GN Hear & Now

RESOUND EXPERT

BINAURAL DIRECTIONALITY II - 

With over 20 years in the hearing industry, I have witnessed the huge evolution of digital signal processing in hearing aids. Some modern hearing instruments can now do amazing things I would never have thought possible when I started out as an audiologist, such as categorisation of the listening environment, suppression of multiple moving noise sources through adaptive directional polar plots, removal of acoustic feedback and so on. One thing that hearing instruments still cannot do, however, is to predict the listening intent of the wearer. In other words, hearing instruments cannot predict which sounds in the environment the wearer wants to attend and listen to.

By and large, hearing instrument manufacturers design directional strategies which make assumptions about the signal of interest. This may involve focusing the directional microphone beam towards the loudest speech signal in the environment, or it may involve narrowing the focus of the directional beam to directly in front of the wearer when the hearing instruments detect speech in a particularly noisy environment.
Sometimes assumptions made by hearing instruments will align with listening intent, but in reality, what we want to listen to in any given environment is unlikely to remain static. 

Take, for example, a situation like a busy restaurant. At some points the hearing instrument wearer may be in conversation with a person directly opposite them. If a waitperson approaches the table from the side or behind, their voice will then become the signal of interest. Perhaps someone two seats down to the side is loudly relaying a particularly interesting turn of events, and the listener may want to focus on that conversation. Later that loud talker’s conversation simply becomes competing noise.

When hearing instruments employing assumptive directional strategies make the wrong assumption, they are unlikely to provide directional benefit. In fact they may do the opposite. They may create a directional deficit by focusing the directional beam away from the actual signal of interest.

In the situation described above, hearing instruments which form a narrow frontal directional beam when they detect speech in loud noise might be good for hearing the person opposite the listener, but would potentially create a ‘tunnel hearing’ effect, not conducive to hearing the waitperson or even being aware of the interesting conversation two seats down. Hearing instruments which focus on the loudest speech signal may mean the loud person two seats down is the only thing the hearing instrument wearer picks up, whether or not this is the conversation they want to hear.

Binaural Directionality II with Spatial Sense is ReSound’s premium directionality strategy. It encompasses a binaural microphone steering strategy which provides awareness of all surrounding speech and allows the wearer to choose what to focus on.

Front and rear speech detectors on each hearing instrument estimate the location of speech with respect to the listener.
The environment is also analysed for the present or absence of noise.

The two hearing instruments exchange information in order to decide whether to switch microphone mode in one or both hearing instruments. Possible outcomes are a bilateral omnidirectional response (with Spatial Sense), a bilateral directional response and an asymmetric directional response (i.e. one hearing instrument in directional mode and the other in omnidirectional mode).

Table 1 provides a justification for each of these binaural microphone configurations.

Binaural directionality
Pattern
Research Finding
Bilateral Omnidirectional
with Spatial Sense

In quiet environments, a bilateral omnidirectional response is strongly preferred by users.

 

Bilateral
Directional


A bilateral directional response provides the greatest benefit in noise when the speech signal is predominantly in front of the listener.

 

Asymmetric Omnidirectional
and Directional
A directional response for one hearing instrument and an omnidirectional response for the other hearing instrument can improve ease of listening and awareness of surroundings as compared to bilateral fixed directional fittings, without significantly degrading directional benefit.  Further, when speech is to the side of the listener in a noisy environment, the best intelligibility can be achieved if the hearing instrument on the same side as the speech is in an omnidirectional mode and the opposite hearing aid is in a directional mode.


Table 1: Research study findings on optimal binaural microphone response which form the basis of the Binaural Directionality binaural microphone steering strategy.

The advantage of Binaural Directionality II over binaural beamforming strategies is greater awareness of speech from all directions, allowing the hearing instrument wearer to detect and quickly orient themselves to the target speech more easily and quickly. Once oriented towards the target speech, there are significant signal to noise ratio improvements in both the bilateral directional mode and the asymmetric directional mode.

What is Spatial Sense?
Any experienced hearing instrument fitter will be familiar with end user reports of difficulty localising sounds. Clinicians have it drummed into us that one of the advantages of a binaural fitting is better sound localisation. That may have been the case back in the days of linear, omnidirectional amplification, but let’s think about what happens with modern amplification strategies.

The brain determines the direction of an incumbent sound by analysing interaural time differences (ITD) for low frequencies and interaural level differences (ILD) for high frequencies. Hearing aids are likely to preserve ITDs because any processing delay should be the same between the left and right hearing instruments. ILDs, however, can be grossly distorted by hearing instruments. Different polar plots in the left and right hearing instruments at any given time is likely to distort the ILD.

Wide dynamic range compression (WDRC) also interferes with ILDs. Sounds reaching the ear furthest from a sound source will be less intense than sounds reaching the ear nearest to the sound source, so WDRC will apply relatively more gain to the softer sound at the far ear. This reduces the ILD to the hearing instrument wearer so sound localisation will be less accurate.

Spatial Sense analyses the ILD at the input stage to the hearing instruments, and following compression, it applies a correction to restore the original ILD in the output signal. A second element to Spatial Sense involves restoration of spectral pinna cues which are normally lost with placement of the microphones above the pinna in BTE and RIE hearing instruments. Spatial Sense is active when Binaural Directionality II places the hearing instruments in a bilateral omnidirectional mode, which is about 78% of the time for the average hearing instrument wearer.

In laboratory trials, hearing impaired listeners using Spatial Sense achieved significantly better localisation for sounds coming from multiple angles, including improved front/back localisation, when compared with standard omnidirectional processing. Click here to see how Spatial Sense has benefitted blind triathlete Jonathan Goerlach.

To clarify directional options in Aventa for ReSound’s 9 level technology:

(1) ‘Binaural Directionality II’ incorporates Spatial Sense when it places both aids in an omnidirectional mode.

(2) ‘Spatial Sense’ fixes both aids in an omnidirectional mode incorporating Spatial Sense


 


ReSound Expert, Jane Fitzgibbons 

Area Manager (QLD/NT/nNSW)