THE PHOTOPHONE - Historical research revived with 21st century technology

Hoving, J.H.K. (2012) THE PHOTOPHONE - Historical research revived with 21st century technology.

Abstract:This report is the result of a research assignment, part of the bachelor study Applied Physics. The project is done at the Biomedical Photonic Imaging Group (BMPI) of the University of Twente. The main goal of the project is to create a device to demonstrate the photoacoustic effect. This setup is based on the Photophone, invented by Professor Alexander Graham Bell in 1880. The device has to transmit an audio signal (speech) by modulating light intensity and convert the signal back to audio waves making use of the photoacoustic effect. The final design has to have a robust and simple construction so it can be used for educational purposes. To keep as close as possible to the original design by Bell, the target is to design both modulator as receiver without electronics. The research is split in two parts. First, to convert modulated light into sound, research is done on absorber material influence, light intensity response, absorption cell dimensions influence and frequency response. Second, to modulate light with audio waves with a mechanical setup, research is done on different modulations techniques, frequency responses and modulation depths. From research on the absorption cell can be concluded that the modulated light beam has to have an AC signal of at least 50 mW to be audible without electronic amplification. From research on modulation techniques can be concluded that it is difficult to create a modulation of larger than 5 mW AC signal, due to optical safety requirements and modulation technique limits. The research concludes that the modulated light signal is too weak to make the photoacoustic effect audible. Instead, for the construction of the final demonstration setup, electronic light modulation is used. A 5 Watt LED (with optical power of approx. 250 mW) is directly modulated by an electronic audio signal with a small amplifier. The receiver is slightly optimized on basis of the results from the research. The demonstration setup produces a clearly audible sound.
Item Type:Essay (Bachelor)
Faculty:TNW: Science and Technology
Subject:33 physics
Programme:Applied Physics BSc (56962)
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