Hearing Aid Dispenser Mandatory Revision Questions and Correct Answers

Frequency - - How many times per second a sound wave alternates pressure - Measured in Hz or kHz Period - - Time taken for a repetitive sound wave to finish one cycle. - = 1/f Phase - - Describes the timing of a sound Two waves are out of phase when... - there waveforms are proportional, but all frequencies have a 180 degrees phase shift (opposite polarity). Wavelength - The distance a sound wave travels in one period. Sound Pressure Level - - Number of dB by which any sound pressure exceeds the reference (2x10^-5 pa). When pressure doubles, how much is added? - 6 dB When pressure increases 10 times, the SPL increases by... - 20 dB An octave is... - a doubling of frequency Fourier Analysis - A mathematical procedure by which any signal can be separated into component sine waves at different frequencies. Combining these sine waves will reproduce the original signal. Gain is calculated by... - Output level minus input level Is compression linear or non-linear - Non-linear Name the essential parts of a HA - - One or more mics to convert sound into an electrical signal - An amplifier to increase the strength of the electrical signal - A mini loudspeaker (receiver) to turn the electricity back to sound - Means of coupling the sound source to the ear canal. - A battery How do directional microphones work? - - Suppress noise coming from on mic/port while retaining good sensitivity to the sound coming from another direction. Name some sensitivity pattern of a HA with directional mics enabled - Cardioid (has a heart shaped pattern), super cardiod, hyper cardiod, figure 8 Why is distortion bad? - Degrades intelligibility What are distortion products? - - Areas that have been peak clipped from the original sound signal. Total Harmonic Distortion (THD) - - Measurement of the harmonic distortion present; Defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. What is a negative result of peak clipping? - Distortion