Research studies have shown that digital wireless phones interact with some hearing aids creating a buzzing noise that reduces speech intelligibility for some hearing-impaired individuals. To develop a standard interference signal that can be used to test all hearing aids, hearing aid output interference was converted to an equivalent acoustic input referenced interference spectrum (IRIS). For this study, it was shown that the IRIS signal has a similar spectral pattern defined by the phone technology for all hearing aids. IRIS signals for three phone technologies, CDMA, TDMA-50 Hz, and TDMA-217 Hz, were generated. Five IRIS signal levels (35 dB SPL, 45 dB SPL, 55 dB SPL, 65 dB SPL, and 75 dB SPL) for each of the three phone technologies were mixed with speech at 65 dB SPL to test the speech intelligibility of 24 hearing-impaired people subjected to wireless phone interference. A "No Noi se" condition was also tested. Annoyance ratings (short-term annoyance before, long-term annoyance during and short-term annoyance after speech intelligibility) were also collected for each test condition.
Speech intelligibility scores for the TDMA-217 Hz IRIS phone signal at low speech-to-noise ratios (<10 dB) were significantly lower than those for CDMA and TDMA-50 Hz at the same level. The CDMA and TDMA-50 Hz IRIS phone signals had a similar effec t on speech intelligibility. For all three phone technologies, speech intelligibility scores at speech-to-noise ratios of 20 dB and 30 dB were similar to those for the "No Noise" condition.
Annoyance ratings increased as a function of noise level. Long-term annoyance ratings collected during speech intelligibility testing were higher than the short-term annoyance ratings collected before and after speech intelligibility testing. Short-term annoyance ratings increased following long-term exposure to noise. Ninety percent of the annoyance ratings collected during speech intelligibility testing at 35 dB SPL were between "No Interference" and "Not Annoying."
Different spectral weighting methods, A-weighting, C-weighting, linear weighting, speech interference level (SIL), and articulation index (AI) were applied to the IRIS signal to determine the best single index that represented the impact of the interference on speech intelligibility. Among the above weighting schemes, both SIL and AI were found to have a good correlation with speech intelligibility. However, SIL uses a more limited frequ ency range. Therefore, from the results of this research, AI represents the best index for predicting the impact of wireless phone interference on speech intelligibility, irrespective of the type of IRIS phone signal. An AI value less than 0.3 was found to reduce speech intelligibility to 50% of the "No Noise" baseline conditions, and an AI greater than 0.9 was found to provide speech intelligibility no different from the "No Noise" condition.