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Figures for Chapter 3 Hearing aid systems Dillon (2001) Hearing Aids
Custom ITE construction Photo removed to minimize file space Figure 3. 1 (a) A faceplate with components mounted, next to a shell far an ITC hearing aid. After gluing the two parts together, all material outside the cutting mark is removed. (b) A close-up of the components mounted on the faceplate. Source: Dillon (2001): Hearing Aids
Analog and digitally-programmable block diagrams Analog hearing aid microphone Signal path receiver Programmable hearing aid Signal path Digital controller and memory Source: Dillon (2001): Hearing Aids programmer interface programmer unit Figure 3. 2 Block diagram of an analog hearing aid (top) and a digitally programmable hearing aid (bottom).
Induction loop signal path Current Magnetic field Sound wave Voltage Sound wave Figure 3. 3 The complete chain, from sound wave in to sound wave out, for a magnetic loop induction system. Source: Dillon (2001): Hearing Aids
Current and magnetism Magnetic Flux Current Source: Dillon (2001): Hearing Aids Figure 3. 4 Lines of magnetic flux flowing around a conductor carrying a current.
Addition of magnetic flux inside loop Figure 3. 5 A complete induction loop system, showing how lines of magnetism from all parts of the loop add constructively within the region enclosed by the loop. Source: Dillon (2001): Hearing Aids
A two-turn magnetic loop Resistor Figure 3. 6 The connections needed to make a loop of two turns using a single run of cable that has two separate wires. The location of an optional series resistor is also shown. Source: Dillon (2001): Hearing Aids
Audio amplitude Frequency modulation (a) Unmodulated carrier amplitude Time Modulated carrier amplitude (b) (c) Figure 3. 7 The waveform of a carrier before (b), and after (c), it has been frequency modulated by a sinusoidal audio signal (a). Source: Dillon (2001): Hearing Aids
Speech-operated switching Rx Control signal Figure 3. 8 An FM system with Speech Operated Switching (SOX) in the receiver to select either the local microphone signal or the FM signal (from the aerial) for amplification by the hearing aid. Source: Dillon (2001): Hearing Aids
Sound field amplification system Rx Tx Figure 3. 9 Block diagram of a sound-field amplification system comprising a transmitter worn by the teacher, a receiver and amplifier mounted somewhere convenient in the room, and four loudspeakers distributed around the room. Source: Dillon (2001): Hearing Aids