Zoom out Search Issue

ManualsBrandsContents Manualsaudio & home theatreZoom in

100

IEEE SIGNAL PROCESSING MAGAZINE [93] MARCH 2015

Figure 2 shows measured isolation

curves of different types of headphones,

where the black solid line is the isolation

curve of an open-back circum-aural (CA)

hi-fi headphone, the green dashed-dotted

line is the isolation of a closed-back

supra-aural (SA) headphone, the red

dashed line shows the passive isolation

curve of an in-ear (IE) headphone, and

the purple dashed line shows the active

isolation of the same IE headphone, i.e.,

when the ANC is turned on.

ANC actively reduces the ambient noise

leaked into the ear canal by introducing an

antinoise signal. Ideally, the antinoise sig-

nal has the same magnitude and opposite

phase as the noise signal inside the ear

canal. The two basic operation principles

of ANC are feedforward and feedback con-

trol [2]. The main difference between these two types is the posi-

tion where the ambient noise is captured. Feedforward ANC

typically uses two microphones: an external microphone captur-

ing ambient noise, and an error microphone inside the headset,

which is used to adaptively tune the ANC filtering. A feedback

ANC uses only the internal error

microphone both for capturing

ambient sounds and for adapting the

ANC system. Moreover, the feedback

and feedforward structures can also

be used simultaneously.

At its most basic, the frequency

response equalization can be a bass

boost to compensate for the auditory

masking occurring at low frequen-

cies. However, more intelligent systems also exist, e.g., for speech

in mobile communication applications in noise [3], for car audio

[5], and for headphone listening in noisy environments [4]. The

goal in [3] is to maximize the speech intelligibility index by

enhancing the clean far-end speech signal for the near-end listener

who is situated in a noisy environment, hence the abbreviation

NELE, which stands for “near-end listening enhancement.” NELE

techniques typically exploit psychoacoustically justifiable spectral

resolution, such as Bark or equivalent rectangular bandwidth

(ERB) scale, and an auditory masking model.

A similar idea is suggested in [4], where a clean wideband

music signal is enhanced based on the content of the music, the

characteristics of the headphones, and the ambient noise around

the user. This system uses a microphone outside the headset to

tics of the headphones, which are measured beforehand, are used

to estimate the level of the music and noise at the eardrum. These

levels depend on the frequency and isolation responses of the

headphones, respectively. The music and ambient noise are ana-

lyzed in Bark bands. The goal is to estimate the masking thresh-

old from the captured ambient noise signal, and then to enhance

only those frequency bands of the music signal which are below

the masking threshold (completely masked) or just above it (par-

tially masked), as shown in Figure 4. Using the unmasking pro-

cess, the timbral balance of the music is enhanced in the

presence of ambient noise and at the same time the volume

increase is minimized, since only

those bands that actually need

amplification are boosted [4].

VIRTUAL REALITY LISTENING

Virtual auditory content can be dis-

played spatially, e.g., for guidance or

navigation, by processing the sound

so that it mimics the experience of

listening to a real, physical sound

source. Rendering a virtual sound source in such a way is referred

to as binaural synthesis [6].

Binaural synthesis works by providing localization cues to the

listener’s ears [7], [8]. The delay and attenuation due to sound

propagation, resulting in time and level differences between the

ear signals, are the two main cues. They are termed the interau-

ral time difference (ITD) and interaural level difference (ILD),

respectively, and depend on the relative direction and, for nearby

+Head Tracking

+Binaural Synthesis

Headphone

Listening

+Mic

+ANC

+NELE

+Mic

+Mixing

Augmented

Reality

Audio

+Acoustic Signal

Processing

Modified

Reality

Audio

Virtual

Reality

Audio

Listening

in Noisy

Environments

[FIG1]

The main use cases of a headset are natural listening, listening in noise, virtual

reality, augmented reality, and modified reality.

100 1 k 10 k

Frequency (Hz)

Attenuation (dB)

Open-Back CA

Closed-Back SA

IE with ANC

[FIG2] Measured isolation curves of different headphones.

IN THE NEAR FUTURE,

A HEADSET WILL BE A “HEARING

AID FOR THOSE WITH NORMAL

HEARING,” WHICH CAN IMPROVE

LISTENING CONDITIONS

FOR EXAMPLE IN A

NOISY ENVIRONMENT.

THE WORLD’S NEWSSTAND

THE WORLD’S NEWSSTAND