Figures for Chapter 3 Hearing aid systems Dillon (2001) Hearing Aids

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 Custom ITE construction Photo removed to minimize file space

Figure 3.2 Block diagram of an analog hearing aid (top) and a digitally programmable hearing aid (bottom). Analog hearing aid microphone receiver Signal path Programmable hearing aid Signal path Digital controller and memory programmer unit programmer interface Source: Dillon (2001): Hearing Aids Analog and digitally-programmable block diagrams

Voltage Magnetic field Sound wave Sound wave Current 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 Induction loop signal path

Figure 3.4 Lines of magnetic flux flowing around a conductor carrying a current. Current Magnetic Flux Source: Dillon (2001): Hearing Aids Current and magnetism

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 Addition of magnetic flux inside 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 A two-turn magnetic loop

Unmodulated carrier amplitude Audio amplitude Time (a) (b) Modulated carrier amplitude (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 Frequency modulation

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. Control signal Rx Source: Dillon (2001): Hearing Aids Speech-operated switching

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. Tx Rx Source: Dillon (2001): Hearing Aids Sound field amplification system