GAIN11

The Art & Science of Audio & Video

Haas Effect; (Precedence Effect)

The Haas effect, first discovered in 1946 and named after Helmut Haas, also commonly referred to as the Precedence Effect and/or law of the first wave front. The Haas effect is a psychoacoustic effect having to do with the auditory phenomena that allows us to localize sounds emanating from anywhere around us. This is achieved by using a combination of sensory responses to the varying physical differences between the sounds we hear such as arrival times, level of intensity and phase interactions. Our use of varying arrival times to determine location is a result of simple anatomy. The geometry of the human head with our ears spaced apart and a barrier in between means that direct sound will arrive at the ear closest to the source first and then arrive at the ear furthest away allowing us to locate the source of the sound.

So first let’s stop and break down what we just read. You’re probably thinking, okay that all sounds great but what does it mean? Let’s build a firm foundation to work from by defining some of the terms used above. The precedence effect and law of first wave front refer to the first sound source to arrive at the listener’s position taking precedence over all sounds that follow within 20 – 40 ms (approximately). Depending on what source you refer to this may vary by up to 10ms. Psychoacoustics refers to the science or study of a human’s subjective perception of sounds. It’s essentially using the field of physics to examine how the human psyche interprets and relates to the acoustic environment around us and the infinitely changing variables within it.

Let’s imagine a scenario where you are standing on the fifty yard line in the center of a football field. Now we take two speakers and place one on the 30 yard line and one on the 20 yard line. We are now going to play identical sounds from both speakers simultaneously at the same level of intensity or same dBspl. I think it goes without saying that the sound produced by the speaker on the 30 yard line will reach your ears before that produced by the speaker on the 20 yard line. To you, our listener, it will appear that the sound is being produced solely by the speaker on the 30 yard line. This phenomenon is known as “involuntary sensory inhibition” simply meaning that your perception of the later arriving sound is suppressed.

In our scenario above the sound produced by the speaker on the 20 yard line will arrive roughly 26.5ms after that of the speaker on the 30 yard line. So knowing that sound travels at 1130ft per second we divide 1130ft by 1000 telling us that sound will travel approximately 1.13ft per millisecond therefore; [(10yds = 30ft)(30ft / 1.13ft = 26.5ms)]. The speed of sound is affected by its environment (i.e. temperature, humidity, etc.) but not to such extremes that we need be concerned with for our current topic. (Just because I know someone will feel the need to comment on that statement, should you need to take acoustical measurements somewhere above the arctic circle or in a vacuum let me know and I’ll give you a different formula to use.)

As we increase the time difference in arrival time beyond 40ms you will inherently become able to discern between the two distinct sound sources or two speakers. Your auditory sensory mechanism will have the necessary time to detect the time difference that we commonly know as delay. This is in essence what you experience when you hear an “echo” in a gymnasium or stadium for instance. Due to the size of the acoustical environment the distances traveled from the sound source to the surfaces creating reflections causes the arrival times to appear at such lengthy intervals that it is what we know as an echo.

We most commonly use the Haas effect to our advantage in performance venues such as auditoriums or theatres. Due to the size of the rooms it is often necessary to employ the use of delay speakers to adequately cover the rear of the venue or possibly a balcony. It would obviously be extremely distracting for a spectator towards the rear of the room if the sound appeared to originate from above their head and only fifteen feet in front of them when the stage may be 70ft away. This is a perfect scenario in which we would want to apply a 20 – 30ms delay to the signal being sent to the second set or “delay” speakers so that the sound from the main speakers arrive at the listener’s position before the sound from the second set. This will maintain the perception that the combined sound is emanating from the main speakers only or from the performance area. So we’ve identified our acceptable window of delay but what about the sound level? Fortunately there is an established window of variance for this as well. Using your dBspl meter test the level intensity at the listening position and as long as the level from the second speakers does not exceed 10dBspl above that of the main speakers you will be able to maintain the perception that the main speakers are the sole sound source.

I’d like to pause again to review for those that haven’t had to opportunity to participate in an installation consisting of a main cluster and delay speakers using digital delay and multiple channels of discrete amplification. You probably are asking yourself questions such as how do you determine the proper delay time and how do you correctly adjust the gain for the individual speakers. We’ll address the delay first. If you recall we discussed earlier the speed of sound and how it translates in to distance traveled per millisecond. So let’s apply that once again to determine our proper delay settings. First determine the distance from the main speakers to the secondary speakers. This can be done several ways either by using advance equipment that is able to calculate numerous arrival times (TEF or Smaart Live), or physically measuring from point A to point B with a tape or if you have the luxury of properly scaled drawings and you know the speakers are installed according to the print locations you can take a scaled measurement directly from the prints. Once you have determined the actual distance measured in feet simply divide that number by 1.13 and that is the distance in milliseconds. Let’s say our secondary or “delay” speakers are 70ft from the main speakers that is equivalent to a time difference of 62ms. I would begin by adding an additional 20ms and work from there. Rely on your ears to tell you what is sound right. Test equipment is great and can provide a wealth of information but it can not under any circumstances, regardless of what anyone tells says, tell you what sounds good!

Now for the gain adjustment it is just as easy. After your main speakers have been properly adjusted to provide adequate level across the intended coverage area you will use that as your reference. Move back or up to a listening position to be covered by the “delay” speakers measure the level of intensity of the main speakers at that location as well. Now adjust the gain on the amplifier channel associated with the “delay” speaker until you achieve a level that is equal to or exceeds the level of the main speakers by no more than 10dBspl.

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July 6, 2008 - Posted by | Class: E=MC2(+/-3db) | , , , , , , , , ,

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