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Amplification/Sensory Systems SPA 4302 Summer 2006 Hearing Aid Development • • • • • thru the 1800s: Acoustic Devices Circa 1900: Carbon Devices 1930s: Vacuum Tube Devices 1950s: Transistor Devices 1980s: Digital Devices Hearing Aid Circuit Overview • Analog: – mic – amp – filters – atten – receiver • Digital: digital – mic – AtoD – processing – DtoA – receiver • Compression: circuitry or programming to reduce the amplification of loud sounds to keep them below UCL • Tele-coil: direct pick up from a telephone’s electromagnetic field Characteristics of Hearing Aids • OSPL: – Maximum output of aid • Acoustic Gain: – dB difference between output and input • Frequency Response: – range of frequencies amplified – shown as curve, and calculated • Distortion: – equivalent input noise – harmonic distortion Binaural Amplification • Monaural fittings may permit auditory deprivation effects in the unaided ear • Binaurally fitted pts may show improved – listening in noise – localization • HOWEVER, – this has been difficult to document clinically – some (particularly elderly) pts will actually do worse with binaural than monaural fitting. Types of Hearing Aids • • • • • • • Digital/Analog/Hybrid Behind the Ear In the Ear Canal Completely in Canal Bone conduction Implanted Cochlear Implants • • • • Mic - Processor - electrodes Electrical stim of neurons via up to 22 electrodes Latest generation of devices are successful with both postand pre-lingually deafened pts. Selecting Hearing Aid Candidates • More than just the audiogram: • Expectations and Motivation • Communicative Demands • Vocation/Education/Financial Resources Dispensing Hearing Aids • An Historical/Hysterical Issue: • Audiologists are the persons best qualified to fit hearing aids. Selecting HAs for Adults • Ideal HA fitting should (Carhart, 1975): – Provide a restoration of adequate sensitivity for speech and environmental sounds too faint to hear without the hearing aids – Provide a restoration, retention, and or acquisition of the clarity (including intelligibility and recognition) of speech and other sounds within ordinary, relatively quiet environments – Achieve the same when these sounds are in noisier environments – Ensure that higher intensity sounds are not amplified to an intolerable level Verifying Hearing Aid Performance • • • • Probe microphone “Real Ear” measurements With hearing aid in place (in situ) Without aid in (real ear unaided response) Insertion loss: presence of aid in ear plugs it and alters the resonance of the pinna and canal. Hearing Assistance Technologies • • • • Assistive Listening Devices Visual/Vibratory Alerting FM/Infrared transmission Telephony