Directional microphone technology for hearing aids was introduced in the late 1960s in Germany, a few years later in the United States, and for decades has been considered the best solution using an ear-worn instrument to improve speech understanding in background noise. 

For some patients, however, the use of directional technology has presented a style selection dilemma. If small and discreet instruments (eg, ITCs or CICs) are a high priority, and this form factor is preferred by the patient, until recently, effective directional processing was not possible. The patient, or the hearing care professional, would then have to decide what is most important—least visible hearing aids or better speech understanding in background noise? In the past year, however, through the use of new beamforming technology, a solution to this long-standing quandary has become available.

While originally in BTE products, directional microphones have been used in custom instruments (full-concha ITEs) since the mid-1970s. In later years, when dual microphone processing became the norm for directional technology, researchers compared the directional benefit for ITEs versus BTEs. Their findings revealed that the benefit obtained with directional ITE instruments was similar to that of their BTE counterparts, and the absolute performance (speech recognition in background noise) was essentially the same.

Directional technology is most effective in custom instruments when the faceplate is flush with the exterior rim of the concha—in other words, a large full-concha ITE. This style typically is not very cosmetically appealing, and goes against the very reason why most patients want custom instruments. 

In the past, there have been several issues preventing the successful implementation of directional technology in the smaller ITC and CIC styles:

• There must be room within the instrument itself for the microphones and circuitry required for the directional processing.
• There must be effective port spacing to achieve the desired directivity. Consider that, in a CIC instrument, the spacing can be no greater than the diameter of the ear canal (about 7 mm), with no more than 4-5 mm to work with after the dimensions of the exterior shell are subtracted.
• The third issue to consider when designing directional processing in small custom instruments is that the deeper the faceplate lies within the concha, the more the attenuation and reflections of sound caused by the pinna disrupt the timing and amplitude differences of the sound entering the two ports—sounds from the front could strike the back port first. It is the precise analysis of the timing between ports that makes directional hearing aids directional.
• Given all the factors working against effective directionality in small custom instruments, it’s probably not surprising that these products have not been available. This is changing, however, thanks to recent advancements in signal processing.  The most recent development regarding directional hearing aid processing involves wireless communication between the hearing aids, which allows for the exchange of audio data received by the microphones of both the right and left instruments, which then can be used to achieve narrow beamforming. This new beamforming technology from Siemens, termed “Narrow Directionality”, was described in detail by Kamkar-Parsi et al, and clinical research with this technology has revealed significant benefit for speech recognition in background noise, even when compared to individuals with normal hearing (for details, see Froehlich et al and Powers and Froehlich).

Binaural Beamforming in ITCs and CICs

Following the success of the initial research, the next logical step was to take the clinically proven binaural beamforming approach to smaller custom instruments, where until recently, effective directionality was not available. With ITC products, it is still possible to use two inlet ports, and therefore, the beamforming algorithm operates much like it does with the BTE and larger ITE models. However, with the CIC instrument, where space allows for only one inlet port, the directionality task was more challenging.

The solution was binaural OneMic directionality (see Herbig and Froehlich for review). With this approach, each hearing device in a bilateral pair operates with the exchange of audio signals detected and processed by both hearing aids. It is possible to then design a binaural adaptive beamforming algorithm that incorporates head-shadowing effects by carefully weighting and combining both available microphone signals, and by imposing an appropriate optimization criterion for the adaptive weighting rule. This results in an enhanced output signal, where interfering lateral noise sources can be efficiently attenuated while the frontal desired speaker signal remains untouched.

Measures of sAI-DI

Previous papers have explained how the effectiveness of this beamforming technology can be evaluated, and product comparisons can be made through the measure of the sequential articulation-index-weighted directivity index, or sAI-DI. These sAI-DI measures are based on the previous work of Hagerman and Olufsson, and extends the approach suggested by Wu and Bentler. It is described in detail in a recent publication of Aubreville and Petrausch.

Using the sAI-DI laboratory approach, it was possible to compare the newly designed ITC and CIC beamforming instruments to the previously researched mini-BTE/RIC models. These findings revealed an sAI-DI of 10.5 dB for the ITC, slightly higher than the 9.2 dB for the RIC instruments. As expected, because of factors discussed earlier, the sAI-DI for the CIC OneMic design was somewhat smaller at 5.1 dB, but still considerably superior to the sAI-DI of -0.5 dB for the traditional CIC omnidirectional.

While the laboratory sAI-DI measures were encouraging and provide an excellent method to make comparisons among products, there is only an indirect relationship between these values and SNR improvement for speech recognition in background noise. It was therefore also necessary to conduct clinical behavioral measures to assess the efficacy of these instruments. This testing was conducted at two different independent sites.

Many patients prefer to use custom instruments rather than BTEs. In the past, one downside of the small custom products was that the patient would then be deprived of advanced directional beamforming available in larger hearing aid styles. As we have shown here, that is no longer a concern. Clinical speech testing at two different independent sites reveals that a significant improvement in speech recognition is available for advanced directional processing for both the ITC and CIC styles. For both of these styles, the binaural beamformer is automatic and adaptive, only activating when the listening situation indicates the need. In other situations, the omnidirectional processing will be present to maintain spatial awareness.

Based on the Siemens e2e wireless 3.0 system, the power consumption is so minimal that these benefits can be provided in the standard universal program. In this way, the overall practical benefit for the user is maximized.