Directional listening
In normal daily life people use various mechanisms to locate sources of sound.
Firstly, the sound of a source to the right side of the listener (e.g. the right loudspeaker) not only
reaches the right ear but, attenuated and delayed, is also heard by the left ear. The level of attenuation
and the delay time of this crossfeed signal provide important directional information.
Secondly, the soundwaves are partly absorbed and partly reflected by the tissues of the head.
Reflections at the oracles (pinnae) interfere with the soundwaves that directly enter the ear-channel
and amplify or attenuate specific frequency components. Since these reflections depend on the
direction of the soundwave the “color” of the sound changes with the direction of the source.
Thirdly, reflections of the soundwaves from the walls, ceiling and floor of our listening room produce
reverberation that conveys an extra feeling of space.
The information obtained by these mechanisms is further refined by movements of the head. Changes
in sound levels, delay times and sound color refine the sense of direction. For a demonstration,
blindfold a friend and ask him to locate a ticking clock that you have hidden in the room. He will start
turning his head although he can’t see anything. With his head in a fixed position an exact localization
is much more difficult.
Listening by headphones
All these mechanisms are missing when we listen to music by headphones. The sound at the right ear
will no longer reach the left ear and pinnae-reflections no longer interfere with the original soundwave.
Moreover, the headphones are directly attached to our head, and so head movements no longer add
information. Reverberation is also not present.
As a result, the sound heard by headphones seems to stick to the inside of our head and to our ears
and an unnatural soundfield is created. The brain misses logical clues for direction and this
subconsciously results in mental stress. Some people cannot tolerate this stress and are unable to use
headphones.
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