Summary: Can't make sense of all the different audio formats out there? 5.1, 6.1, 8.1 - what's the point of all these speakers? How do you make sense of what kind of hardware is needed for a home theater system and how to place all the speakers? Check out Alexis' primer on what it all means.
A little bit about how we hear
With stereo, identical sounds from both speakers will sound like they are coming from between the speakers if the sounds reach your ears at the same time. This makes sense, but we must consider that the sounds from two speakers can only reach your ears at the same time if you are more or less equidistant from the speakers.
The next evolution came with the addition of the center channel in between the front speakers. This was primarily used for movies to carry to voices of the actors. Center channels were traditionally placed near the TV screen so that actor’s voices would appear to be coming from the screen. It turns out that this new channel now carried nearly 70% of all sound during a movie. Many people loved the center channel as it allowed voices to be reproduce without the interference from ambient sounds that were fed through the other speakers, others hated it because it robbed the imaging and soundstage that they had with two speakers before. One of the reasons for this is that many center channels are not as well designed and built as a main speaker. In addition, the aesthetic horizontal placement of the center channel wasn’t ideal and the TV screen affected the sound.
Overall, the consensus was that the center channel was a good development. It allowed people across the entire sofa to properly place sounds in space when watching a movie and made it so people didn’t have to sit directly in between the main speakers.
Around this same time, people began adding a subwoofer to their sound system. At the time, most subwoofers were actually just woofers that made up for the inadequate bass response of smaller speakers. BOSE is most famous or infamous for its design, but JBL and others had sub-sat systems in the 1970s. The reason that subwoofers have more freedom of placement is because the low frequency sounds are non-directional. This is because the wavelength of the sound is much bigger proportionally to the distance between our ears and the temporal sensitivity of our ears. It’s high school physics all over again. This is on the part of the truth however, as subwoofer placement is just as important as placement of any other speakers. Room resonances begin to play an important factor in frequencies below 80hz. A subwoofer that has a perfectly flat response in an anechoic chamber may not perform as nicely in a room. The shape of the room, the volume, the material and stiffness of the walls can all determine its resonance peaks although predicting it beforehand is difficult. Subwoofers placed in a corner sound louder because they have the reinforcement of two walls and the floor. This is similar to cupping your hands when you yell.
That said, there is not a perfect location for a subwoofer. If you want the most output, then corner placement would be the first thing to try; the problem is that the bass can become boomy and inaccurate. Placement in the middle of a room avoids wall reinforcement but can result in tepid bass. This all gets twice as complicated if you add a second sub and need to deal with interactions between the subs and the room. Best thing to do with subs is to buy a Sound Pressure Level meter and some test cd’s to find places in the room without resonances or to use an equalizer to tame the room resonances.
SIDEBAR: NHT’s new line of subwoofers includes different models intended for either corner placement or middle of the room placement
So let’s recap: we’ve got two front speakers, two rear speakers, a center channel and a subwoofer. This kept us happy for quite a few years as we slowly added a discrete signal for the subwoofer and created a stereo rear channel. Some companies started to get fancy and add additional speakers in the back. Yamaha was one of the first, and added a pair of rear speakers calling them surround effects speakers. Since there was no format that directly supported these speakers, the built in DSP sorted it all out and to create a matrixed pair of channels. They also came out with a pair of speakers in the front called the front effects channel where the speakers were placed lateral to the main speakers. Things were calm until George Lucas came out with Star Wars Episode 1.
This is what we have so far
Then there was another
With Star Wars Episode 1 came a rear center channel. This was more than just another set of speakers playing the same signal in the back, but a new discrete speaker just like we have in the front. With this came a great amount of hype and a movie with great sound effects. How much the addition of a rear center channel added to this, we’ll never know. But with this came a big race to bring that rear center channel to the home theater. This brought the 6.1 multichannel format.
Three’s company in the back
Why stop at one
Since it was difficult to buy a single rear speaker, manufacturers came up with the 7.1 format, with, you guessed it, an additional rear speaker. Still the data only coded for a single “rear surround” channel but manufacturers DSP’d their way to a 7.1 system. Then Yamaha decided to keep their front effects channel so we have a 8.1 system too, since they only use a single rear channel
SIDEBAR: Audio Review – User reviews of audio and home theater equipment
The best system I think is one where all the satellite speakers are identical plus a subwoofer. This makes the tonal quality of the sound the same as the sound moves around the room. Problems to this approach would be aesthetic. It is hard to place a large speaker as a center channel or as the rear channels. Center channels are often just a speaker placed on its side. This placement is so that it blends in better with décor, but if the speaker was not designed to be put on its side, then it won’t sound as good. The placement of a tweeter above a woofer is to better define the stereo image. Most of our spatial acuity when it comes to sound is left to right and front to back, based upon the placement of our ears. With up and down sounds we aren’t as good because the sounds usually reach both ears at the same time. When a speaker is on its side, the sound from the tweeter and woofer may not reach our ears synchronously. In addition, the dispersion pattern is changed and you may have interactions with the TV screen. The lesson here is to experiment with placement if your priorities are sound and not looks.
Back it up
In the beginning the rear channel was primarily an ambience channel where you want indistinct imaging and a diffuse sound field. That is why rear speakers were made as dipoles or bipoles. With dipole speakers, the sets of drivers fire out of phase, with bipole they are in-phase. Therefore, dipole speaker drivers both move in the same direction at the same time, while bipoles have one driver moving out while the other moves in. This helps in placing the speakers in different locations, some speakers allow you to switch between this two modes. With dipole speakers, there is a null zone of sound between the drivers, so these speakers are best placed to the side of the listener with the drivers firing forward and backward to create a diffuse sound field. Bipoles are designed to be placed at the back of the room, behind the listener, firing parallel to the back wall, creating a wider sound field with localization to the back.
The stock SVS driver in my hands
Larger enclosures usually make for louder and lower playing subs as well. Manufacturers that are able to use small enclosures often use some form of active equalization to smooth out the frequency response. Interestingly, the greatest technology in speakers is in the sub. Servo controlled technology is basically a quality control system for subwoofers. A sensor on the surface of the driver senses driver motion and sends that signal back to the brains of the sub. The sub then compares the driver movement with the incoming signal. In a perfect world, they would be the same as your driver would be a perfect electronic to mechanical energy converter. If the compared signals are different, the sub can modify the incoming signal to better make the cone move like it is supposed to. The feedback from the servo sensor on the driver doesn’t even have to be that fast since the incoming signal is only 80hz or less. With servo technology the sub should in theory be more accurate, it’s similar to the concept behind crc encoding where you double check to see if the other side got what it was supposed to. That said, technology can’t overcome a poorly designed driver and many passive subs are well designed and do great without servo technology.
Subwoofers can also handle much more power because their voice coils are often beefier than other speakers. They are also designed to take a little more abuse, compared to a frail tweeter. Since the subwoofer operates in a frequency spectrum that we aren’t very sensitive to, you can get away with using a slightly lower fidelity amplifier, but one with more power. That’s why you see high-efficiency digital amplifier technology like BASH in subs but not yet in regular power amps.
SIDEBAR: The key with subs is their stroke volume, not the diameter Are we suggesting size doesn’t matter as much as motion? –ed
The main thing with front speakers is whether you want “large” or “small” ones. In multichannel lingo, large speakers get the full signal range, while small speakers are designed to only play above 80hz with the subwoofer taking over the lower frequencies. This crossover frequency is traditionally 80hz because that is the THX standard and because sounds under 80hz tend to be non-directional. To get front speakers that play down to 80hz you really need at least a 5 inch woofer. The small, stylish satellites that you see in stores really can’t do this and should be crossed over higher at 125hz usually. As you give the subwoofer more frequencies to take care of, the chance of localizing the sub increases. Many receivers have the option of adjusting your crossover frequencies. The really good pre-amps let you adjust the crossover for each channel.
A great way to test out a front channel speaker is to just listen to it in two channel mode, preferably with some music you are very familiar with. Speakers that sound good when you have two of them should sound even better when you have six of them.
Amp me up
So we’ve taken care of the speakers that we need for multichannel audio, next is figuring out the input sources. You’ll need either a receiver with amplified outputs for all channels or a power amp for each channel. Some receivers only come with 5 speaker outputs but give you a pre-amp out to make your system into a 6.1 or 7.1 system. This is a nice option for upgrading down the road.
We’ve got all the hardware we need now, the speakers and the amp. Next time we’ll discuss the software behind multichannel audio. Just like in the computer world, good hardware can’t make up for poor software and vice versa. Lately, there have been many new software players who want to fight for the signal to your surround sound system. We’ll discuss what you need to know about all this in Multichannel Audio part deux. Don’t worry, there won’t be a quiz in the meantime.
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