Carbon nanotube coaxial thermophone for automotive exhaust noise cancellation

Abstract

Exhaust noise from automobiles is a major concern and needs to be controlled. Current noise control systems, mainly mufflers, have significant size, weight, and performance limitations at low frequencies. Passive control systems are unable to cancel noise efficiently over the entire frequency spectrum. Active noise control system helps to overcome the above limitation. This paper deals with the design and testing of a coaxial carbon nanotube speaker for active cancellation of automotive exhaust noise. Carbon nanotubes are virtually massless and work on the thermo-acoustic principle. Their ability to sustain heat makes them suitable for use in elevated temperature environments such as automotive exhausts. Analytical calculations are performed using transfer matrix method to evaluate the effect of coaxial thermo-phone and side branch speakers on the sound pressure level in the exhaust tailpipe. Design of the coaxial CNT thermophone and testing of it by mounting on an automotive exhaust tailpipe are discussed. Low frequencies (20 Hz–1000 Hz) are considered to evaluate the performance of the thermophone. For active cancellation of exhaust noise, Filtered-X Least Mean Square (FXLMS) algorithm is developed and implemented using NI LabVIEW.