Andrew R. Barnard

Feasibility of a high-powered carbon nanotube thin-film loudspeaker

The thermophone, conceived in 1917 by Arnold and Crandall, was a unique thermoacoustic loudspeaker. The high heat capacity per unit area (HCPUA) of thin-film materials at that time limited the usefulness of thermophones. Recently, researchers of carbon nanotubes (CNTs) have developed techniques to create a super-aligned thin-film of multi-walled CNTs, possessing extremely low HCPUA. This paper will discuss CNT thin-film loudspeaker theory as well as some initial investigations into the feasibility of a high-powered audio CNT speaker. The advantages of such a loudspeaker include: Ultra-lightweight, compact, no moving parts, low cost, and independence from expensive rare-earth materials.

Experimental quantification of the true efficiency of carbon nanotube thin-film thermophones

Carbon nanotube thermophones can create acoustic waves from 1 Hz to 100 kHz. The thermoacoustic effect that allows for this non-vibrating sound source is naturally inefficient. Prior efforts have not explored their true efficiency (i.e., the ratio of the total acoustic power to the electrical input power). All previous works have used the ratio of sound pressure to input electrical power. A method for true power efficiency measurement is shown using a fully anechoic technique. True efficiency data are presented for three different drive signal processing techniques: standard alternating current (AC), direct current added to alternating current (DCAC), and amplitude modulation of an alternating current (AMAC) signal. These signal processing techniques are needed to limit the frequency doubling non-linear effects inherent to carbon nanotube thermophones. Each type of processing affects the true efficiency differently. Using a 72 W(rms) input signal, the measured efficiency ranges were 4.3 × 10(-6) - 319 × 10(-6), 1.7 × 10(-6) - 308 × 10(-6), and 1.2 × 10(-6) - 228 × 10(-6)% for AC, DCAC, and AMAC, respectively. These data were measured in the frequency range of 100 Hz to 10 kHz. In addition, the effects of these processing techniques relative to sound quality are presented in terms of total harmonic distortion.

Advancements toward a high-power, carbon nanotube, thin-film loudspeaker

The carbon nanotube (CNT) thermophone has been explored as a novel loudspeaker. Potential advantages of this technology in the audio industry include ultra-lightweight, low production cost, compact size, and independence from rare-earth materials. In this paper, progress toward a practical CNT loudspeaker is presented. Large, high quality CNT thin-film assemblies are designed and built. Design guidance for these types of assemblies is provided. Maximum sound output level, total harmonic distortion, and power efficiency tests are performed. A maximum source level of 111 dBA at 1 m is achieved at 2 kHz with the new sources. The main hurdle to this technology remains power efficiency. Several paths forward are discussed as this technology continues to advance to a position where it may be able to compete with current state-of-the-art, moving-coil loudspeakers.

Airframe structural damage detection: A non-linear structural surface intensity based technique

The non-linear structural surface intensity (NSSI) based damage detection technique is extended to airframe applications. The selected test structure is an upper cabin airframe section from a UH-60 Blackhawk helicopter (Sikorsky Aircraft, Stratford, CT). Structural damage is simulated through an impact resonator device, designed to simulate the induced vibration effects typical of non-linear behaving damage. An experimental study is conducted to prove the applicability of NSSI on complex mechanical systems as well as to evaluate the minimum sensor and actuator requirements. The NSSI technique is shown to have high damage detection sensitivity, covering an extended substructure with a single sensing location.

Design and implementation of a shielded underwater vector sensor for laboratory environments

Underwater acoustic vector sensors, for measuring acoustic intensity, are typically used in open water where electromagnetic interference (EMI) is generally not a contributor to overall background noise. However, vector sensors are also useful in a laboratory setting where EMI can be a limiting factor at low frequencies. An underwater vector sensor is designed and built with specific care for EMI immunity. The sensor, and associated signal processing, is shown to reduce background noise at EMI frequencies by 10-50 dB and 10-20 dB across the entire frequency bandwidth, as compared to an identical unshielded vector sensor.

Multi-physics modeling of conformal, solid-state, and thin-film thermophones

Thin-film thermophones are thermoacoustic loudspeakers made from new materials such as aligned carbon nanotubes or graphene. They are solid state devices, in that there are no moving parts associated with the sound generation mechanism. Lumped parameter models have been developed for planar thermophones and agree well with experimental results. One benefit of thin-film thermophones is that they can be designed such that the transducer can be conformal to complex geometries. However, the lumped parameter models are not appropriate for complex geometries or electrical/thermal transients. An electrical-thermal-acoustic multiphysics model of these transducers has been created using COMSOL Multiphysics to address the issues present with the lumped parameter models. Development and validation of the model will be discussed.

Near field acoustic holography measurements of carbon nanotube thin film speakers

Carbon nanotube (CNT) thin film speakers produce sound with the thermoacoustic effect. Better understanding of the physical acoustic properties of these speakers will drive future design improvements. Measuring acoustic properties at the surface of the CNT thin film is difficult because the films, themselves, do not vibrate, are fragile and have a high surface temperature. In order to measure the surface particle velocity and sound pressure level (SPL), near field acoustic holography (NAH) has been used by employing probe microphones. NAH images the acoustic quantities of the source system using the set of acoustic pressure measurements on a hologram parallel to the source surface. It is shown that the particle velocity at the surface of an open-air, double-sided speaker is nominally zero, as expected. However, the SPL distribution is not uniform on the source surface, contrary to common lumped parameter model assumptions. Also, particle velocity and sound intensity distributions on the hologram have been obtained in this study. Finally, measured directivity patterns of the planar CNT speaker are reported.

Evaluation of crowd noise levels during college football games

Crowd noise at sporting events is a highly publicized acoustic phenomenon in the mainstream media because it is widely believed to influence the game. However, very little rigorous scientific data exist on the topic. Perhaps the sport where crowd noise can have the biggest impact on the game outcome is American football. Football crowd noise can make it difficult for teams to communicate on the field and even induce penalties. In 2007 and 2009, researchers from Penn State University�s Graduate Program in Acoustics measured crowd noise at three Penn State football home games. Noise generated by the crowd was rigorously quantified and synchronized to events during the game. Peak levels of 123�140 dB and max 10-second equivalent levels of 109�114 dB were recorded. Maximum possible communication distances were computed around the field using measured A-weighted equivalent sound pressure levels. Communication distances are shown to be limited to less than 3 meters on the field depending on which team (home or visitor) has the ball and the location on the field. The subjective loudness psychoacoustic metric, measured in sones, was used to show that the crowd noise experienced by the visiting team is subjectively 2�3 times louder than that experienced by the home team. This results in a distinct, measurable home field advantage. SPLs outside the stadium at distances of 200�800 m were shown to be 20�30 dB below SPLs inside the stadium. Recommendations are provided for other researchers who wish to make similar measurements for comparison to the results presented here

PERFORMANCE OF HARD DISK DRIVES IN HIGH NOISE ENVIRONMENTS

Digital information stored in rotational media, such as hard disk drives (HDD) needs to be reliable and readily accessible. Although various studies have been performed on the modal analysis of HDDs and the noise emitted during operation, few researchers have addressed the effects noise can have on the performance of HDDs. Analysis performed by Siemens Corporation, IBM and Tyco Fire Protection Products shows that HDDs are sensitive to external disturbances such as tones, broadband noise with tones, and broadband signals. This paper focuses on identifying the critical frequency ranges and the levels of noise where the performance of the drives reduces. A series of tests has been performed using filtered noise in one-third octave frequency bands at sound pressure levels ranging from 80 dB to 130 dB (re 20 mPa) in an anechoic chamber. A detailed description on the test set-up along with the methodology of testing has been provided. Read/write speeds have been used to measure HDD performance. A completely random read/write workload with varied data packet size has been incorporated through a custom LabVIEW program. Serial Advanced Technology Attachment (SATA) drives have been used for the analysis. General trends in performance curves of enterprise HDD, laptop drives, helium filled HDD and non-helium filled HDDs are shown. HDD performance is shown to be sensitive to sound pressure levels as low as 85 dB (re 20 mPa) at some frequencies and the frequency range from 4 kHz to 10 kHz has been found to be most sensitive to HDDs performance.

Platinum films to carbon nano structures: The history of the thermophone transducer

The thermophone is a device which creates sound using rapid heat oscillations on its surface. This results in a thin and lightweight loudspeaker with no moving parts. Braun was the first to address this phenomenon in the late 1800s. Arnold and Crandall developed the first theory and correlated experimental results in 1917. Unfortunately, the materials necessary to make an efficient thermophone did not exist in their time. In 2008, Xiao rediscovered the thermophone effect using carbon nanotube thin films, a material much better suited to efficient thermophones. Since then many researchers have been working on developing thermophone technology using carbon nanostructures including nanotubes, nanofibers, and graphene. This talk will cover the history of the thermophone from its early days through today and give a broad overview of the application areas for this technology, which span from underwater transducers to consumer electronics and automotive applications.