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IEEE SIGNAL PROCESSING MAGAZINE [97] MARCH 2015

the given sequence while the external microphone signal is fil-

tered with an FIR filter having the allpass tail impulse response.

ASSISTED LISTENING APPLICATIONS

We limit the scope of assisted listening to such applications that

help listening in a noisy environment or that employ augmented

and modified reality technologies. Nevertheless, by far the most

common application of assisted listening technology is the hear-

ing aid. In this context, a hearing aid can be interpreted as a modi-

fied reality system that enhances ambient sounds mainly by

amplifying them in a desired manner. However, the focus of this

article is on other modern assisted listening applications, which

are aimed at people with normal or nearly normal hearing.

LISTENING TO MUSIC

In a noisy environment, everyone suffers from hearing problems

caused by the auditory masking effect. A typical situation is listen-

ing to music or a movie soundtrack in a vehicle, such as in a bus

or in an airplane. ANC and NELE methods help to cope with such

situations, as discussed previously.

There are additionally other situations where music listening

can be enjoyed better by using augmented reality technology.

Examples of systems, which are easy to implement, include the

silent disco and the silent concert. Both applications require the

use of headphones. In a silent disco, music played by the DJ is

delivered to the audience via a wireless network or FM radio.

Everyone can then decide whether to listen to the music or not

and can also adjust the volume to her/his liking. The actual sonic

environment in the disco is therefore fairly quiet, with noises

mainly coming from conversations and dancing. The silent con-

cert is a similar concept but with the important difference that

singers and acoustic musical instruments will be heard also with-

out headphones. However, a proper mix of the music can only be

enjoyed through the headphones.

In music festivals, the listening position of most people is far

from optimal, and assisted listening technology can enhance the

experience. Larsen et al. have built and tested a system in which

the music from the stage is transmitted via FM radio to mobile

devices of the listeners to be played through headphones [29]. The

usability of such a system is critical with respect to the delay such

that the actual audio from the public address system is heard

simultaneously with the content transmitted via radio. Thus, a

localization method and an intelligent delay control, based on the

distance from the stage, are required. Overall, the opinions of test

users were positive.

The LiveEQ application described by Rämö et al. captures

ambient sounds around the user, provides a user-controllable

graphic equalization, and plays back the equalized ambient signal

to the user with headphones [30]. This modified reality hear-

through system can be used for example in a loud concert to limit

the noise exposure caused by the live music. If the headset attenu-

ates the ambient sound well, the user can mix her/his own version

of the music by boosting selected frequency ranges with the real-

time equalizer. Still, the sound pressure level of the mixed music

can be lower than that of the original live music.

AUDIO-AUGMENTED REALITY

In audio-augmented reality, the real soundscape and virtual audi-

tory events mix seamlessly together [31]. In such applications

sound is typically used to deliver information that assists the user

in performing certain tasks, or to enhance the perception of the

environment. A major advantage of auditory over graphical display

is that the user can perceive acoustic information from any direc-

tion without being required to turn toward the acoustic source.

Therefore, an important application area of audio-augmented real-

ity are scenarios where the user cannot or should not look at a dis-

play to obtain information, due to the user’s vision being either

impaired or occupied with a primary task, e.g, while walking or

driving. Similarly, audio-augmented reality may convey informa-

tion about the immediate surroundings that lie outside the user’s

field of view, for instance, the approaching of a quiet electric car.

Most often, the application scenarios are mobile such that the

user can freely move around and the augmented audio content is

determined based on the user’s location. One typical application is

navigation in which the user always gets accurate spatialized

instructions on how to proceed to achieve the target location. This

same concept is valid in a wide range of use scenarios covering,

e.g., walking in a city, driving a car, or taxiing an airplane at an air-

port. However, the group of people that benefits most from audio

in navigation are the visually impaired [32], especially in

0 20 40 60 80 100

–0.1

Sample

Amplitude

Given

Sequence

All-Pass

Tail

Impulse Response

100 1 k 10 k

–60

–40

–20

Frequency (Hz)

(a)

(b)

Magnitude (dB)

Magnitude Response

Total Allpass

Given Sequence

All-Pass Tail

–0.05

[FIG6]

An example all-pass filter design for acoustic transparency [28]: (a) The engineered impulse response contains the leaked sound

in the beginning and a designed all-pass tail, and (b) their combination has a flat magnitude response.

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