Probe-tube measurements of ALDs: uniformity, linearity and frequency response differences

A Madsen IGO 1000 was used to assess six assistive listening devices (ALDs) (Sound Plus Infrared, Minicon Induction Floor Loop, Phonic Ear Phonear, Radio Shack Realistic Personal Amplifier, Williams Sound PockeTalker with induction neck loop and Williams Sound PockeTalker permanently wired) worn by a single subject. In situ measures determined product uniformity of three samples of each of the same ALDs, volume control linearity, ear coupling effects on canal resonance and frequency response differences. ALDs were generally uniform, although the two PockeTalker models showed gain variability at all frequencies. Frequency response changes controlled by high boost switches in the Realistic were also found not to be uniform. The earbud had almost no effect on canal resonance, while lightweight headphones caused a slight loss of canal resonance. Non-telecoil coupled devices had broader/flatter responses than telecoil coupled devices. Average differences between telecoil and non-telecoil coupling were minimal. Probe-tube measurements were found to be a feasible means of assessing ALDs in situ.