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IEEE SIGNAL PROCESSING MAGAZINE [98] MARCH 2015
short-distance navigation and object
avoidance in which spatialized sound
can play a crucial role to help people
advance safely.
A typical use case of audio-aug-
mented reality is a museum guide
[33], [34]. With the augmented real-
ity techniques it is possible to have
each piece at an exhibition to act as a virtual sound source such
that a visitor can hear the attraction introduce itself. Another
interesting application domain for augmented reality is gaming.
An example of an audio-only augmented reality game is Guided by
Voices, where an overlay of virtual objects and game characters
onto the real world is created entirely using sound [35].
USABILITY ISSUES
Although the presented techniques have many attractive appli-
cations, they are not completely free of usability problems. The
observed sound quality and naturalness are on a very high
level, but user comfort should be improved [36]. Tikander has
shown that the user’s own sounds are challenging, too, since
current audio-augmented reality systems aim at natural hear-
through of ambient sounds [36]. For example, when the user
reads aloud or eats crispbread, the technology alters the experi-
ence such that it may cause annoyance. This is caused by the
blocking of the ear canals—the occlusion effect—and the
inability of the techniques to alter bone-conducted sounds,
such as the user’s own voice.
Another related question is social acceptability. When one is
wearing headphones, others often assume that she/he is not listen-
ing, although with augmented reality headphones the case might
be the opposite, and one’s listening can actually be more intense
than without the headphones.
CONCLUSIONS
This article has reviewed signal processing methods and applica-
tions related to assisted listening. Headphones are commonly used
with a mobile phone or another portable device, and the ambient
noise disturbs listening by masking some of the audio content.
Active noise control helps to improve the attenuation of noises
while NELE methods improve the audibility and intelligibility by
modifying the audio signal itself. An unmasking method devel-
oped for headphone listening was described, which estimates the
levels of music and noise in the ear by accounting for the attenua-
tion characteristics of the headphone. It then computes a masking
threshold from the noise signal and compares the spectrum of the
music signal against it. An adaptive equalizing filter is then
adjusted to boost those frequencies in the music, which would
otherwise be completely or partially masked by the noise.
Virtual reality audio is an extension of regular headphone lis-
tening in which the user can hear transmitted or recorded sounds
seemingly from his environment. This is achieved by using head-
tracking and binaural synthesis techniques, which help to keep the
virtual sources at their prescribed locations even when the user is
moving. Interpolation techniques and a complete signal processing
system to implement time-varying
binaural synthesis were discussed.
Audio-augmented reality mixes
real and reproduced sounds, requir-
ing external microphones and a
hear-through function to cancel the
attenuation caused by the headset.
A new method for designing a filter
to achieve colorless, or allpass-type, hear-through system
was described.
While there are many audio applications for virtual and
augmented reality, such as navigation tools and museum
guides, several relevant applications belong to the category of
modified reality. These systems reproduce through the headset
a processed version of the ambient sound field, such as in
enhanced concert applications or in the LiveEQ system, which
provides a real-time equalizer to concert audiences. The
increase in computational power of mobile devices will enable
even more advanced new devices and applications, such as
adaptive intelligent headphones that will observe the environ-
ment continuously and modify the audio mix and content to
deliver the most relevant information to the user according to
her/his personal preferences.
AUTHORS
Vesa Välimäki ([e-mail]) received the M.Sc. and
the doctor of science in technology degrees from the Helsinki
University of Technology, Espoo, Finland, in 1992 and 1995,
respectively. He is a professor of audio signal processing in the
Department of Signal Processing and Acoustics, Aalto Universi-
ty, Espoo, Finland. In 2008–2009, he was a visiting scholar at
Stanford University. His research interests include headphone
audio, digital filters, audio effects processing, sound synthesis,
and acoustics of musical instruments. He is an associate mem-
ber of the IEEE Audio and Acoustic Signal Processing Techni-
cal Committee. He is a Fellow of the IEEE and of the Audio
Engineering Society.
Andreas Franck ([e-mail]) received the diplo-
ma degree in computer science and the Ph.D. degree in electri-
cal engineering, both from the Ilmenau University of
Technology, Germany. Since 2004, he has been with the
Fraunhofer Institute for Digital Media Technology (IDMT),
Ilmenau, Germany. In 2014, he joined the Institute of Sound
and Vibration Research, University of Southampton, United
Kingdom, as a postdoctoral research fellow. The work for this
article was performed while he was with Fraunhofer IDMT. His
research interests include spatial and object-based audio, in
particular efficient sound reproduction algorithms, variable
digital filters, and fast convolution techniques.
Jussi Rämö ([e-mail]) received the M.Sc. degree
in communication engineering in 2009 from the Helsinki
University of Technology, Finland, and the doctor of science in
technology degree from Aalto University, Finland, in 2014. His
major in both degrees was acoustics and audio signal process-
ing. Since 2009, he has worked as a researcher in the
THE INCREASE IN
COMPUTATIONAL POWER
OF MOBILE DEVICES WILL ENABLE
EVEN MORE ADVANCED NEW
DEVICES AND APPLICATIONS.
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