Table 1.

The Characteristics of Adaptive Directional Microphones Implemented in Selected Commercially Available Hearing Aids^*

	Oticon-Syncro	Phonak-Perseo	ReSound-Canta	Siemens-Triano	Widex-Diva
Signal Detection and Analysis	Global modulation detector to determine the continuous signal to noise ratio. Parallel processing to calculate the resultant signal-to-noise level in each possible microphone mode and polar configuration Level detector to determine the overall sound pressure level of the incoming signal Front-back ratio detector to estimate the location of the dominant input signal Wind noise detector to estimate the level of wind noise	Level detector to determine the overall sound pressure level of the incoming signal Front-back ratio detector to estimate the location of the dominant input signal Analysis of the amplitude modulations, temporal fluctuation, and spectral center of gravity of the incoming signal to infer the presence of speech and noise	Front-back ratio detector to estimate the location of the dominant sound source	Level detector to determine the overall sound pressure level of the incoming signal Wind noise detector to detect the presence and the level of wind noise	Front-back ratio detector to estimate the location of the dominant input signal Noise classification to determine noise type: wind, circuit or environmental Level detector to determine the overall sound pressure level of the incoming signal
Decision Rules for Determining the Microphone Mode	Surround Mode: If the omni-directional mode in all 4 bands provides the highest SNR If the incoming signal is at soft to moderate levels with no or low background noise If the dominant speaker is from the back If strong wind is detected Split-Directionality Mode: If the omni-directional mode in the lowest frequency band and directional mode at the upper 3 bands provide the highest SNR If the incoming signal is at a moderate level with some background noise If a moderate level of wind noise is detected Full-Directionality Mode: If the directional mode in all 4 bands gives the highest SNR If the incoming signal is high with background noise If no wind noise is detected	Omni Mode: speech only Directional Mode: The decision rules for switching to directional microphones can be adjusted by the clinician on the basis of user priority for speech audibility or comfort: Audibility: the algorithm switches to directional microphone mode when speech in noise is detected, but not when speech alone or noise alone is detected Comfort: the algorithm switches to directional microphone mode whenever there is noise, regardless of whether speech is present or not. If only speech is present, it is not activated	User determined	Omni Mode: Analysis indicates that primary signal is speech or wind noise The intensity of incoming signal is below a predetermined level (e.g., 60 dB SPL). The trigger level varies depending on the hearing aid model. Directional Mode: Minimal wind noise is detected Level of the incoming signal exceeds the predetermined level	Omni Mode: If incoming signal is from the front only If environmental noise is insignificant If wind noise is the dominating noise source Directional Mode: Minimal wind noise is detected Level of the incoming signal exceeds the predetermined level Background noise is detected
Adaptation Speed for Omni-Directional and Directional Switch	2–4 sec, depending on the hearing aid's Identity setting, i.e., the life style of the hearing aid user in the fitting software	Variable/programmable by clinician, from 4–10 sec, based on “Audibility” or “Comfort” selections in the hearing aid fitting software	Not applicable because switch is user determined	6–12 sec, depending on the settings of the listening program	5–10 sec, depending on the settings of the listening program
Decision Rules for Determining the Polar Patterns	The analyzer unit calculates the signal-to-noise level at each azimuth from 90° to 270° for each polar pattern across the four bands. The internal delays of the polar patterns that generates the best SNRs are adopted	The internal delay that yield the minimum power output from the directional microphone is adopted	The internal delay that yield minimum output from the directional microphone is adopted	The weighted sum of a bidirectional and cardioid pattern is calculated and the internal delay that yields the minimum output (weighted sum) from the directional microphone is adopted	The analyzer unit receives the sounds from the front and back omnidirectional microphones and adopts the internal delay that would give the lowest output at the directional microphone output
All hearing aids: Any polar pattern with nulls between 90° to 270° is possible
Adaptation Speech Between Different Polar Patterns	2 sec/90°, speed may vary depending on the hearing aid's Identity setting	100 ms between polar patterns	Analysis of environment every 4 ms, changing of polar pattern every 10 ms	50 ms/90°	Typically less than 5 sec
Polar Pattern when Multiple Noise Sources Exist	Cardioid in uniform noise field Multiband directionality enables different polar patterns to reduce the level of the multiple noise sources if they have different frequency contents	Cardiod	Hypercardioid	Hypercardiod	Hypercardioid
Low Frequency Equalization	Automatic	Programmable in fitting software via “Contrast” feature	Programmable in fitting software	Automatic	Automatic for each polar patterns
Information Source(s)	Oticon, 2004; Flynn, 2004, personal communication	www.Phonak.com (a); Ricketts and Henry (2002); Fabry (2004), personal communication	Groth (2004), personal communication	Powers (2004), personal communication. Powers & Hamacher (2004)	Kuk et al., 2002a; Kuk, 2004, personal communication
Clinical Verification	Flynn (2004): compared to the first-order fixed directional microphone implemented in Adapto, Syncro's Full-Directionality mode combined with its noise reduction algorithm yielded about 1–2 dB better SNR-50s for hearing aid users with multiple broadband noise sources in the back hemisphere. It is unclear how much of the improvement is solely generated by the adaptive directional microphone	Unavailable. See text for the evaluation of the first-order adaptive directional microphone implemented in Phonak Claro	Unavailable	Bentler et al. (2004a): the hybrid second-order adaptive directional microphone has improved the SNR-50s of hearing aid users for 4 dB. No significant difference in SNR-50s between the first-order and the hybrid second-order adaptive directional microphones. Ricketts et al. (2003): Significant benefit was observed using the second-order directional microphone compared to its fixed directionality mode in moving noise	Valente and Mispagel (2004): Compared to the omni-directional mode, the adaptive directional microphone improved SNR-50s for 7.2 dB if a single noise source was located at 180°. The improvement in SNR-50s decreased to 5.1 dB and 4.5 dB when noise was presented at 90° + 270°, and 90°+180°+270°, respectively

^*These hearing aids are selected to demonstrate the range and the differences in implementation methods of adaptive directional microphone algorithms in commercially available hearing aids. SNR = signal-to-noise ratio.