The ear has some natural directional characteristics, mainly due to the presence and shape of the pinna (outer ear). These pinna effects are important for the ability to decide if a sound is coming from in front or behind, especially for frequencies around 2-8 kHz.
One example is the pinna shadow effect. Sounds coming from the front reach the ear canal almost directly, whereas sounds coming from behind are obstructed by the pinna and therefore attenuated before they reach the ear canal.
This pinna shadowing is most pronounced for frequencies around 2 to 5 kHz, where sounds coming from behind are attenuated by 3-4 dB relative to sounds coming from the front. This natural 3-4 dB sound level difference is important for the listener to know whether a sound source is in front or behind.
Microphone location can have a negative effect on hearing aid users’ ability to determine where the sound is coming from. Specifically, behind-the-ear (BTE) hearing aids, where the microphones are located behind the pinna, will not preserve the pinna shadow effect, and the hearing aid user therefore loses some of the ability to distinguish front and back.
Digital Pinna simulates the natural pinna shadow effect
In BTEs with Digital Pinna, the natural pinna shadow effect is simulated by means of a restriction on the adaptive locator.
More specifically, we can re-create the natural attenuation of sounds coming from behind by setting the frequency bands from 2 kHz and up (bands 10-15) in fixed directional mode (hypercardioid, which picks up sound at the front and eliminates most sound from the sides and rear), while leaving the lower bands (1-9) in omni-directional mode.
The microphone system in hearing aids with Digital Pinna remains an adaptive system when Digital Pinna has been activated. In quiet listening environments, the lower bands (1-9) will be in omni-directional mode, whereas the upper bands (10-15) will be in hypercardioid as described above.
In noisy listening conditions, the microphone mode of the lower bands can adapt any characteristic from omni-directional to bipolar, while the upper bands may move from hypercardioid towards bipolar to provide the highest possible speech intelligibility.