Growing up, I always admired people with ear auricles (or more correctly, Pinnae) that are not sticking out. To hide my own insecurity, I grew my hair so long so people would not see my pinnae.

It was a revelation for me during my first anatomy class when I learned the pinna also serves a very important role in front-back localization (or the ability to tell if a sound originates from the front or from the back). Turns out, I was lucky to have big ear auricles!
The pinna helps front-back localization in the following way. When a sound is presented directly in front of a person, it is “funneled” directly into the ear-canal. However, when the same sound is presented from behind the listener, the same pinna that funnels the sound into the ear-canal now stands in the way of the sound.

This casts a shadow and reduces the level of the sound that reaches the ear-canal. That is why we hear people less well when they call us from behind than when they talk to us directly from the front.

The brain is able to take advantage of the sound pressure level difference between sounds presented from the front and the back, and inform the listener of the location (or direction) of the sounds.

The trouble with front/back localization

People with normal hearing typically have no problems with front/back localization. This is not the case for people with a hearing loss.

A recent study conducted at the hearing aid research facility ORCA-USA showed that the average front/back localization accuracy of hearing impaired listeners was only 30 percent (compared with those with normal hearing, who have 90-100 percent accuracy). What is more startling is that even when these listeners wore hearing aids, their front/back localization ability did not improve. In some situations, it became worse.

One possible reason for the loss in front/back localization with hearing loss is the loss of sound inputs to the brain. The style of hearing aids that a hearing impaired person wears could also affect the kind of sounds that goes to the brain.

Imagine the case of a Behind-the-ear (BTE) hearing aid that sits over the pinna. The microphone on the hearing aid sits on top of the hearing aid. If it is an omnidirectional microphone, it will pick up sounds from the front and from the back with equal sensitivity. In this case, the level difference between sounds from the front and the back would be reduced to zero. The cues that normal hearing people use to help them distinguish front and back is now eliminated.

This lack of cues is another reason why people wearing a BTE are more likely to have front/back confusions than normal hearing people or hearing impaired people wearing custom hearing aids.

The beauty of the digital pinna

Fortunately, there is a digital signal processing (DSP) technique that electronically compensates for the loss of front/back level cues in BTE hearing aids. This is the Digital Pinna (DP) developed by Widex. The DP is an advanced microphone system that creates a sound pressure level difference electronically by picking up less sound from the back microphone than the front microphone.

Indeed, the processing is so advanced that the level difference created by the DP is almost twice as large as the level difference created by the natural pinna (See figure 1).
Figure 1: Comparing the directivity indices (DI) of the natural ear (unaided), aided ear with an omnidirectional microphone (omnidirectional), and with the Digital Pinna (DP). The DI provided by the DP is approximately 4 dB, over twice as much as that provided by the unaided ear. (< 2 dB)

The ORCA-USA team studied the benefits provided by the DP feature. In a peered reviewed journal article, Kuk et al (2013) showed that the hearing impaired listeners’ ability to distinguish front and back improved from 30% to an average of 60% accuracy (see Figure 2). This validates the efficacy of such a feature. As a side benefit, the ability of the listeners to understand speech in noise was also improved.

Figure 2: Comparing front/back localization accuracies in hearing impaired listeners (n=18) between using an omnidirectional microphone and the Digital Pinna (From: Kuk F, Korhonen P, Lau C, Keenan D, Norgaard M. (2013) Evaluation of a pinna compensation algorithm for sound localization and speech perception in noise. Am J Audiology. 22(6):84-93)
In another study, the ORCA-USA team showed that such front/back localization ability can be further improved by training the hearing impaired listeners for 4 weeks while they wore the Widex hearing aids with the DP (Kuk F, Keenan D, Lau C, Crose B, Schumacher J. (2014)

Evaluation of a Localization Training Program for Hearing Impaired Listeners. Ear Hear 35(6):652-666.). The results of the study showed that while DP provides the localization cues, additional training on how to use the cues can further improve front/back localization ability of hearing aid wearers.

These studies showed that the Digital Pinna is a necessary feature in a BTE hearing aid to ensure proper localization cues. While this feature alone provides the front/back localization cues, the utility of such cues can be further enhanced through the use of a home training program.

Contact fkuk@widex.com to download the training program. 

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