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TUBE ULTRAFEX T1954
4.2 Psycho-acoustic Background
The term psycho-acoustics refers to the psychological aspect of hearing in contrast to the physiological
transfer of impulses (transmission of nervous impulses). Psycho-acoustics examines the effect of sound on
the listener and the reasons for certain sonic impressions. How a sound is interpreted is influenced by a lot of
factors, most of them can hardly be measured yet are fairly important. For instance, those portions which are
responsible for the spatial localization of a sound. Nevertheless, they determine the quality of a recording to an
extraordinarily large extent.
There are also portions of the audio spectrum which we perceive as presence or naturalness. If this kind of
information is missing, the recording suffers from a loss in freshness, liveliness and spatial transparency.
Furthermore, natural harmonics are essential components of the sound. Often enough, they only represent a
minor portion of the signal and are easily lost. It´s the harmonic structure that makes a tone colour unique.
Without this structure, different instruments would not be distinguishable. When comparing acoustic musical
instruments, for instance acoustic guitars, you will note that even two instruments from the same series have
a different sound. Numerous factors determine the sound of an instrument: the design and materials to name
but two, but with such bearing on the eventual sound produced by that instrument.
From a physical point of view, a guitar produces a tone by means of a vibrating string which, in turn, sets air in
motion. The subsequent propagating sound waves reach the ear and are identified by the brain as a tone. Since
the string vibrates within itself, the tone consists of not only the fundamental oscillation but also innumerable
upper harmonics which are based on the fundamental wave.
The complex vibrations of the string are transferred to the body which, in turn, is also set in motion. The
combination of string and body produces the sound of the instrument. For example, certain harmonics may be
amplified due to resonance effects in the body, while other frequencies may be canceled due to the properties
of the wood.
This phenomenon creates complex sounds and is underlined by the fact that a combination of harmonics can
produce additional tones, known as interference or residual tones. All of these tiny sound portions contribute to
the sound of certain instruments. The human ear, which is highly sensitive, can detect even minimum changes
in the harmonic structure of a sound.
By experiencing the CD quality of 32-bit converters etc., we have made considerable advances toward the
naturalness of sound, yet still recordings do not sound like the music in a concert hall. Why is there a
difference?
Here, the keyword is intelligent hearing: the visual contact with the musicians enables us to concentrate our
attention on a certain instrument which results in an intensification of the sonic experience. The listener sitting
in front of a speaker system lacks this spatial experience and at the same time the visual feedback aspect of
listening to live music. The perceived positioning of instruments is made even more difficult since the disper-
sion of the sound is not homogeneous, i.e., widely panoramic, but usually reduced to two sound sources.
In particular, the loss of upper harmonics during the transmission of the sound additionally affects the per-
ceived positioning of the instruments and the transmission of room ambiance. The reason for this loss in sound
quality is the inadequacy of the sound recording and reproduction processes.
Each link in the transmission chain from the microphone via mixers, effects devices, tape recorders ampli-
fiers etc., to the loudspeakers causes a loss in sound quality. Each time the sound is processed, it
becomes audibly less natural.
4.3 On Psycho-acoustic Devices
Although the psycho-acoustic effect of enhancers and exciters etc., has been known for several decades, the
function of these devices has been deliberately surrounded in mystique, to increase their appeal.
However, it is fairly clear that all devices in this field are based on certain technically repeatable methods of
functioning. Basically, three principles apply:
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Sound improvement by means of dynamic frequency correction.
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The generation of a wider sound with the help of phase shifting with respect to delay times.
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The enrichment of the program material with artificially generated harmonics.
Independently of each other, each of these methods produces a certain effect which is perceived as a subjec-
4. TECHNICAL BACKGROUND