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9.1 Sensitivity to Time-Related Information
It is widely known that the human ear/brain system is extremely sensitive to time-related
distortions. This can be understood when one realizes that directional and spatial information
is provided by inter-aural time (and phase) differences. During the period of man's evolution,
the ability to accurately determine the direction and distance of sound sources conferred a
decided survival advantage, hence our present day aural sensitivity to time-related
information.
This sensitivity to time-related information can be observed when listening to audiophiles
discussing the quality of a system's bass reproduction. Many of the terms refer to temporal
properties. A system with poor transient bass response is described as "boomy", "heavy",
"sluggish", or "slow". When the transient response is accurate, the bass is characterized as
"tight", "clean", "quick", or "fast".
"Fast Bass"
The term "fast bass" would seem to be an oxymoron on the surface. After all, it is the
"slowness" of a note that makes it a low frequency. Nonetheless, the term provides an
accurate description of our subjective impression. Many people have erroneously ascribed
"fast bass" to the use of a light diaphragm or the use of a powerful energizing system.
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In
fact, it is not how fast the diaphragm can be set into motion that imparts a speaker with "fast"
bass. Rather, it is how fast that motion can be stopped, how quickly the stored energy can be
dissipated, that results in the sensation of "fast" bass.
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The acceleration of an object is equal to the force exerted upon it, divided by the mass of the
object. Since a loudspeaker is used above its fundamental resonance, it operates in what is known
as the mass-controlled region. In this region, high acceleration (large driving force and/or small
driven mass) does not imply extended high frequency response or fast transient response. Instead,
high acceleration confers high efficiency.