Phase,masking and hass effect

Just to reap let me remind you that sound is always a complex phenomenon but for us to understand what is going on in a scientific way we need to break it down in to simpler events that are repeatable. Usually we analyze how individual frequencies interact with each other. we call simple sounds of a single frequency sine wave. One of the effects we have discover of the interaction of 2 sine waves of the same frequency is cancellation or addition depending on the moment in the cycle in which they interact. We assign a measure of degree depending in to the time difference between the start point of each cycle over time.
The moment in the cycle the 2 sounds meet will change the amplitude of the combined sound. It  could  be added making the level of that frequency louder  if in the positive or canceling making the frequency lower if in the negative. Meaning if the Phase difference is higher than the 180o mark the frequency will cancel out. This is specially important when dealing with speaker and microphone placement and has a more noticeable effect in the low end.( see fig:9)

Fig 10: When a louder sound interacts with a softer sound the louder sounds makes difficult to hear or perceive the softer sound. Depending on phase and frequency they would interact more or less.

This is in part due to the way we perceive sounds. WE DONT HEAR IN REAL TIME… This was discovered during the Speech experiments conducted by Helmut Hass in 1951 . Among other discoveries it realized that our brain has a 30ms buffer time in which 2 sounds will be perceived as a single hit in our brains. Any 2 sounds spaced more than 30ms apart will be perceived as a echo. this has a effect on the way we perceive the sound inside of a space as a sound will be combined with the reflections and will be defined by the RT60 of the room. (see Fig:11)

Fig 9: Phase difference is the time difference between 2 sine waves of the same frequency with a difference in time /distance.

When 2 complex sounds interact the way they will blend will be dependent in the Phase interaction of the sounds components. But also are going to be dependent in the overall loudness and harmonic content of the sounds. Lower frequencies will mask higher frequencies  and inharmonic sounds will mask harmonic sounds. A example of this is when you are watching Tv and someone starts vacuuming the floor, the vacuum's sound will "Mask" the Tv. (See Fig:10)

Fig 11: Graph of the relation of a room in sound .