Echolocation: evaluation of finite element analysis of ultrasonic transducers

Echolocation is the process of building an acoustic image of the environment by sensing the ultrasonic echoes that are bounced off objects in the environment. It is naturally observed in bats and dolphins, who use it for navigation and orientation. The Finite Element Method is one of several numerical techniques that simplify the abstract equations of calculus and obtain approximate solutions for real-world physical problems. This thesis evaluates the suitability of various numerical \u27Element Methods\u27 for the modelling and design of ultrasonic transducers and the study of their radiated and scattered sound fields. To achieve this, it determines the selection criteria to choose between the alternate methods and identifies the essential set of elements that are required to model the problem. Further, it studies various commercial FEM software packages and identifies software language features necessary to implement the software package. It arrives at conclusions regarding the basic requirements to develop a minimal software package capable of modelling ultrasonic transducers. On the basis of this thesis, software can be developed to provide a structure upon which future researchers can build and develop more complex models, such as those involving transducers and their interaction with the environment

Learning to write in networked public: children and the delivery of writing online

This investigation explored how three children (together with parents) developed networked publics that were diverse, well-connected, and powerful in the world. It was framed in response to calls in the field to better understand the new literacies young writers develop online and outside of school, and to increase literacy educators’ attention to the role of public audiences in writing and how writing is circulated. Performative case study methodology, ethnographic methods, and digital methods were employed to track and describe the online networks of three children (ages 11-13). These focal children were actively involved with their parents in social media, and had developed widespread networks with shared interests in children’s books and book reviews (Case 1), baseball (Case 2), and helping the homeless (Case 3). The children’s online networks were conceptualized as networked publics, drawing on Warner’s (2002) notion of publics as ongoing discursive relations among strangers, and on Actor-Network Theory’s notion of networks as assemblages of diverse interests that mobilize toward a common goal (Callon, 1986) and that develop stability in relation to ongoing circulations of texts (Latour, 1986; Spinuzzi, 2008). Research questions were framed broadly around the rhetorical canon of delivery [now digital delivery (Porter, 2009)], and were concerned with how writers distributed texts online, how those texts circulated, how the networked publics become more stable and powerful, and what instabilities children and parents had to negotiate in order to accomplish all of this. Data sources included interviews with 15 children and 28 adults, and fieldnotes observations of approximately 1,700 screen-captured webpages and other online artifacts. Findings showed that the young writers and their parents initiated and sustained networked publics through distribution practices that were oriented toward building trust; their texts displayed: interest, appreciation, reliability, service, credibility, and responsiveness. Both grassroots and commercial entities circulated texts in these networks, as they were sources of the ongoing renewal these different groups all needed in order to thrive. Sources of instability included conflicts over standards of writing quality, matters of profit, and the constancy of the demand to generate new interest and writing online. Children and their parents responded to these instabilities by welcoming and negotiating heterogeneous perspectives and partnerships. Implications of the study call for further research and teaching about the art of networked public discourse and digital delivery.Curriculum and Instructio

Sensory coding in an identified motion-sensitive visual neuron of the locust (Locusta migratoria)

Visual environments may contain a complex combination of object motion. Animals respond to features of complexity by generating adaptive behavioural responses. One important feature of a complex visual environment is a rapidly expanding object in the visual field (looming) which may represent an approaching predator or an object on a collision path. Many animals respond to looming objects by generating avoidance behaviours (Maier et al. 2004; Santer et al. 2005; Oliva et al. 2007) and neurons involved in the detection and relay of looming stimuli are present in birds (Sun and Frost 1998) and many insects (Simmons and Rind 1992; Hatsopoulos et al. 1995; Wicklein and Strausfeld 2000). One of the most widely studied visual pathways is found in the locust. This visual pathway, which includes the lobula giant motion detector (LGMD) and its post-synaptic target, the descending contralateral motion detector (DCMD), signals the approach a looming visual stimulus (Schlotterer, 1977; Simmons and Rind, 1992; Hatsopoulos et al., 1995). The DCMD descends through the ventral nerve cord and synapses with motorneurons involved in predator evasion and collision avoidance (Simmons, 1980; Simmons and Rind, 1992; Santer et al., 2006). Previous studies have suggested that this pathway is also affected by more complicated movements in the locust’s visual environment. For example, Guest and Gray (2006) demonstrated that the approach of paired objects in the azimuthal position and approaches at different time intervals affect DCMD firing rate properties. In my first objective of this thesis (Chapter 2), I tested locusts with computer-generated discs that traveled along a combination of non-colliding (translating) and colliding (looming) trajectories and demonstrate how distinctly different DCMD responses result from different trajectory types. In addition to estimating the time of collision and direction of object travel, the presence of a discernable peak associated with the time of object deviation suggests that DCMD responses may contain information related to changes in motion. Previous studies suggest that LGMD/DCMD encodes approaching objects using rate coding; edge expansion of approaching objects causes an increased rate of neuronal firing (Schlotterer, 1977; Hatsopoulos et al., 1995; Judge and Rind, 1997; Gabbiani et al., 1999). Based on observations of DCMD responses to simple looming objects that showed oscillations in DCMD responses (for example, Fig. 1D Santer et al., 2006) and the fact that bursting occurs in many other sensory systems (Yu and Margoliash, 1996; Sherman, 2001; Krahe and Gabbiani, 2004; Marsat and Pollack, 2006), it was hypothesized that the DCMD may show bursting activity. In my second objective of this thesis (Chapter 3), I tested locusts with simple looming stimuli known to generate behavioural responses in order to identify and quantify bursting activity. Results show that the highest frequency of bursts occurred at intervals of 40-50 ms (20-25 Hz). The behavioural significance of this frequency is related to the average wingbeat frequency of the locust’s forewing during flight (~25 Hz; Robertson and Johnson, 1993). Based on previous evidence of DCMD flight-gating (see, for example, Santer et al., 2006), bursting may gate information into the flight circuitry, thereby providing visual feedback that may be modified to generate an avoidance response during flight. Single spiking and bursting occurred throughout object approach up until the late stage of approach, where burst frequency rapidly increased. Results predict that the DMCD may use a bimodal coding strategy to detect looming visual stimuli, where single spiking at the beginning of approach may result in subtle course changes during flight and bursting near the time of collision may initiate an evasive glide. Taken together, these results illustrate that the encoding of visual stimuli in single neurons is dynamic and likely much more complicated than previously thought