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IEEE SIGNAL PROCESSING MAGAZINE [82] MARCH 2015

concept of a personal sound zone, i.e., reproducing sound

within a desired region of space with a reduced sound level else-

where. Microsoft researchers later demonstrated their “Personal

Audio Space” project at Microsoft Research TechFest 2007,

where a linear loudspeaker array consisting of 16 drivers was

used to enhance the sound in one area while canceling sound

waves in another area within the same physical space. By step-

ping even a few paces outside the target region, users reported

that they could not hear the reproduced music. Researchers fur-

ther extended this concept to develop personal audio for per-

sonal computers and televisions [6], as well as for mobile

devices [7] and automobile cabins [8].

A way to create personal sound zones is to formulate a

multizone sound control problem within the same physical

space as illustrated in Figure 1. Here, multiple microphones

and loudspeakers are used to control the reproduced sound

fields. A preference is to use a single array of loudspeakers

rather than separate arrays for each zone. This improves free-

dom and flexibility, allowing sound zones to be positioned

dynamically and listeners to freely move between zones. When

the system is implemented in reverberant enclosures, loud-

speaker designs and audio processing are two key aspects to

control sound radiation and to deal with the complexity and

uncertainty associated with sound field reproduction. This

article aims at reviewing these techniques to support the goal

of establishing personal sound zones.

MULTIZONE SOUND CONTROL

In a general formulation, sound fields are produced over

Q sound

zones. Here M pressure controlling microphones are placed

within each zone so that the zone sound fields are controlled by a

total of

QM matching points. The sound pressures measured at

the microphone positions in each zone q are represented as a vec-

tor ()[( ), , ]xpx pp

,,,, q Mqq

f ~~= and given by

,pHg

= (1)

where [( , ), , ( , )]gy ygg

f~~= denotes the vector of loud-

speaker driving signals at a given frequency

~ to create personal

audio sound scenes and H

represents a matrix of acoustic transfer

functions (or acoustic impedances) between the loudspeaker driv-

ers and the microphones in zone

.q Sound control techniques can

broadly be classified into two categories, acoustic contrast control

(ACC) and pressure matching (PM), and we consider each in turn.

ACOUSTIC CONTRAST CONTROL

Choi and Kim [9] first formulated the personal audio problem by

creating two kinds of sound zones: the bright zone within which

we want to reproduce certain sounds with high acoustic energy,

and the dark zone (or the quiet zone) within which the acoustic

energy is kept at a low level. The principle of ACC is to maximize

the contrast in the acoustic energy between the bright zone and

the dark zone. Among the

Q sound zones, we specify the first

zone as the bright zone and the remaining Q 1- zones as the

dark zones. The acoustic energy in the bright zone is defined from

the sound pressures measured at the

M matching points, that is

pHgE

bb b

== with HH

= and · denoting the

norm. Similarly, the acoustic energy in the dark zones is repre-

sented as

pHgE

dd d

== with [ , , ]HH H

f= and

()

$ represents the Hermitian transpose.

In [9], the acoustic contrast, defined as a ratio between the

average acoustic potential energy density produced in the bright

zone to that in the dark zones, is maximized. The acoustic con-

trast maximizing method may perform well over the dark zones

but may be unrobust to providing the desired maximum energy

in the bright zone. To ensure the sound energy within different

zones are optimized simultaneously, the problem can be refor-

mulated as maximizing the acoustic energy in the bright zone

with the constraint that the energy in the dark zone is limited to

a very small value

In addition, a limit is imposed on the

loudspeaker power consumption, i.e., ,g E

# also known as

the array effort. These constraints ensure that sound leakage out-

side the

Q zones is not excessive and also that realized

g(y

) g(y

)

g(y

)

g(y

)

Zone 1

Zone q

Zone Q

(a) (b)

[FIG1]

(a) An illustration of personal sound zones in an office environment. (b) A loudspeaker array is used to create multiple sound

zones for multiple listeners.

THE WORLD’S NEWSSTAND

THE WORLD’S NEWSSTAND