Active Noise-Canceling of Vehicle Exhaust Noise

Active Noise-Canceling of Vehicle Exhaust Noise
Applicant name: Eric Hardester
Applicant email and NetID: erichardester@yahoo.com, ericrh
Purpose
The purpose of this project is to create a noise-canceling system for road vehicles by measuring
the tone of the exhaust fumes and broadcasting a new tone to create destructive interference and lower
the noise pollution caused by road traffic.
Why Active-Noise Canceling is Important
Noise pollution is a major problem in the world, particularly in urban areas. It is known to increase
stress and hypertension, both of which are harmful conditions. By being able to accurately and effectively
measure sound output of vehicles, noise-canceling systems can be employed to decrease if not eliminate
many sources of noise pollution. This will lead to improved psychological and physiological health of the
general population.
Main Proposal Body
With the growth of urban cities and the infrastructures required to maintain those cities, noise
pollution levels are on the rise. The World Health Organization states on its website,
“Excessive noise seriously harms human health and interferes with people’s daily activities at
school, at work, at home and during leisure time. It can disturb sleep, cause cardiovascular and
psychophysiological effects, reduce performance and provoke annoyance responses and changes in
social behaviour.” i
This shows that not only are loud noises annoying, but they are affecting the health of our
population. A common source of this noise pollution is the vehicles that transport us and transport our
goods. Examples of this include trains, commercial jets, and the semi-trucks driving along University
Avenue in Provo, UT. To combat this, some companies have instituted what is called active noisecanceling
technology. An audio company called Bose has created a set of headphones that incorporate
this technology, called “Bose Quiet-Comfort” ii . Intended for noisy environments, particularly commercial
aircraft cabins, these headphones employ microphones that sample the sound waves nearby and then
use a processor and speaker to produce sound waves in the ear that eliminate unwanted frequencies.
This technology could theoretically be applied to eliminate the unwanted frequencies from a car’s exhaust
pipes.
Noise-canceling systems are based off of wave physics. Waves, particularly sound waves, can be
added together to form new sound waves. Depending on the different frequencies and amplitudes of the
waves added together, there can be two different outcomes. The first is constructive interference. This
means that the waves line up in a way that allows them to build on each other and increase in amplitude.
The other outcome is destructive interference which is the outcome we are interested in. This means that
the waves line up in a way that allows them to cancel each other out and decrease the amplitude. This is
ideally accomplished by have two waves that are the same and setting them out of phase by 180
degrees, or PI rads. This can be done by placing two speakers next to each other and broadcasting two
signals that are out of phase by 180 degrees, or it can be done by placing a distance between the two
speakers equal to half the wave length of the frequencies which puts them 180 degrees out of phase.
This project will involve placing a speaker next to a microphone which will sample the frequency
of the exhaust tone coming from the exhaust pipe of a car. Placing the speaker next to the microphone
will simulate placing two speakers next to each other. This means that the speaker will then theoretically
be able to produce a signal that is 180 degrees out of phase with the signal received by the microphone

and cancel out that signal. Lower amplitudes, or ideally a complete canceling, will occur and little or no
sound will be heard from the exhaust pipe by those around.
The project can be separated into three distinct parts- measuring the sound frequency and
amplitude, using a computer program to create an inverse wave based off of the measurements, and
broadcasting that new sound wave through a speaker to create destructive interference and cancel out
the sound waves.
The sound will be measured with a microphone placed just outside of the exhaust pipe of an
Infiniti G35 sedan. The best type of microphone to use to gather data will be determined between a
condenser, dynamic, and piezoelectric styles. Once the most effective type of microphone to use is
determined and an accurate precise measurement is accomplished, the microphone will be moved farther
into the exhaust pipe. This will simulate where the microphone would be in a working noise-canceling
system.
LabVIEW will be used to gather the data from the microphone onto a laptop computer. Tests will
be performed with the car in a stationary position at various RPM levels to measure the sound
frequencies and amplitudes at each respective level. The signal will be inverted and the new signal will be
sent to an amplifier and a speaker. The term “active” in active noise-canceling comes from this process of
the computer continuously sampling the frequency and amplitude of the exhaust tone and reprocessing
the inverted signal.
The speaker will be placed just in front of the microphone and will broadcast the new signal. The
placement will be such that the new signal is not measured by the microphone. Adjustments will be made
in speaker position to allow for optimum noise-cancellation. The speaker will also be moved farther back
with the microphone, but will be located in a separate pipe next to the exhaust instead of in the actual
exhaust pipe.
To measure the affect of the speaker on the exhaust tone, a second microphone will be placed a
short distance from the exhaust pipe to measure the amplitude of the sound waves now exiting the pipe.
A comparison will be made between the amplitudes of the samples taken with the speaker and without
the speaker to determine the difference.
Outcomes
Once the project has been completed, a detailed research paper will be printed up to describe the
results of the project. It is expected that a simple system will be completed to measure and cancel out the
exhaust tones that will also be presented in this paper. It may even be possible to create some sort of
prototype device once the research paper is complete that could lead to further research and
development of the idea. The main goal, however, is to develop a method of accurately measuring and
recording the frequency and amplitude of the exhaust tones and be able to reproduce an inverse signal.
Project Timetable
By February 1 st , the type of microphones needed for sampling will be determined. By March 1 st , the
method of acquiring and analyzing the samples taken by the microphone will be determined. By March
15 th , the experiments will be completed. By March 20 th , the first draft of the research paper will be
completed. By March 25 th , the second draft will be completed. On April 4 th , the final draft will be
finished and turned in.
i
http://www.euro.who.int/en/what-we-do/health-topics/environmental-health/noise
ii
http://www.bose.com/controller?url=/shop_online/headphones/noise_cancelling_headphones/index.jsp
Other sources:
http://www.who.int/occupational_health/publications/noise6.pdf
http://www.chrisruckman.com/ancfaq.htm