732 Chapter 25
were largely relegated to the world of communications, where they stayed until the mid-
1950s, when they underwent something of a revival.
Why the change? In a nutshell, stereo. Although stereo was developed in the thirties by
Alan Blumlein, it was not to see commercial introduction, via the stereo microgroove
record, until the 1950s. Hi-fi , as we now know it, had been developing even before the
introduction of the LP, with the 78 rpm record as its source, but the introduction of the
microgroove record and then stereo were defi nitely two large shots in the arm for this
emergent science of more accurate sound reproduction. Stereo was also a major stimulus
to headphones, and although it may seem an obvious thing now, it took an American
Henry Koss to think of the idea of selling stereo headphones as “ stereophones ” and
creating a whole new market for quality stereo headphone listening. Needless to say, Koss
has not looked back since and neither has Sony since the introduction of its fi rst Walkman
personal cassette player. Ironic, perhaps, because stereo was originally developed by Alan
Blumlein for loudspeaker reproduction!
25.2 Pros and Cons of Headphone Listening ................................................................
Good headphones, having no box resonances, can produce a less colored sound than
loudspeakers, even today. Headphones have the further acoustic advantage of not exciting
room resonances and thus giving the listener a more accurate sense of the recorded acoustics.
However, headphones do not seem capable of producing the sheer impact available from
loudspeakers, and unfortunately, stereo images are formed unrealistically inside the head
due to the way stereo is recorded for loudspeaker reproduction.
None of these disadvantages matter much if your prime requirement is privacy or, in the
case of Walkmans, portability. You can cheerfully blast away at your eardrums without
infl icting your musical tastes on others. The closed back type of headphone in particular
is very good at containing the sound away from others and insulating the listener from
outside sounds.
25.2.1 Dummy Heads
One way of overcoming the sound-in-the-head phenomenon that occurs when listening
to normal speaker-oriented stereo through headphones is to record the signal completely
differently using microphones embedded in artifi cial ears in a dummy human head.