Introduction to Acoustics

cross the head, in opposite directions, and the left ear
will hear the right speaker arrival, and vice versa, a process
sometimes referred to as loudspeaker crosstalk. The
time delay between arrivals is typically on the order of
250 µs, resulting in a small, subtle, but audible notch
in the resulting response around 2 kHz. Some temporal
smearing of sharp transients may also be evident.
A final difficulty with virtual imaging of stereo systems
is the relative inability to position virtual images
outside the range of the speakers. This is another consequence
of loudspeaker crosstalk, limiting the maximum
interaural time difference to that corresponding to the
position of the speaker, which again is about 250 µs,
and similarly limiting the maximum interaural amplitude
difference. One way around this would be to put
the speakers in different rooms and sit between them,
one ear in each room, with a custom-fitted baffle doorway
to block any crosstalk, which is a little impractical.
Another approach is to employ a crosstalk cancellation
(virtual-speaker) system, to cancel the second arrival at
each ear, but such systems are themselves very sweetspot
dependent, and even then usually limited to creating
images on the horizontal plane. The most direct way to
provide sound from additional directions is to position
additional speakers around the listener, including above
and possibly below, assuming additional source channels
are available to feed them, but that is no longer
a stereo sound system.
18.6.3 Binaural
Like stereophonic sound, binaural makes use of just
two channels, but it uses headphones instead of loudspeakers
to avoid the crosstalk problem. On paper, this
system seems like it should be even more perfect than
stereo. However, the effect is often to place the sound
images within the head, with little or no front/back
differentiation.
Part of the problem appears to be that the presentation
does not get altered when the listener’s head moves,
so that the image follows the head movement, which is
rather unnatural. There has been some work exploring
the use of tracking headphones, which alter the signal
in response to head motion, but these have not yet
been completely successful, and require more than two
channels to be available.
Another issue with binaural reproduction seems to
be the lack of pinna cues, those alterations in timing and
spectra that are imparted by the pinnae to an arriving
sound as a function of direction. Efforts to impart pinna
cues electronically have had some success, but they tend
Audio and Electroacoustics 18.6 Complete Audio Systems 777
to be as individual as fingerprints, making it difficult to
have a single binaural track which works for everyone.
There has also been some success in producing a universal
binaural track by imparting a direction-dependent
room reverberation characteristic to the signals, although
some purists object to the addition, and the effect is still
somewhat listener dependent.
One experimental system that mitigates some of the
shortcomings of two-channel binaural is four-channel
binaural. This employs two dummy heads or equivalently
arranged directional microphones, with one
head/microphone-pair in front of the other, preferably
with an isolating baffle separating them. The resulting
four-channel recording is played over four speakers,
preferably placed close to the listener in fixed positions
around the listener’s head, corresponding to the
microphone positions, possibly with a left–right isolating
panel or crosstalk-canceling circuits to minimize
crosstalk. Because the front–back pair each preserve
ITD and IAD, the left–right space is properly preserved,
while front–back differentiation and externalization are
provided by using separate front and back external
drivers. Dynamic IAD and ITD cues with head motion
are also at least crudely preserved by the use of
front and back channels. The resulting presentation can
exhibit good spatial fidelity.
Although it may never be suitable for general audience
presentation, the binaural format has compelling
qualities for the single listener, and remains an area of
active investigation.
18.6.4 Ambisonics
This system, devised by Michael Gerzon, approximates
the sound field in the vicinity of a listener using an array
of precisely positioned loudspeakers, It is sweet-spot
dependent, so not amenable to audience presentation,
but can provide compellingly good spatial reproduction.
18.6.5 5.1-Channel Surround
The 5.1-channel format is a well-thought-out response
to the shortcomings of stereo. The format employs three
front speakers, two surround speakers placed to the side
and/or back of the listening room, and the 0.1 lowfrequency
channel that only occupies a small fraction
of the total bandwidth.
The addition of the center-front speaker anchors the
front left–right spread, significantly mitigating the problems
stereo has with virtual images, even with audience
presentation. The surround channels provide sound more
Part E 18.6