Active Noise and Occlusion Effect Cancellation in Headphones and Hearing Aids

The perception of one’s own voice is distorted when telephoning with headsets, or
wearing hearing aids. The reason for this is the so-called occlusion effect, which
occurs when ear canals are completely or partially closed by the headset or hearing
aid. The occlusion causes amplification at low frequencies, and attenuation at
high frequencies of one’s own voice. The unnatural perception of one’s own voice
and of noise caused by chewing and swallowing are among the most common
complaints of users. Furthermore, environmental noise might impair perception. In
this thesis, both the unnatural perception of one’s own voice and the disturbance
by environmental noise are tackled by a novel signal processing approach.
The proposed solution solves the problem of the occlusion effect by actively
emitting a compensation signal through the integrated loudspeaker. The approach
is called Occlusion Effect Cancellation (OEC) and significantly improves the perception
of one’s own voice and of the acoustic environment. This novel approach
combines methods of active noise cancellation (ANC, Noise Cancelling Headphone)
with a personalized design. The bilateral headset contains two additional microphones
per side, one inner and one outer, to acquire signals for the calculation of
the compensation signals. A correctly balanced processing of the two microphone
signals results in a „digital ear opening“ and a much more natural perception of
both one’s own voice and of the environment. The extent of the digital ear opening
is controllable. The system can also be operated as a noise cancelling headphone
by changing the parameters to a conventional design to create an acoustic isolation
from the environment.
This thesis proposes a novel robust approach based on digital filtering to solve
the described problems. A combination of feedback and feedforward filter design
allows for either approaching personal silence or a natural perception of one’s own
voice and the acoustic environment.
The main contributions are:
• Novel design concept for ANC / OEC systems which are robust w. r. t. acoustical
variations of ear canals and earpiece fittings
• Novel structure for combined feedforward-feedback filters with adaptive
stability control
• Analysis of variability of acoustic front-end (headset) as well as electronic
back-end (digital signal processing incl. AD/DA-conversion) and implication
on ANC/OEC performance
• Real-time implementation of algorithms on a PC-sized dSPACE system as
well as a mobile integrated circuit (Analog Devices ADAU 1777)
• Objective instrumental and subjective auditive evaluation of the concepts
The achieved ANC performance is comparable to that of a commercial reference
system. The novel OEC algorithm revealed in both objective measurements and
subjective listening tests, significant improvements of the own-voice perception.
To conclude, this thesis provides an in-depth discussion of the underlying
problems of active noise and occlusion effect cancellation, design methods for digital
control filters, an analysis of the implementation requirements for a real-time system
as well as an evaluation based on both measurements and listening tests.
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Abstract
on ANC/OEC performance
• Real-time implementation of algorithms on a PC-sized dSPACE system as
well as a mobile integrated circuit (Analog Devices ADAU 1777)
• Objective instrumental and subjective auditive evaluation of the concepts
The achieved ANC performance is comparable to that of a commercial reference
system. The novel OEC algorithm revealed in both objective measurements and
subjective listening tests, significant improvements of the own-voice perception.
To conclude, this thesis provides an in-depth discussion of the underlying
problems of active noise and occlusion effect cancellation, design methods for digital
control filters, an analysis of the implementation requirements for a real-time system
as well as an evaluation based on both measurements and listening tests.