Development of the Carbon Nanotube Thermoacoustic Loudspeaker

Traditional speakers make sound by attaching a coil to a cone and moving that coil back and forth in a magnetic field (aka moving coil loudspeakers). The physics behind how to generate sound via this velocity boundary condition has largely been unchanged for over a hundred years. Interestingly, around the time moving coil loudspeakers were first investigated the idea of using heat to generate sound was also known. These thermoacoustic speakers heat and cool a thin material at acoustic frequencies to generate the pressure wave (i.e. they use a thermal boundary condition). Unfortunately, when the thermoacoustic principle was initially discovered there was no material with the right properties to heat and cool fast enough. Carbon nanotube (CNT) loudspeakers first generated sound early in the 21st century. At that time there were many questions unanswered about their place in the sound generation toolbox of an engineer. The main goal of this dissertation was to continue the development of the CNT loudspeaker with focus on practical usage for an acoustic engineer. Prior to 2014, when this effort began, most of the published development work was from material scientists with objective acoustic performance data presented that was not useful beyond the scope of that particular publication. For example, low sound pressure levels in the nearfield at low power inputs was a common metric. Therefore, this effort had three main objectives with emphasis placed on acquiring data at levels and in nomenclature that would be useful to acoustic engineers so they could bring the technology to market, if adequate. Investigation into the true power efficiency of CNT loudspeakers Investigation into alternative methods to linearize the pressure response of CNT loudspeakers Investigation into the sound quality of CNT loudspeakers Overall, it was found that CNT loudspeakers are approximately four orders of magnitude less power efficient than traditional moving coil loudspeakers. The non-linear pressure output of the CNT loudspeakers can be linearized with a variety of drive signal processing methods, but the selection of which method to use depends on a variety of factors (e.g. amplification architecture available). In general, all methods studied are on the same order of magnitude power efficiency, but the direct current offset and amplitude modulation drive signal processing methods are superior in terms of sound quality

Evaluation of acoustic noise in magnetic resonance imaging

Magnetic resonance imaging (MRI) is a technique in which strong static and dynamic magnetic fields are used to create virtual slices of the human body. The process of MR imaging is associated with several health and safety issues which may negatively affect patient and radiological health workers. Potentially hazardous are biological effects of both the static and dynamic magnetic fields, the torques of the magnetic fields acting on ferromagnetic objects, thermal effects, and the negative effects of high acoustic sound pressures. The subject of this dissertation is the evaluation and modification of acoustic noise generated during MRI

Magnetostrictively induced mechanical resonance of electrical steel strips

Extensive research has been carried out over the years to reduce the acoustic noise resulting from vibration of electromagnetic cores mainly caused by magnetostriction. This project presents the results of a basic experimental study of magnetostriction in strips of magnetic materials commonly used in electromagnetic cores which gives an important new understanding of the phenomenon. The presence of mechanical resonance in the laminations is highlighted here for the first time. A standard magnetising system was built and a new method of measuring magnetostriction was used. A single point laser vibrometer was used to measure magnetostrictive vibration of the samples. The magnetostriction of grain-oriented materials cut at various angles to the rolling direction, non-oriented samples with different silicon content and nickel iron strips was measured over a wide range of magnetising frequencies and at peak flux densities up to 1.O Tesla. Magnetostriction measurement results were used to identify magnetisation induced mechanical resonance of the samples. The magnetising frequency at resonance was derived from the relationship of velocity, frequency and wavelength of an electromagnetically excited strip. Theoretical value of the fundamental resonant frequency and its harmonics were calculated and compared with measured values. The variation of the acoustic noise in a three- phase transformer core under no-load condition with various switching frequencies and different modulation indices was measured under pulse-width modulation and sinusoidal voltage excitation. All measurements were repeated at least five times to assess experimental accuracy and uncertainties. Results suggest that under resonance, transformer cores can produce excessive noise and potentially long term deterioration of lamination coating and possible core failure. Extrapolation of the results to larger cores infer that the phenomenon can possibly occur in cores with different length laminations leading to variability of noise output according to how close the magnetising frequency or predominant harmonics are to the resonant frequency. These findings demonstrate the importance of the interaction between basic magnetostriction and geometrical factors that are contributing towards the total noise output and care that must be taken when characterising the basic magnetostriction of samples of different sizes magnetised at different frequencies.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Navigation/traffic control satellite mission study. Volume 3 - Selected navigation/ traffic control satellite system analysis and equipment definition Final report

L band and VHF voice communication in satellite navigation and traffic control networ

Personalized ambient parameters monitoring: design and implementing of a wrist-worn prototype for hazardous gases and sound level detection

The concentration is on “3D space utilization” as the concept and infrastructure of designing of a wearable in ambient parameters monitoring. This strategy is implemented according to “multi-layer” approach. In this approach, each group of parameters from the same category is monitored by a modular physical layer enriched with the respected sensors. Depending on the number of parameters and layers, each physical layer is located on top of another. The intention is to implement a device for “everyone in everywhere for everything”

Telstar I, volume 1

Telstar satellite design, construction, ground facilities, and use

Magnetostrictively induced mechanical resonance of electrical steel strips

Extensive research has been carried out over the years to reduce the acoustic noise resulting from vibration of electromagnetic cores mainly caused by magnetostriction. This project presents the results of a basic experimental study of magnetostriction in strips of magnetic materials commonly used in electromagnetic cores which gives an important new understanding of the phenomenon. The presence of mechanical resonance in the laminations is highlighted here for the first time. A standard magnetising system was built and a new method of measuring magnetostriction was used. A single point laser vibrometer was used to measure magnetostrictive vibration of the samples. The magnetostriction of grain-oriented materials cut at various angles to the rolling direction, non-oriented samples with different silicon content and nickel iron strips was measured over a wide range of magnetising frequencies and at peak flux densities up to 1.O Tesla. Magnetostriction measurement results were used to identify magnetisation induced mechanical resonance of the samples. The magnetising frequency at resonance was derived from the relationship of velocity, frequency and wavelength of an electromagnetically excited strip. Theoretical value of the fundamental resonant frequency and its harmonics were calculated and compared with measured values. The variation of the acoustic noise in a three- phase transformer core under no-load condition with various switching frequencies and different modulation indices was measured under pulse-width modulation and sinusoidal voltage excitation. All measurements were repeated at least five times to assess experimental accuracy and uncertainties. Results suggest that under resonance, transformer cores can produce excessive noise and potentially long term deterioration of lamination coating and possible core failure. Extrapolation of the results to larger cores infer that the phenomenon can possibly occur in cores with different length laminations leading to variability of noise output according to how close the magnetising frequency or predominant harmonics are to the resonant frequency. These findings demonstrate the importance of the interaction between basic magnetostriction and geometrical factors that are contributing towards the total noise output and care that must be taken when characterising the basic magnetostriction of samples of different sizes magnetised at different frequencies