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The Haas Effect

Haas Effect Also called the precedence effect, describes the human psychoacoustic phenomenaof correctly identifying the direction of a sound source heard in both ears but arriving at different

times. Due to the head's geometry (two ears spaced apart, separated by a barrier) the direct soundfrom any source first enters the ear closest to the source, then the ear farthest away. The HaasEffect tells us that humans localize a sound source based upon the first arriving sound, if thesubsequent arrivals are within 25-35 milliseconds. If the later arrivals are longer than this, thentwo distinct sounds are heard. The Haas Effect is true even when the second arrival is louder thanthe first (even by as much as 10 dB!). In essence we do not "hear" the delayed sound. This is thehearing example of human sensory inhibition that applies to all our senses. Sensory inhibitiondescribes the phenomema where the response to a first stimulus causes the response to a secondstimulus to be inhibited, i.e., sound first entering one ear cause us to "not hear" the delayed soundentering into the other ear (within the 35 milliseconds time window). Sound arriving at both earssimultaneously is heard as coming from straight ahead, or behind, or within the head. The HaasEffect describes how full stereophonic reproduction from only two loudspeakers is possible.(After Helmut Haas's doctorate dissertation presented to the University of Gottingen, Gottingen,Germany as "ber den Einfluss eines Einfachechos auf die Hrsamkeit von Sprache;" translatedinto English by Dr. Ing. K.P.R. Ehrenberg, Building Research Station, Watford,Herts., England Library Communication no. 363, December, 1949; reproduced in the UnitedStates as "The Influence of a Single Echo on the Audibility of Speech," J. Audio Eng. Soc., Vol.20 (Mar. 1972), pp. 145-159.)

[From the Rane Professional Audio Reference, compiled by Dennis A. Bohn.].....................

A Few Notes On Music In The Final Mix

By Randy Thom, C.A.S.[excerpt]

...Music mixers do have their frustrations, though, and one of them is music copied off cd's whichis supposed to be put into the film in that two-track form. You can eq it, add some reverb, ride itto keep it out of the way of dialog, but not much else. It often winds up being assigned to only theleft and right channels of the final mix, which presents a problem in the form of the "HaasEffect." If you are equidistant from the left and right speakers it will sound ok. But if you areeven slightly closer to the left speaker than the right, then any material (like maybe the vocal)which in the source mix is equally loud in the left and right will seem to be coming only from theleft speaker. Because the sound arrives at the left ear only a few milliseconds before it arrives atthe right ear, our brain tells us that the source of the sound is to the left.

To all the people sitting on the left side of the theater (even one seat away from the center) anymono material in the two channel mix will seem to be coming only from the left. To those sittingon the right, that stuff will seem only to be coming from the right.

This problem can't be entirely fixed if you are stuck with a two channel source that has monoelements, but one way to make it slightly better is to bleed some of the two channels into thecenter speaker. You'll make the music less stereo this way, but at least the vocal won't jump sodramatically from one side of the theater to the other when you cross the center line.

The Haas Effect obviously isn't limited to music. Any mono sound that you send equally to twoor more widely separated speakers will sound like it is coming exclusively from whatever speakeris closest to the listener.

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Haas Quantising Effect, by Charles Macchia

Remember this?....

...OK, so you've got timing problems. And what has you're music teacher done? He's forced to

spend $50 on a metronome and put aside time every night tapping a pencil to it whilst in a Zenlike trance. So you give up watching the late show because it interferes with your "tapping time"and for this sacrifice what do you discover?

For the most part your timing isn't all that great as expected, usually you hit ahead of themetronome click and you hear an obvious flam between it and your pencil tap. However, everynow and then the mysterious thing happens...

Suddenly you hear a very loud pencil tap and the metronome disappears! This naturally throwsyou off so much your timing goes to pieces for the next 10 or so metronome clicks but it's nothingcompared to those really weird occasions when the metronome suddenly gets louder and your tapdisappears.

Guess what ? You've just become an unwilling victim of the

Haas effect.

Time for my attempt at a theory....

Whenever an audible system is hit or struck it goes into a very characteristic form of chaos for ashort time before it settles into a more stable periodic vibration that we've come to know as thesustained or pitched sound. Over millions of years of evolution we've come to use this chaoticattack (or transient) as the definition of the beginning of an individual sound out from thebackground of noise, and this makes very good sense.

Our brains actively listen for chaos points and without our knowing it and labels them as thebeginning of new distinct sounds. Here's where it gets interesting...Because the chaos watchdog in our head can't usually pull apart two attacks under approx. 35 msapart, an audio illusion actually occurs, two similar sounds will disappear into one about twice asloud if played under this, rule of thumb, 35 ms barrier. This is known as the Haas effect (35ms isthe accepted amount for most sounds, however for extreme pin point transients like metronometaps the Haas effect may not kick in till you're under 20 ms or less). What is most interestingaboutthe Haas effect is what determines which sound gets masked, remember the tapping to themetronome? Yes, it's actually rather simple, the order the transients are recieved in determineswhich sound masks another. If you're like most people you'll discover that you generally playslightly ahead of the beat so in those instances when you're so close the Haas effect kicks in you'llfind that the metronome disappears. This is because you're under the 20ms or so Haas effect

barrier for those two sounds, however your pencil click is the transient that came first, maskingthe attack of the metronome, thus the two sounds collapse into the audio illusion ofone very loud pencil tap. In those, for most persons, rare instances when the pencil tap disappears,you've played behind the metronome so it's transient dominates collapsing both sounds into onevery loud metronome click. You're hand is on the pencil and you just whacked the table with thething but you didn't hear it! Yup, this is spooky but now you know why.