June 12, 2007
By Chad Sapieha, CBC News
So you finally splurged on a fancy flat-screen television that rivals the size of your living-room window. Nice. Your eyes are in for a treat. But how about your ears?
Conventional 5.1 surround sound: A standard 5.1 surround-sound system employs five discrete channels (centre, front right, front left, rear right, and rear left), as well as a dedicated low frequency emitting channel broadcast through a subwoofer.
Sure, the salesman talked up the wattage of the set's on-board speakers and showed off some sort of simulated "3-D sound" settings, but do you really feel like the Millennium Falcon is roaring across the room when you watch The Empire Strikes Back?
To make it sound as though Han Solo is dodging not only the asteroids onscreen but your living-room furniture as well, you'll need a surround-sound home theatre system. But before you go charging off to the store, let's take a moment to understand just how it is that surround sound makes you want to peek behind the couch to check for swooping TIE fighters.
How you hear
Here's a quick refresher on what hearing is all about. Sound is created when an object vibrates, causing the air particles around it to move. These vibrating particles influence neighbouring particles, creating a cascade effect commonly known as a sound wave.
Our eardrums — pieces of thin, taut skin located deep inside our ears — are extremely sensitive to these waves, detecting and translating even the smallest air vibrations into neural stimuli that our brains interpret as sound. The brain can also tell the direction from which sound waves emanate.
It's easy enough to understand how it distinguishes sounds coming from your left or right; after all, your ears are positioned on either side of your skull. They can sense split-second discrepancies in the amount of time a sound wave takes to reach each eardrum, and are also capable of discerning subtle differences in volume. But how about sounds coming from in front, behind, above, or below?
It turns out that timing and amplitude play a role here as well. So does the stuff called cartilage that gives our ears their squiggly appearance. As sound bounces off the various bits of your outer and inner ear a pattern is created in the wave that helps the brain recognize it as stemming from a particular direction.
Virtual surround array (sound reflecting): Some virtual surround systems deliver as many as five distinct channels by reflecting sound waves off the walls of a room.
Scientists have a pretty clear picture of how we hear. It just so happens that they also have a good idea of how to manipulate sound to alter our perception of it.
This field of research is called psychoacoustics, and there are psychoacoustic technologists who spend their days dreaming up new ways to make our brains believe that sounds are coming at us from the four corners of our living rooms — sometimes with as few as just two speakers.
True surround sound
True surround sound systems go easy on psychoacoustic trickery. If you hear the sound of a truck idling behind you with a surround-sound setup, it's probably because there are a couple of speakers back there rumbling away. And if you seem to hear that truck start to drive off to the right, then turn and zoom across the floor in front of you, it's likely because the soundtrack you're listening to is cleverly building up the volume of the truck's engine in one speaker before fading it down and building it up in another, repeating the process in speakers scattered throughout the room to cause a gradual shift in the direction and timing of the sound waves reaching your ears.
Consumer surround-sound systems come in a variety of configurations. The most basic, known simply as 3.0, has been around for about a quarter of a century and is composed of three speakers — placed front left, front right, and rear.
Of course the more speakers, the greater the potential for more realistic surround sound. Modern high-end systems employ as many as seven distinct channels in a so-called 7.1 configuration, which involves speakers placed front centre, front left, front right, left, right, rear left, and rear right. The .1 refers to an eighth channel delivered through a subwoofer, which outputs extremely low frequencies that other speakers in the system are incapable of generating in order to add some extra rumble.
However, in order to take full advantage of this type of eight-speaker system you need to have a 7.1-channel audio source, which is rare. Most major movies on DVD today offer Dolby Digital 5.1 surround sound, which means that sound designers encoded the film's audio using a surround standard created by Dolby Laboratories that employs five distinct channels (plus the low frequency subwoofer channel). These soundtracks have been expertly spread across all of the channels to make the audio follow and flatter the onscreen action. If the movie is played back using a speaker system designed to decode a Dolby Digital 5.1 source, as most modern surround systems are, the soundtrack has the potential to sound exactly as the filmmakers intended.
To achieve true 7.1 surround sound you'll need not only a system designed to decode a 7.1-encoded digital signal, but also source content encoded with eight discrete audio channels. Thanks to the emergence of the new high definition audio-video optical formats Blu-ray and HD-DVD, which support the 7.1 standard, movie companies now have a means to deliver films in true 7.1 surround sound. The trick is finding a Blu-ray or HD-DVD disc that has actually been encoded in 7.1 — studios have yet to embrace this new surround standard.
Fewer speakers, same result (more or less)
While true surround sound delivers a highly realistic audio experience, many people don't want to go through the hassle of running speaker wires throughout their living rooms. That's where virtual surround sound comes in.
Philips' Ambisound SoundBar: Philips' virtual surround SoundBar is an example of applied psychoacoustics. By tampering with specific sound wave properties it can make certain channels appear to bend behind a listener without reflecting sound waves off any walls.
These systems involve a little more psychoacoustic ingenuity, since they strive to create a surround-sound experience using only speakers that are positioned directly in front of the listener. Given what we've learned about how sound works, this might seem a bit far-fetched, but the science behind it is sound — so long as the listening environment is configured within precise parameters.
The most common kind of virtual surround-sound systems use room walls as reflectors. The sound wave begins its life emanating from a speaker heading toward a side wall in the room. To simulate a rear speaker, the sound is deflected at a shallow angle backwards toward the room's rear wall, where it then bounces once more towards the centre of the room. When the apex of the sound wave finally reaches the listener's ear, it is, in fact, coming from behind him, creating the illusion that it originated from the rear. To mimic side speakers the wave bounces off the side wall at a sharper angle so that it arrives at the centre of the room before hitting any other walls.
The problem with the bouncing method is that its success is largely dependent on room symmetry. A wall protrusion or tall piece of furniture could interfere with the paths of reflecting sound waves. Certain surfaces, like sound-absorbing curtains, also tend to have an adverse effect on sound reflection.
Another method of achieving reliable virtual surround is to make sound waves bend around the listener. Or at least appear to. This approach involves some hardcore psychoacoustics, and companies that have researched such highly sophisticated systems are understandably hesitant to give up their secrets. Suffice it to say these virtual surround setups use complex digital algorithms to tamper with the sound waves. By adjusting properties of the wave, a sound can be created that, say, carries with it the pattern of a sound originating from a different direction, or a property that effectively cancels out certain sounds not meant to be heard by one or both ears.
For a clearer explanation, we went to Philips to see if they could explain their new Ambisound SoundBar system, which employs some serious wave processing along with creative placement of six speakers in a single bar measuring less than a metre across that is placed at the front of the listening area. Alas, a 20-minute discussion with a Philips engineer did little to shed additional light on the Ambisound enigma — there was talk of "notches of silence" directly in front of the listener in which certain sound waves were simply "absent" (see illustration), but just how it creates these notches and bends sound is something of a proprietary mystery. However, if the awards won by the SoundBar at this year's Consumer Electronics Show in Las Vegas are any indicator, there seems to be method to Philips' psychoacoustic madness.
Better with than without
Virtual surround sound can be kind of trippy and a little disconcerting, but in the right kind of home-theatre environment and with a properly positioned audience these systems can be surprisingly effective. However, if you want the most authentic surround experience (and are willing to put some time and effort into wiring and positioning several speakers), a system involving six or more discrete channels is still your best bet.
If all this technical garbledegook isn't your cup of tea, just place your trust in the audio engineers and follow the setup instructions. The first time you flinch away from the sound of an explosion coming from behind your couch, you'll realize that it was worth the effort.