroller will take that signal, and shift it 180 degrees out of phase, creating the anti signal, so that when combined with the ambient noise that enters the users ear, both signals will cancel out, letting no noise enter the users ear. The microcontroller will then convert this anti signal from digital back to analog, and play it through the headphone speakers into the user s ear as the ambient noise is also getting to the ear. This is all shown in the block diagram in figure 2. 4
Figure 2: Block Diagram The microcontroller, speaker, and microphone should all fit nicely on the user s headphones, with no external components as shown in the product sketch in figure 3. I will demonstrate this project in the labs and will only require a source of noise that I will play to show that the cancellation is working and a computer to plug the headphones into and play a song through. Figure 3: Noise-cancelling headphones product sketch 5
Bibliography [1] Kinetis K65 Microcontroller User Manual, Rev.1, Freescale, Austin, TX, 2015. This is the user manual for the Kinetis K65 which is the microcontroller I am planning on using for this design. [2] William Harris. How Noise-cancelling Headphones Work [Online]. Available: http://electronics.howstuffworks.com/gadgets/audio-music/noise-canceling-headphone3.htm This is information on the theory behind how noise-cancellation works and is implemented. [3] NIDCD. (2015, May 15) Noise-Induced Hearing Loss [Online]. Available: http://www.nidcd.nih.gov/health/hearing/pages/noise.aspx This is an article done by the National Institute on Deafness and Other Communication Disorders on how loud noises can cause hearing loss. [4] Temple University. (2000, February) Noise Sources and Their Effects [Online]. Available: https://www.chem.purdue.edu/chemsafety/training/ppetrain/dblevels.htm This is a report done by the Temple University Department of Civil and Environmental Engineering that notes how loud many sounds are in db. [5] OSHA. (2011) Occupational Noise Exposure [Online]. Available: https://www.osha.gov/sltc/noisehearingconservation/ This is an article done by the Occupational Safety and Health Administration on occupational noise exposure and noise impacts on people s hearing. [6] Bose. (2015) Quiet Comfort Acoustic Noise Cancelling headphones [Online]. Available: https://www.bose.com/products/headphones/over_ear_headphones/quietcomfort.html This is the website of a competing noise cancelling headphone product. 6