Suraj M Prabhu – smprabhu@mtu.edu
Dr. Andrew Barnard – arbarnar@mtu.edu
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
174^th 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.

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

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

Lukasz J. Nowak – lnowak@ippt.pan.pl
Institute of Fundamental Technological Research, Polish Academy of Sciences
Pawinskiego 5B
02-106 Warszawa, Poland

Popular version of paper 1pEAa4, “An Experimental Study on the Role and Function of the Diaphragm in Modern Acoustic Stethoscopes”
Presented Monday afternoon, December 04, 2017, 1:45 PM, Balcony N
174th ASA meeting, New Orleans

Acoustic stethoscope, invented over 200 years ago by French physician, Rene Laennec, is the most commonly used medical diagnostic device and also the symbol of medical professionals. Thus, it might sound surprising, that the physics underlying the operation of this simple, mechanical device is still not well understood. The theory of operation of the stethoscope is widely described in the medical literature. However, most of the presented statements are based on purely intuitive conclusions, subjective impressions or on the results of experiments which do not reflect the complex mechanical problem of the chestpiece – patient interaction. Some recently published findings[1,2] suggest, that the state of the art in the field should be verified.

One of the main challenges in determining acoustic properties of stethoscopes is the fact, that under patient examination conditions (i.e. the only case that actually matters for the considered problem) the chestpiece of the stethoscope is mechanically coupled with a body. The effects of this coupling significantly alter the sought parameters. Thus, you cannot simply replace a patient with a loudspeaker, in order to use harmonic test signals, as in the normal case of measuring acoustic parameters of any standard audio device. You have to perform the analysis and draw the conclusions based on the sounds from the inside of the body of a patient, and those are relatively quiet, noisy, and variable in nature.

The present study focuses on the role and function of the diaphragm in modern acoustic stethoscopes. During auscultation, the diaphragm is excited to vibrate by the underlying body surface, and thus it is the source of sound transmitted through the hollow tubes of a stethoscope to the ears of the physician. The higher are the velocity level values distributed across the surface of the diaphragm, the louder will be the perceived sound. Loudness is a crucial parameter, as the auscultation sounds are very quiet in general, and the diagnosis is often obtained based on the distinction of very subtle changes in those signals. Different stethoscope manufacturers use various materials, shapes, sizes and attaching means for the diaphragms, claiming that specific solutions provide optimal sound parameters. However, no objective data regarding this problem are available, and thus, such statements cannot be accepted from the scientific point of view.

A detailed experimental methodology for determining vibroacoustic properties of different kinds of diaphragms is introduced in the present study. A laser Doppler vibrometer is used to measure the velocity of vibrations of various points on the surface of a diaphragm during heart auscultation (Figure 2). At the same time, an electrocardiography (ECG) signal is also recorded. The ECG signal is used to extract only a subset of clean and uncorrupted velocity signals, without noise and other, interfering body sounds (see Figure 3). The parameters of the extracted and selected fragments are statistically analyzed.

The box plot in Figure 4 shows the values of velocity of vibrations determined at the center and close to the edge for various types of diaphragms encountered in modern acoustic stethoscopes. The first two boxes on the left correspond to the case without a diaphragm. In general, the higher values, and the lower differences between the center and the edge – the better. As it can be seen, the results differ significantly between various diaphragm types. The drawn conclusions are especially important from the physicians’ point of view, as the acoustic efficiency of a stethoscope translates directly into the quality of the diagnosis. An open question remains if and how it could be possible to significantly improve the efficiency of the existing solutions? The analysis of the obtained results states a good foundation for further investigations in this direction, as it allows to better understand the phenomena underlying auscultation examination and to formulate some general assumptions regarding the most promising solutions.

Figure 1. A modern acoustic stethoscope with a diaphragm chestpiece

 
Figure 2. The laboratory stand used for experimental investigations on the vibroacoustic parameters of various kinds of stethoscope diaphragms

 
Figure 3. All the vibration velocity signals extracted from a single recording, including noisy and corrupted ones (top), and the corresponding subset of signals selected for further analysis (bottom)

Figure 4. Box plot presenting the distribution of the measured velocity of vibrations values at the center and edge points for each of the considered cases

[1] Nowak, L. J., and Nowak, K. M. (2017). “Acoustic characterization of stethoscopes using auscultation sounds as test signals,” J. Acoust. Soc. Am., 141, 1940–1946. doi:10.1121/1.4978524

[2] Nowak, K. M., and Nowak, L. J. (2017). “Experimental validation of the tuneable diaphragm effect in modern acoustic stethoscopes,” Postgrad. Med. J., , doi: 10.1136/postgradmedj-2017-134810. doi:10.1136/postgradmedj-2017-134810