Signal Processing and Acoustics Equalization Techniques for Headphone Listening

Aalto University, P.O. Box 11000, FI-00076 Aalto www.aalto. fi Author Jussi Rämö Name of the doctoral dissertation Equalization Techniques for Headphone Listening Publisher School of Electrical Engineering Unit Department of Signal Processing and Acoustics Series Aalto University publication series DOCTORAL DISSERTATIONS 147/2014 Field of research Acoustics and Audio Signal Processing Manuscript submitted 13 February 2014 Date of the defence 31 October 2014 Permission to publish granted (date) 6 August 2014 Language English Monograph Article dissertation (summary + original articles) Abstract The popularity of headphones has increased rapidly along with digital music and mobile phones. The environment in which headphones are used has also changed quite dramatically from silent to noisy, since people are increasingly using their headphones while commuting and traveling. Ambient noise affects the quality of the perceived music as well as compels people to listen to the music with higher volume levels. This dissertation explores headphone listening in the presence of ambient sounds. The ambient sounds can be either noise or informative sounds, such as speech. The first portion of this work addresses the first case, where the ambient sounds are undesirable noise that deteriorates the headphone listening experience. The second portion tackles the latter case, in which the ambient sounds are actually informative sounds that the user wants to hear while wearing headphones, such as in an augmented reality system. Regardless of the nature of the ambient sounds, the listening experience can be enhanced with the help of equalization. This work presents a virtual listening test environment for evaluating headphones in the presence of ambient noise. The simulation of headphones is implemented using digital filters, which enables arbitrary music and noise test signals in the listening test. The disturbing effect of ambient noise is examined with the help of a simulator utilizing an auditory masking model to simulate the timbre changes in music. Another study utilizes the same principles and introduces an adaptive equalizer for mitigation of the masking phenomenon. This psychoacoustic audio processing system was shown to retain reasonably low sound pressure levels while boosting the music, which is highly important from the viewpoint of hearing protection. Furthermore, two novel hear-through systems are proposed, the first of which is a digital augmented reality headset substituting and improving a previous analog system. The second system is intended to be worn during loud concerts as a user-controllable hearing protector and mixer. The main problem in both of the systems is the risk of a comb filtering effect, which can deteriorate the sound quality. However, it is shown that the comb filtering effect is not detrimental due to the passive isolation of an in-ear headset. Finally, an optimization algorithm for high-order graphic equalizer is developed, which optimizes the order of adjacent band filters to reduce ripple around the filter transition bands. Furthermore, a novel high-precision graphic equalizer is introduced based on parallel secondorder sections. The novel equalization techniques are intended for use not only in headphone applications, but also in wide range of other audio signal processing applications, which require highly selective equalizers.