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Understanding Home Theater Audio

Aug 11, 2010 10:58 AM, By Bob McCarthy

Putting large-scale and small-scale audio in perspective.


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Home theater audio

I grew up in the age of stereo. Yes, there was a time when stereo was such a novelty that records would have a label on them to indicate this special feature. I listened to these recordings in my living room, bedroom, and later, dorm room, all the while preparing myself for my ultimate goal: to see the band in concert. The concert inevitably set up a compare-and-contrast scenario to my home listening experience. In the world before music videos (the inverse equivalent of the silent movie), the concert setting provided a personal level of engagement that the recorded experience could not match. On the other hand, the sonic intimacy of the home stereo was never translated into the theaters and arenas. The sound experience was fundamentally unmatched, with the big hall sound being both larger and smaller than my home experience. At home, the mix of the band was in front of me, stretched out along a line from the left loudspeaker to the right, but nowhere else. In the concert setting, the band originates from a small pinpoint, and then surrounds us with complex reverberation from all directions.

At the time, I surmised that these differences could be lessened by superior loudspeaker technology, and indeed there have been tremendous advances in the field. Today, the age of stereo has long faded to the age of earphones, and as a result, the disparity between live and Memorex is as great as ever. What I did not understand as youth, but grasp now, are the laws of physics and psychoacoustics at play that preclude the possibility of making the arena concert experience the same as our home theater, or earbud, frame of reference. These laws are not subject to repeal.

Figure 1. Compare and contrast the stereo experience in the home with that of the arena at 100Hz. Top: The home listening area falls almost entirely within the ±5-millisecond area, leaving open the possibility of panoramic stereo. Bottom: The wide spacing of the arena loudspeakers leaves only a small fraction of the listeners inside the central ±5-millisecond window. See a larger image.

To scale or not to scale

That is the question. Some aspects of sound transmission and the listening experience can simply be rescaled from small to large. The most notable of those is level. If the shape of my living room is the same as that of the arena, the SPL distribution over the space is largely unchanged. The difference in direct sound level from the middle of the room to the far end would be about 6dB in both cases, even though the difference in actual length might be more than 50 meters. This is because SPL distribution functions as a ratio (-6dB per doubling of distance), which makes it self-scaling to venue size. The experience for individuals within the space may vary because the size of the humans does not rescale. Whereas the home scenario has all listeners very near the center of the room, the arena has listeners scattered from front to back and side to side. Visualize home listeners within an inch of the loudspeakers and pushed up against the walls, and you have begun to grasp the level distribution challenges in an arena.

The next parameter to consider is time. In contrast to the ratio-based (logarithmic) world of level, time is strictly about differences (linear). If we compare the reflection paths of our home and arena scenarios, we can see no discernible difference. Angle of incidence equals angle of reflection, and around the room we go. The difference, however, is the difference: the amount of time elapsed between the direct sound and a given reflection. A back wall reflection at home arrives at a central listener within a 5-millisecond-to-10-millisecond window of the direct sound. The same path in the arena can easily exceed 250 milliseconds. The direct/reflected level component is almost the same in both cases (assuming comparable surface reflectivity), but the time gap between the direct and reflected sound is a world of difference in our minds. Our brains use timing information to discern between tonal modifying short reflections (cup your hands and talk to demonstrate) and long reflections that create discrete echoes...echoes...echoes. The walls of our living room are close enough that their reflections are perceived as fused to the direct sound. It is highly unlikely that we will consciously perceive the individual wall surfaces as sound sources. The identical geometry in an arena creates reflections that can become unhinged from the direct sound. Transient responses can be localized wherever glass and steel are found, creating a widespread marching band out of a single drummer. Steady state responses, such as that of the bass guitar, will be held in the room until long after the band has moved to the next note.

Wavelength

The working range of most sound systems runs from around 18kHz down to 30Hz. This is a ratio of 600:1, and that number corresponds to the difference in size of the wavelengths over that range. The sizes of these wavelengths run from fingernails to intermodal shipping containers. The wavelengths in your living room are the same size as in the arena, but the way they fit into the room shape is vastly different. How many shipping containers can you fit into your home listening area? That is how cramped 30Hz feels in your home. The arena is the open road by comparison, and this opens up a mixed bag of possibilities. The first is that we have enough space in an arena to exert some control over the low frequencies, and the second is how urgently we need to. Control of sound requires a loudspeaker system with extended length and/or depth, both of which are in short supply in the home environment. Therefore, the distribution/control of low frequencies in the home is, in most cases, little more than wishing and hoping and perhaps some expenditures on bass traps, etc.

In the arena, we must get control of the sound because the reflection timing is so long that it is destructive to the intelligibility. Control is exerted over the wide range of wavelengths by use of horns, waveguides, coupled loudspeaker arrays, and even exotic cardioid steering configurations. With large arrays, it is possible to mold the spatial distribution of the sound to the shape of the space, over a very wide frequency range. There is no practical equivalent in the home. The lows are sure to run wild through the room. Fortunately, the consequences are less dire.



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