Headphones 733This “ dummy head ” -recording technique effectively ensures that each listener’s ear
receives a replica of the sound at the dummy head’s ear positions. The result is much
greater realism, with sounds perceivable behind, to the side, and in front of the listener,
but not inside the head.
The exact location of sound images depends to some extent on how similar the listeners ’
ears are to the dummy’s, but even using a plastic disc for a head-like baffl e between two
microphones gives very passable results.
Why then has this not been taken up more widely? In a word, the answer is loudspeakers.
Dummy head recordings do not sound at all convincing through loudspeakers due to the
blending of sounds between the microphones that occurs around the dummy head during
recording. The only way to listen to dummy-head recordings with loudspeakers is to sit
immediately between them and listen as if they were headphones, which is not exactly practical.
Nevertheless, listening to good dummy-head recordings through fi ne headphones is an
experience that is not easily forgotten. The sound can be unnervingly real.
25.2.2 Cross-Blending
Other ideas used to try to reduce or eliminate the unwanted images in the head with
headphone stereo have centered around cross-blending between the two stereo channels
to emulate the infl uence the listener’s head would normally have. In other words, to
reproduce and imprint electronically the effect of the listener’s head on the stereo signals.
The head has two main effects. First, it acts as a baffl e, curtailing the high frequencies heard
by the ear furthest from the sound. Second, it introduces a time delay (or rotating phase
shift with increasing frequency) to the sound heard by the furthest ear from the source.
Naturally such a simulation can only be approximate. Basic circuits simply cross-blend
fi ltered signals between channels to emulate the absorptive factor. More complex circuits
have been devised to introduce a time delay, in addition to high-frequency absorption,
into the cross-blended signals. This is reported to be more effective, lending a more
natural out-of-the-head sound to headphone listening with normal stereo recordings.
25.2.3 Biphonics
There were attempts in the 1970s to emulate dummy-head listening when using
loudspeakers. The technique was known as biphonics and the idea was to cross-blend