HOME THEATER ACOUSTICS.docx

1、HOME THEATER ACOUSTICSHOME THEATER ACOUSTICS-Volume 1- - Were in a downtown theater. The lights are dim and the curtains drawn. - Let the show begin. But when were at home, its not quite the same. - And so begins the challenge for those AV specialists who work in home theater. -Playwrights, actors,

2、and directors have long known that to entertain, they must capture the imagination of the audience. Composers, musicians, and conductors also know that to bring satisfaction into the performance, they must enrapture the audience. To achieve such abandon involvement, the audience must be cultivated i

3、nto a mental state such that they forget themselves and where they are, being conscious solely of the show, and, thus, becoming part of the show themselves.In the movies or live performances, this audience involvement effect is developed by two means. First, good material and a good performance is n

4、ecessary. These factors lie in the hands of those who produce the show. Secondly, a good setting must be provided for experiencing the show and this lies in the hands of those who put on the show. There are numerous elements that go into and compose a good setting for a show. A light dimmer might be

5、 one. A pull cord for the curtain is another. Comfortable seats in an even temperature and draft-free room are needed. Air conditioning noise needs to be reduced just as the excess light from windows. And the list goes on.The A/V specialist in home theater is the person in charge of putting on the s

6、how. Part of this responsibility is to get the equipment operating properly, and the other part is to create a proper setting so that distracting effects are eliminated. For the most part, the program material available these days for home theater is pretty good, and so is the equipment used to proc

7、ess it. What separates the good from the bad and ugly in home theater is, in most cases, the distraction factor of the room. By controlling distractions, the A/V specialist can bring into play that final touch and end up presenting a really good show without breaking a leg.The customer does go to th

8、e store to buy both program material and the equipment. The third element is, however, a customer service commodity. Its the personal touch part of the sale that only the A/V specialist can implement. Not unlike high end audio, the home theater sale goes beyond the equipment-in-a-box mentality to in

9、clude the setup. To maximize customer satisfaction and loyalty, good equipment must be set up in a room that has been organized for minimal distraction effects. We now turn our attention in some detail to describing various distraction effects that exist in home theater.A/V DISTRACTIONSThe home thea

10、ter audience generally has access to all five senses: sight, sound, smell, touch, and taste. Being at the downtown movies can keep all five senses pretty busy, especially if we like popcorn. We can be easily distracted from the movie by our senses. Lets say we take a seat - only to discover that the

11、 person next to us is devouring hot popcorn by the handful. If we dont care about popcorn, only the noise is distracting. But if we love popcorn, the smell drives us crazy and well probably get up and go buy some popcorn. The point being that sensory distraction can keep us from being involved in th

12、e movie.There are two kinds of distraction: primary and secondary. For the movies, the medium of communication involves sight and sound. Primary distractions would be things we see or hear that are not the sight and sound of the movie. Secondary, but not lightly dismissed, distractions are the comfo

13、rt of chairs, temperature settings, and drafts in the room. Primary distractions from the movie program material involve auditory and visual distractions. The A/V specialist will work to minimize A/V distraction from the A/V program material.Home theater is a mix of sight and sound. Two out of our f

14、ive basic senses are fed coordinated signals that result in the audio/visual message experience. Imagine for a moment a video projection screen displayed inside a room, whose entire surface is covered with mirrors. The floor, ceiling, and walls would all be mirrors. Consider the visual confusion you

15、 would experience in such a house of mirrors when the video screen lights up, Which image would be the real image? The one ahead? Maybe its to the left or again to the right? Is it above on the ceiling or behind you? To some degree, it may not matter which image is real and which one virtual, as eac

16、h looks like the other. On a practical basis, it is the confusion and distraction that comes from having so many images presented at one time. Some of the images may be simple, clear visions, of the real screen, but looking in another direction, well see the stacked, multiple image effect of paralle

17、l mirrors that fades away from us into virtual infinity.No one would really propose a house of mirrors setting for home theater. Fortunately, the painted surfaces of most rooms are far from a mirror surface; they are partially light absorbing, and due to their rough surface, they are partially light

18、 diffusing. Whatever light reflects off the wall is diffusely scattered in practically all directions with nearly equal intensity. A blue wall stays a blue wall no matter where you are located in the room. Blue is the color reflected, but the reflecting surface of the wall is so rough that blue ligh

19、t is scattered everywhere.We are lucky that light is pretty well behaved in rooms, but were not so lucky with sound. The room does become a house of mirrors as far as the speakers are concerned. For sound, the walls are acoustically flat like a polished mirror. If we could visually See the speaker i

20、mages, they would be found all over the walls. Unfortunately, with sound, the room really does become a house of mirrors.A good room for home theater will be set up to control both visual and sonic distraction effects. Before we can go very far with guiding the home theater room away from behaving l

21、ike a house of mirrors, we need to understand the principle of sensory fusion. Sensory fusion is part of our natural condition and comes as a mixed blessing. Without this fusion process, movies as we know them today would be impossible. Yet, because of it, we become susceptible to sensory input over

22、load. But first, lets review some aspects of sensory fusion. There are two kinds of sensory fusion; one involves time and the other involves space. There is always a limit as to how close events can occur and still be distinguished as individual events.TEMPORAL FUSIONConsider video. It is actually a

23、 rapidly flashed sequence of still images like movie frames. These stills are flipped up to us at a rate just a little faster than our ability to perceive separate visual events in time. Hence, this staccato of stills seems to the viewer to create a continuous motion. The visual images are flashed a

24、t a rate of 60 times/second (every 1 sec/60 or 17 ms) and at that rate we cannot separate one image from the next. These well-defined still frames are being flashed to us at a rate faster than our visual reaction time for discrimination of separate events in time. This is called temporal fusion, the

25、 time period of sensory fusion.The reason we are slow to perceive the rapidly flashing visual images is because we employ a slow speed biochemical sensor (not a speed of light electronic photocell sensor) in our eye/brain system. The relative slowness of our electro/chemical visual sensors results i

26、n the visual fusion of actually separate-in-time events. It is not a weakness, but it is the nature of our biochemical being that multiple events are perceived as separate events, only if they occur sufficiently separated out in time. If separate events arrive too quickly, they are perceived as one

27、continuous event. Without this vision fusion process, video as we know it today would be like watching a strobe light show - a novelty - but not an entertainment medium.Lets utilize this card flipping process to introduce the sense of sound into temporal fusion. Most of us have done something like t

28、his when we were young. We used a clothesline clip to position a card into the spokes of the bike wheel and we got the sound of a motorcycle. Try an experiment. Take a deck of cards in your hands, arch them back and then with the thumb, release the entire deck in one second. What do we hear? A breat

29、hy, fluttering type of sound, but a tone nonetheless. If we flip through 50 cards in one second, we get 50 separate positive pulses of air per second. But we hear this process as if it was a breathy 50 Hz tone, which is a bass note whose location is about four keys up from the bottom end of the pian

30、o keyboard.If we flip one card per second, we hear distinct snaps. If we flip 50 cards per second, we do not hear 50 snaps per second, but perceive a continuous tone of 50 cycles per second. Because we are human, and our detection systems are biochemical, our experiences with sight and sound are qui

31、te similar. Rapidly flipped cartoon cards create the impression of continuous motion and rapidly snapped playing cards create the impression of continuous sound. Both effects occur because of the temporal fusion threshold (time) in our ability to detect separate events. Separate events that occur wi

32、thin 1/20 second are perceived as one event. Multiple events that are spaced closer than 1/20 second apart are perceived as a continuous event.SPATIAL FUSIONAgain, we consider video and find the spatial (location) version of fusion on the video screen itself. At the movies, the image is practically

33、a continuous distribution of colors and shadings because it is a projected photograph (slide shot) of real objects. The smoothness of the image is controlled by the graininess of the film, which long ago was reduced to the levels. Not so with video. We have pixels, dots, or blocks of colors on the screen. The