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|Subject: FAQ #2: Speakers and Speaker Placement||Date: 1/6/2006 9:57 AM|
|Author: JLMoran||Number: 2793 of 10547|
(Re-posted as a dedicated thread, with a few new terms added and some info in the details section, so more eyeballs catch it and it can be properly critiqued)
(WARNING: VERY long posting ahead -- I leave to the likes of dougaha to condense this down)
First some terms:
Tweeter: A metal dome or ribbon used to produce the highest frequency sounds, such as a cymbal crash or whistle. Most speakers have one of these units, though some cheaper models lack one entirely, while others (such as some center channel speakers) have two.
Mid/Bass Cone: A small to medium size (5" to 8" diameter) cone, usually made of carbon fiber or high quality plastic/resin composites. Used to produce the sound of voices, most musical instruments, and many sound effects that fall in the middle of the frequency range. They generally can produce sounds as low as 80 or 100 Hz, and as high as 2 KHz. All speakers have at least one of these units. Some high-end models will have two.
Bass Cone: A dedicated low-frequency unit, usually larger than the mid/bass cone. When a speaker has both mid/bass and bass cones, the mid/bass handles sound down to several hundred Hz, while the bass cone is dedicated to the lowest frequency sounds. These are generally used only in higher-end speaker units, and some ultra high-end models will have two of these along with two mid/bass units.
Break-in Period: The time needed from when a brand-new speaker is first connected to an audio system before the sound it produces is "correct". Most good speakers need to be used for at least 40 or 60 hours (not continuously) before they are properly broken in. Until then, they can sound harsh, tinny, and generally rough. A brand-new speaker will not sound like the demo unit you heard at the store, because (surprise, surprise) those demo units have been thoroughly broken in before being put on the floor. If you only use your A/V system occassionally, it could take several weeks before the speakers start producing their true sound quality.
Mains: Term used for the front left and right speakers, sometimes includes the front center speaker as well. As the name implies, these speakers produce most of the sound in a surround system.
Surrounds: The speakers placed in the rear of the room. Mostly produce background and atmospheric sounds.
Channel: A single sound source in a surround system. Channels include Front Left, Front Right, Front Center, Rear Left, Rear Right, Rear Center, Back Rear Right and Back Rear Left.
Hertz (Hz): A unit of sonic frequency. The human ear is theoretically capable of hearing a range from 20 Hz at the lowest end to 20,000 Hz (20 Kilohertz, or 20 KHz) at the high end. If you're one of the lucky people who can hear the whine of a TV or computer monitor, or get a headache any time you walk into a store with a silent alarm system, you can hear 20 KHz or slightly higher. If you can feel the pressure wave of a popping balloon a split second before you hear the bang, you can hear down around 20 Hz or slightly lower. Low A on a piano is 120 Hz.
Decibel (dB): A unit of sound volume. Decibels are measured on a logarithmic scale, so an increase of 3 dB is actually twice as loud as the original volume.
Sensitivity: A measure of how much sound a speaker will produce when one Watt of power is applied to it. Measured in Decibels. The higher the number, the more sensitive the speaker is and the more sound it will produce at the same number of applied Watts. A speaker with a sensitivity of 92 dB is twice as loud at the same volume setting as a speaker with an 89 dB sensitivity. Net effect: More-sensitive speakers will sound loud at lower volumes, saving wear and tear on the amplifiers in your receiver.
Monopole speakers: Speakers with only one set of mid/bass and tweeter units. All standard speakers are monopole.
Dipole speakers: Speakers with two sets of mid/bass and tweeter units. Each set is mounted opposite to the other in the speaker cabinet. In essence, it's a pair of speakers mounted back-to-back in a single cabinet. These are generally mounted on a wall, so that sound is produced in opposite directions at the same time. These are special (and generally expensive) speakers used for the surround channels.
2-way speaker: A speaker with a single tweeter and a single mid/bass cone. Called "2-way" because the incoming audio signal is split into high and low frequency parts, which get routed to the tweeter and mid/bass cone respectively. Most low-cost and mid-range speakers are 2-way.
3-way speaker: A higher-end speaker that splits the incoming audio signal into high, middle and low frequency parts. The high is sent to the tweeter, the middle to one or more mid/bass cones, and the low to one or more large bass cones. These are considered superior to 2-way speakers since the mid/bass cone(s) are truly only dealing with mid-range frequencies, resulting in less distortion and a cleaner sound.
Subwoofer: A large (usually 10" or 12" diameter) speaker with a dedicated amplifier built in to the cabinet. Used to produce Low Frequency Effects, which are generally 80 Hz or lower. Normal speakers (even high-quality ones) usually can't go lower than 80 or 100 Hz without distortion, unless they were built with an integrated subwoofer unit.
Sonic matching: Making sure that a set of speakers generally all produce the same sound. This is important, as it keeps the sound stage uniform as sounds and dialog move from one speaker to another. Most speaker manufacturers design their home theater speakers to be soncially matched within a given speaker model line. If you're mixing and matching speakers from different brands, they should match fairly well if every speaker in the system has the same sensitivity and frequency response. You can also compare the sizes of the tweeters and mid/bass cones between speakers. If they're the same across a given set of speakers (e.g., the front left/right and center) they should work pretty well together. Subwoofers are generally excluded from this since they're so big and only produce low-frequency sounds.
Now for the details:
Home Theater uses discrete "channels" for sound. All varieties of surround sound have three speakers placed in the front of the room -- left, center, and right. The center channel is where every bit of main dialog is routed, and (surprisingly) many of the sound effects as well. For this reason, the center channel is speaker is often considered the most critical of the entire set, and should have good-quality tweeters and mid/bass cones.
The left and right front speakers do the work of producing peripheral sound effects, as well as any dialog that is panned off to the side of the picture (or completely out of the picture) and much of the musical soundtrack. These speakers are also important, and are often the largest speakers in a home theater setup. These should at minimum have a single tweeter and mid/bass cone.
Front Speaker Placement
Placement of the front three speakers is usually done one of two ways: Either in a straight line or in a slight arc. The arc is considered preferable by purists, because the distance from your seating position to each of the three speakers is identical and there are no timing issues with the sound reaching your ears from each speaker. However, the straight line approach is usually fine since almost all A/V receivers allow you to tell them the distance to each speaker. Once that's done, the receiver will automatically adjust the timing of the signals it sends so there are no synchronization problems.
The other thing to keep in mind with the front left and right speakers is to angle them inward, so the speaker grilles are pointing directly at the center seating position. This is called "toeing in", and it keeps the sound from these speakers tightly focused for people sitting in the main listening area. Obviously, if your front speakers are wall-mounted to be flush with the wall this isn't possible, but for floor-standing speakers or bracket-mounted units, you should try to do this.
The remaining speakers in a surround sound system are the surrounds. In a 5.1 system there are only two surrounds -- rear left and rear right. A 6.1 system adds a rear center channel, while a 7.1 system adds a second pair of rear left and right. The surround speakers are often small, though some people prefer to use a pair of speakers identical to the front left/right units.
Regardless of what type of system you have, all of the surrounds do the same thing: They produce the ambient background sounds in a movie soundtrack, as well as any sounds meant to pan from rear to front. For a few examples:
- In the movie "Pearl Harbor", the big plane attack scene has bullet impacts panning from front to back and left to right, moving in sync with the movement of the planes and the bullet sprays.
- Likewise, in "The Matrix", the big gun fight in the lobby is full of the sounds of empty bullet casings falling to the floor; the metallic clinks primarily come from the rear surrounds and echo through the room just like they would in the lobby.
- In "The Abyss", the rear surrounds constantly have small creaks and water droplets falling as characters are talking in the underwater lab, making you feel like you're really underwater.
- Finally, in the movie "The Others", there's a scene where a girl in a room with her brother closes the curtains, putting the room in total darkness. A few moments later, soft footsteps sound from the right rear surround, quickly growing in pace and volume as they run to the front center, where the curtain is suddenly yanked open -- showing no one there besides the girl and her brother, right where they were before she closed the curtain. Our room was dark when I saw this, making it easily one of the creepiest moments I've experienced in home theater. I really felt like there was someone in the house with us!
Surround Speaker Placement
5.1 Surround Sound Systems
The main thing with increasing the number of surround speakers is speaker type and placement. In a 5.1 system, most people recommend using dipole surrounds. These speakers are almost always mounted on the wall in line with your sitting position (generally in the middle of the room). This puts each dipole surround directly to either side of you, with the speakers firing to the front and rear of the room. This allows the sound to bounce around a bit before reaching your ears, which is very desirable -- you're sitting in what's called "the null", where no sound is directly audible from the speaker and you only hear the sound after it's reflected off of a wall or two. Since the sounds from these speakers are atmospheric, you ideally don't want to be able to pinpoint the sound to any one specific location. It's supposed to be diffuse and hard to pinpoint, just like real ambient sounds in the real world.
Ceiling mounted speakers can also work as surrounds. They give the sound the same opportunity to bounce around a bit before reaching your ears, giving it that desired diffuse quality. The only potential drawback is if the floor is carpeted, since the carpet will absorb much of the sound and not allow it to reflect decently.
Normal monopole speakers can be used as surrounds in a 5.1 system, but they do have the drawback of the sound being more localized instead of diffuse. But for people on a budget (like me), it's certainly better than nothing at all, especially if you have a pair of old but still serviceable speakers from your stereo system handy. One trick that can be used in this situation is to put the speakers in each back corner of the room, and then turn them around so the speakers are firing into the corner. This lets the sound diffuse a little, but it does mean leaving the speaker terminals exposed for all the world to see. And you don't want to face the embarrassment of terminal exposure, do you? ;-)
Surround Placement in 6.1 and 7.1 Systems
In a 6.1 and 7.1 system, standard speakers will often do the trick, since they are placed behind the listener facing towards the opposite wall (or, for the center rear in a 6.1 system, the speaker is sometimes placed pointing up towards the ceiling with the speaker mounted on a floor stand). The greater number of speakers allows for sounds to be more spread out, getting the same diffuse effect as a single pair of dipole surrounds. In fact, using a single pair of dipole surrounds in a 5.1 system is really no different than using four standard speakers in a 7.1 setup. I'd argue that using four dipole units in a 7.1 system is overkill, but others here may feel otherwise.
The subwoofer is the .1 in a 5.1, 6.1 or 7.1 system. It handles the Low-Frequency Effects (LFE) for a movie soundtrack. Examples of LFE include:
- The rumble when the T-Rex makes his appearance in Jurassic Park.
- The deep thrum of the engines as the massive spaceship ponderously moves past in the opening of Space Balls: The Movie.
- Any other deep sound that goes below about 60 or 80 Hz
Subwoofers are designed to handle these ultra-low sounds. They have very large cones (generally 10" or 12" diameter), big cabinets to move all that air around, and dedicated amplifiers to handle the massive power requirements of actually moving that cone through that big volume of air. As a result, they're usually quite heavy (at least 40 or 50 pounds).
Some regular speakers (3-way units with dedicated woofers, or even 2-way units with a well-made 8" diameter mid/bass cone) can handle the job of producing most LFE sounds (down to about 40 or 60 Hz). But to get the deepest sounds (down to 20 Hz) you really need a subwoofer.
When a subwoofer is connected to the system, it automatically routes any sounds that fall below a specified cutoff frequency to the subwoofer (generally 80 Hz, but it can be configured higher or lower to work best with your particular system), while all other sounds go the other speakers in the system. This actually helps keep your main speakers in good condition for a longer time, because they're not being driven to produce all of the deep effects any more, saving wear and tear on your cones and the other innards. That also means that they'll do a better job producing the mid-range and high sounds, making the sound cleaner overall.
Because LFE sounds can't be pinpointed, you can pretty much put a subwoofer anywhere in the room, with a few caveats (described below). It does require a little tinkering to fit it in the room's acoustics just right, but if there's one place that's going to work better than any other because you want to keep the thing out of sight, then just put it there, calibrate it, and forget it.
The first caveat to keep in mind is that putting the subwoofer in a corner makes the sound much more pronounced (the corner acts as a small amplifier of its own). This comes at the expense of making the sound more directional; you can more easily close your eyes and point to where the sound is coming from. Putting the subwoofer along a side wall emphasizes the sound much less, but still gives it some extra "oomph" while keeping the sound diffuse. This is the most common place to put a subwoofer.
The only other caveat with subwoofer placement is the concept of "standing waves". A little physics 101 here: Sound is nothing more than waves cycling through the air. If two sound waves meet up, they can reinforce each other or cancel each other out, depending on where each sound wave is in its up and down cycle.
Think of when you're at the beach. Sometimes, a big wave will be coming in right as another wave is receding back out, and what you thought would be a big crash turns into a whimper. But other times, that big wave comes in a little earlier or later, and you get a huge wave that throws everyone in its path underwater and sends them tumbling head over heels into shore.
Well, standing waves are kind of like this. Because the sound from the subwoofer is constant, that reinforcement or cancellation of the sound waves as they bounce around the room gets "stuck" in certain places in the room. The effect is unpleasant -- buzzing sounds, dead zones, and other weirdness in what's supposed to be a smooth, diffuse rumble.
The way to avoid this is to make sure the subwoofer is positioned so that the distances to the nearby walls are different. Instead of putting it right in a corner, put it a foot or two off-center from the corner so the cabinet is, say, a foot from the near wall but two and a half feet from the adjoining wall. This helps to keep the sound waves from interfering with one another, preventing those standing waves. It can take a fair bit of listening and tinkering with the position by a few inches at a time to get exactly right, but it can worth it over the long run.
I guess calibration is the last thing to mention. All modern A/V receivers allow you to tweak the volume of individual speakers so you get a uniform sound field. The speakers are calibrated through a menu on the receiver, which will play "pink noise" through each speaker so you can adjust it to the same volume as all the others. Some receivers, like the Yamaha RX-V657, include a microphone you place in your listening position while the test tones are playing; the receiver evaluates the sound picked up by the microphone and adjusts the speakers automatically to have the same overall level.
If you don't have such a receiver, you can get the same result by using a Sound Pressure Level (SPL) meter. These can be bought at Radio Shack for about $20, and they're worth it. You hold it upright while playing the test tones, and manually adjust each speaker until each one is outputting a uniform 65 dB (decibels). It's pretty easy; I've done it a few times now as the speakers have broken in.
- Joe -
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