Infrasoniese, GPS-gesinkroniseerde klankopnemers vir die opsporing van olifantdreungeluide

Byker, Rudolf Willem (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The automatic localisation of wild elephants in their natural habitat, without interaction with the animals themselves, has many applications in wildlife conservation and conservation ecology. Current solutions require the attachment of devices to individual animals. Elephant rumbles have strong infrasonic components and elephant habitats offer ideal conditions for the propagation of these rumbles over long distances. An integrated hardware solution for the localisation of these sounds is designed and tested. It consists of sound recorders which are relatively inexpensive, are sensitive to low frequencies, are battery-powered, have low intrinsic noise, can communicate wirelessly, and are synchronised with one another via GPS. A custom recorder is designed since no commercially available audio equipment meets these requirements. The recorder uses a 12 V sealed lead acid battery. Power supplies are designed to power the respective parts of the recorder. A circular array of 6 Invensense ICS-40300 MEMS microphones are used with each recorder. An audio interface is designed, consisting of ADCs, preamps and anti-alias filters. Each audio interface has a high input impedance, a high-cutoff frequency of about 1 kHz, a bit depth of 24 bits per channel, a sample rate of 4800 Hz and 6 independent, synchronously sampled audio channels. A digital audio buffer collects samples from the ADC and sends it to a RPI over SPI, where is it stored on an SD card for later processing. In the future, processing may be done online on the RPI. GPS modules are used to accurately timestamp each sample so that the audio channels from independent recorders may be used as a wide-spaced array of microphones without explicit inter-device communication. Various sound source localisation algorithms are simulated and tested. Multilateration with wide-spaced microphones is investigated, and the conclusion is that at least four wide-spaced sound recorders with a single microphone on each is needed to accurately locate sound sources. A novel direction of arrival estimation algorithm based on the pairwise time delay estimations of all 15 microphone pairs in the 6-microphone array is developed, simulated and tested. This algorithm performed better than multilateration when only two wide-spaced sound recorders were used. In both cases, tests were performed with a loudspeaker playing elephant rumbles or white noise. The results are shown in Figures 10.22–10.27. Suggestions are made for future work towards an end product that can be operated by park rangers and ecologists. The localisation of elephant rumbles with infrasonic sound recorders appears to be a viable solution to various wildlife conservation problems.

AFRIKAANSE OPSOMMING: Die outomatiese opsporing van wilde olifante in hul natuurlike habitat, sonder enige interaksie met die dier self, het verskeie toepassings in wildbewaring en bewaringsekologie. Huidige oplossings vereis dat apparate aan individuele olifante vasgemaak word. Olifantdreungeluide het sterk infrasoniese komponente en olifanthabitat bied ideale toestande vir die propagering van hierdie geluide oor lang afstande. ’n Geïntegreerde hardewarestelsel vir die opsporing van hierdie geluide word ontwerp en getoets. Dit bestaan uit klankopnemers wat betreklik goedkoop is, sensitief is vir lae frekwensies, battery-aangedrewe is, lae intrinsieke ruis het, draadloos kan kommunikeer, en met behulp van globale posisioneringstelsel (GPS) met mekaar gesinkroniseer is. ’n Eie klankopnemer is ontwerp omdat geen kommersieel beskikbare toerusting aan die vereistes van die projek voldoen nie. Die klankopnemer gebruik ’n 12 V verseëlde loodsuurbattery. Pasgemaakte kragbronne is ontwerp om die onderskeie dele van die opnemer aan te dryf. ’n Sirkelvormige skikking van 6 Invensense ICS-40300 mikro-elektromeganiese sisteem (MEMS)-mikrofone word op elke klankopnemer gebruik. ’n Pasgemaakte oudiokoppelvlak, wat uit versyferaars, voorversterkers en teen-oorvleuelingsfilters bestaan, is ontwerp. Elke oudiokoppelvlak het ’n hoë intree-impendansie, ’n hoogafsnyfrekwensie van ongeveer 1 kHz, ’n monsterbreedte van 24 bisse per kanaal, ’n monsterfrekwensie van 4800 Hz, en 6 onafhanklike kanale wat sinkroon gemonster word. ’n Digitale oudiobuffer versamel monsters vanaf die versyferaar en stuur dit via serie-koppelvlak vir randapparatuur (SPI, Serial Peripheral Interface) na ’n Raspberry PI (RPI) waar dit op ’n Secure Digital (SD)-kaart gestoor word vir latere verwerking. Ruimte word gelaat vir intydse verwerking op die RPI in die toekoms. GPS-modules word gebruik om ’n akkurate tydstempel aan elke monster toe te ken sodat die oudiokanale van verskeie onafhanklike klankopnemers as ’n wyd-gespasieerde mikrofoonskikking gebruik kan word, sonder direkte kommunikasie tussen die opnemers. Verskillende algoritmes vir die opsporing van klankbronne is gesimuleer en getoets. Veelsymeting met wyd-gespasieerde mikrofone is ondersoek, en daar is bevind dat ten minste vier wyd-gespasieerde klankopnemers met ’n enkele mikrofoon op elkeen nodig is om akkurate klankbronposisies te bepaal. ’n Nuwe algoritme vir klankbronrigtingberaming, wat op die tydverskilberamings van elkeen van die 15 mikrofoonpare in die 6-mikrofoon skikking staatmaak, word ontwikkel, gesimuleer en getoets. Hierdie algoritme het beter resultate gebied as veelsymeting wanneer slegs twee wyd-gespasieerde opnemers gebruik word. In beide gevalle is toetse uitgevoer met ’n luidspreker wat olifantdreungeluide of witruis speel. Figure 10.22–10.27 toon die resultate. Die opsporing van olifantdreungeluide met infrasoniese klankopnemers blyk ’n haalbare oplossing vir verskeie wildbewaringsprobleme te wees.

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