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3. Simultaneous masking

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Simultaneous masking occurs when a sound is made inaudible by a noise or unwanted sound of the same duration as the original sound. e.g, a powerful spike at 1 kHz will tend to mask out a lower-level tone at 1.1 kHz
Simultaneous masking raises the absolute threshold of audibility of a sound (maskee) by another sound of the same duration as the original sound (masker); the original sound's ATH is typically raised to a level of inaudibility (masked)
While the Fletcher-Munson equal loudness contours represent sensitivity thresholds of the human ear when exposed to pure tones, the addition of masking tones complicates the contour shapes
On-frequency masking:  This refers to a masking signal of both the same duration and frequency as the original sound, and is a best case demonstration of the simultaneous masking effect.  Research has shown that an on-frequency masker will completely mask (render inaudible) a signal only 10db quieter
we know from auditory critical bands that the human ear is sensitive to different frequencies at different points along the Basilar membrane; each critical band is believed to occupy 0.9mm of physical space along the membrane
If two simultaneous tones (a maskee and masker) are stimulating the same critical band, the loudest signal takes precedence, and lower level tones are completely masked)
Off-frequency masking:  A masker that is on-frequency will completely mask a signal only 10db quieter; as the maskee frequency deviates (off-frequency) from the masker, the level of masking reduces, such that frequencies higher than the masker will be masked more effectively than those below; this is due to the overlap and slope of the critical bands within the ear
Looking at the above, it's evident that a masker's threshold at a two octave delta is approximately 30dB; this means that a signal, whether a static tone or a varying one, will completely mask not only on-freq signals only 10db quieter, but also  off-freq signals only 30dB quieter
In the case of a varying tone (music for example), since the two octave delta is in motion, so is its masking effect. i.e. music will mask itself to a level of -30dB (that sound you never heard before, buried in the mix, can't have been quiter than -30dB or so from the loudest note at that time)
This condition is more apparent in musical passage that are dense with simultaneous instrument sounds; a listener exposed to much simpler quartet or chamber music will more easily perceive sounds quieter than 30dB down (prov ided that they are further than two octaves away from any masker signals), as the masker density is less 

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Interesting ... VERY interesting!
I'm thinking that's why I get a better sound stage when I reduce the back channel levels on my sexophonic system with the holography engaged. There's still some very obvious and tasty additions to the mix at the lower levels, but the front of the room just blooms with the slightest turn of the volume knob on the Sansui QRX 9001. I've also got Hafler phasing courtesy of a Dynaco Quadraptor on the four rear speakers, so there's a LOT of information that could be confusing the ear when I push the volume there.
Probably doesn't help that all four of the rear speakers are Bose - we all know how they love to play with reflections and such.

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