Suraj M Prabhu –
Dr. Andrew Barnard –
Dynamic Systems Laboratory
R.L. Smith Building, 1400 Townsend Drive
Houghton, MI 49931

Popular Version of paper 2aEA6, “Carbon Nanotube Coaxial Thermophone for Automotive Exhaust Noise Cancellation”
Presented Tuesday morning, November 5, 2017
174th ASA Meeting, New Orleans

Everyday noise affects human health and is a major irritant for people of all ages. Automotive exhaust noise is the one of the most common community noises. Exhaust noise is generated in the engine, travels down the exhaust pipe with the exhaust gases, and is radiated out to the atmosphere. Due to the presence of enormous numbers of automobiles, people are exposed to significant levels of exhaust noise over their lifetime and, so it is important to control the exhaust noise through engineering noise control technology.

The two types of noise control systems used are passive control system and active control systems. Passive control system uses a muffler to attenuate and/or absorb the exhaust noise. An active control system has a loudspeaker that generates a sound with equal amplitude and opposite phase to cancel the exhaust noise. This works just like noise canceling headphones that are used by many air travelers, except it is done at the source.

Carbon nanotubes (CNT) are carbon nano-structures which when stretched form extremely lightweight, flexible films. These films adhere to any conductive surface to form a thermal speaker and when current is passed through them; their surface temperature fluctuates very rapidly. These fluctuations produce pressure waves in the medium near the film, thereby producing sound. The speaker uses no moving parts to produce sound and hence it is solid state in operation.

Figure 1: Automotive exhaust noise schematic

As the operating temperature of CNT speakers is high as compared to the temperature of the exhaust gases, the speakers can be mounted directly onto the tailpipe. The design of the CNT speaker is a coaxial transducer in the form of a co-axial spool. In addition, compared to the other components of the speaker, the CNT film itself is massless and so the overall weight of the speaker is much less compared to traditional loudspeakers with magnets. When tested in the laboratory for noise cancellation, an average cancellation of 12 – 15 dB was achieved across exhaust frequency ranges.   

Figure 2: Passive control system schematic indicating the location of a single muffler along the tailpipe

Figure 3: Active control system schematic indicating the mounting of the loudspeaker at the end of a side branch and the mounting of the entire setup on the tailpipe.

Carbon nanotubes

Figure 4: Planar CNT speaker having the film stretched between two electrodes and attached to an insulating base with electrical connector

Carbon nanotubes

Figure 5: Coaxial CNT speaker (prototype) having two end plates (white discs), electrodes with wires for connection, CNT (black film) wrapped around the electrodes protected from the atmosphere by a transparent cover

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