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Clarity of sound is a complex phenomena. This article does not attempt to present a scientific argument but rather builds a case based on experience in listening to live music, recorded music, together with a good foundation in the field of psychoacoustics.

Definition of the word clarity for the purposes of the this article: the degree to which a listener can pick out and follow individual parts (voices, instruments, notes) in a musical presentation.

There are four principle dimensions in a musical sound field: the spectral, spatial, temporal, and dynamic dimensions. All four are part of the all-important thing we call "clarity".

  • Spectral: if an amplification system distorts the spectrum or tone of an instrument it can affect clarity. For example, if the high frequency content is reduced or lost, lyrics become more difficult to understand and instrumental notes more difficult to hear, other things being equal.
  • Spatial: When individual voices and instruments come from different directions -- especially directions from side to side as opposed to up and down -- we can more easily hear them. Conversely if multiple voices and/or instruments emanate from the same point in space, they are more difficult to hear. This effect is sometimes called the cocktail party effect.
  • Temporal: If the sound of a voice or instrument is smeared in time by reflections and reverberation the clarity can be reduced. In general, reflections that occur within about 1/10 of a second (100 ms) after the direct arrival from the source are considered helpful and those arriving after are considered harmful. Thus reflections from stage surfaces and side walls often add to a pleasing sense of spatiousness, whereas the long tail of reverberation from rooms with little absorption often interfere with the clarity of voices and instruments.
  • Dynamic: If a sound is too loud, our auditory system distorts and clarity is reduced. If a sound is at or below the ambient noise level, it's clarity is drastically reduced.

Masking Effects

From Wikipedia - Masking Effects

In some situations an otherwise clearly audible sound can be masked by another sound. For example, conversation at a bus stop can be completely impossible if a loud bus is driving past. This phenomenon is called masking. A weaker sound is masked if it is made inaudible in the presence of a louder sound. The masking phenomenon occurs because any loud sound will distort the Absolute Threshold of Hearing, making quieter, otherwise perceptible sounds inaudible.
If two sounds occur simultaneously and one is masked by the other, this is referred to as simultaneous masking. Simultaneous masking is also sometimes called frequency masking. The tonality of a sound partially determines its ability to mask other sounds. A sinusoidal masker, for example, requires a higher intensity to mask a noise-like maskee than a loud noise-like masker does to mask a sinusoid. Computer models which calculate the masking caused by sounds must therefore classify their individual spectral peaks according to their tonality.
Similarly, a weak sound emitted soon after the end of a louder sound is masked by the louder sound. Even a weak sound just before a louder sound can be masked by the louder sound. These two effects are called forward and backward temporal masking, respectively.