What are the differences between typical stereo speakers and home theater speakers?
Speakers perform best when the listener’s head is directly in front of the speaker and at the same height as the drivers. The sound response where the driver is pointing directly at the listener is called the on-axis response. When you move off-axis (to either side or a different height), the speaker will not sound as loud.
Normally, stereo speakers have a better off-axis response than home theater speakers. The sound that the listener hears is a mixture of the left and right channels, combined with the reflections of the both channels off the walls, floor, and ceiling. Stereo speakers sound best when the listening area is centered and in front of the two speakers, but they also sound good from either side of the room. For a well recorded soundtrack, both the left and right signals might contain the same music, but they will not be at the same volume levels. Different instruments will be at different levels in the different speakers. If you are directly in front of one of the speakers, you should still be able to hear some parts of the music coming from the other speaker.
The front speakers in a home theater system are designed to have just the opposite effect. They have a very short vertical off-axis response that limits the dispersion of the sound in the room. This is done to prevent the reflection of the sound off the floor and the ceiling. The reason for this is simple. In a home theater system, there are 2 rear speakers to provide the sound from the back. The reflection from the front speakers will only interfere with the job of rear speakers. When mixing the sound for a movie, the editors want you to feel like you are in the movie. If the actors are in a large room, then the echoes should appear to be deeper and take longer to arrive back at the listener. This is impossible if you can hear the reflection of your front speakers off the back wall of your living room. Ideally, the only reflection you should hear is the false reflection provided by your rear speakers.
What about the rear speakers?
The rear speakers in a home theater system have a greater diffusion than you would get with stereo speakers. Sometimes, rear speakers have drivers firing in multiple directions. This causes a lot of reflection against the walls and makes the rear echoes sound like they are coming from a wide area.
What about the center channel?
The center channel is the most important speaker in a home theater system. It produces all of the voices that come from actors on the screen. Without a center channel, the voices would be coming from the corners of your room. Many home theater speaker systems use two center channels instead of one. It is debatable on whether more than one center channel is necessary. It probably depends most on the size of your television.
What about the subwoofer?
Years ago, the word subwoofer meant a speaker that produced sounds deeper than a regular woofer. A subwoofer driver would have to be at least 12″ wide to produce these frequencies. Now, the term has come to mean the largest driver in a system, regardless of whether the system has a woofer or not. In some small speaker systems, especially computer multimedia systems, a subwoofer can be 6″ or smaller. A subwoofer of this type is completely useless in a good home theater system.
You can get extra bass from a subwoofer through good box design. On average, a ported subwoofer box can achieve an extra 3db, and a bandpass box can produce an extra 5db or more. Many people like this extra bass but there are disadvantages. Ported boxes will usually be slightly larger that a sealed box using the same driver, and a bandpass box can be twice as large. Also, it is difficult to determine when a bandpass sub is being pushed too hard. These subs can self-destruct before any distortion is audible. Finally, ported and bandpass boxes are only louder at certain frequencies. A sealed box may not be as loud but it will have a flatter response curve and a smoother roll-off on the bass end.
Usually, people place the subwoofer in the corner of their room. The placement of a sub isn’t critical because the bass is so deep that the listener shouldn’t be able to determine what direction it is coming from. This is why it is necessary to get a good deep bass subwoofer. Although the corner of the room works fine in most cases, a sub can be placed anywhere in the room. Let your ears be the judge.
Is a subwoofer really necessary?
Depending on how you look at it, subwoofers can radically increase or decrease the cost of a home theater system. A 5 channel system without a subwoofer can produce the same, or more, bass than a system with a subwoofer. Having 4 speakers each with a 15″ woofer will produce a lot more bass than a single subwoofer. The problem is that these speakers would be very expensive and a very large multi-channel amplifier would be necessary to drive them. Having 4 smaller speakers with a smaller amp, and a powered sub would be a cheaper option. Of course, using 4 smaller speakers and no subwoofer is the cheapest option, but this system would not provide a lot of bass.
What are the differences between Dolby Surround, Dolby Pro Logic, Dolby Digital, and DTS?
Dolby Surround and Pro Logic use 4 and 5 speakers respectively. Surround Sound adds rear speakers to the main front pair, and Pro Logic adds an additional center channel. Although these systems use 4 or 5 speakers, the original input signal is still 2 channels: left and right. The 5 channels of sound are encoded into these 2 channels, stored on disc or tape, and then decoded by the home users Dolby decoder. This encoding is done to maintain backward compatibility with older 2 channel systems. A Dolby encoded source will still play correctly when using only 2 speakers without a decoder. The rear and center channels in a surround or pro logic system are not full range. The standard bandwidth for rear speakers is 70Hz to 8kHz, although some soundtracks will contain information as low as 50Hz.
Dolby Digital 5.1, also know as AC-3 because it is Dolby’s 3rd audio code, uses 5 separate channels for encoding the 5 speakers, plus 1 separate channel for a subwoofer. The .1 subwoofer channel is for LFE (Low-Frequency-Effects), and is basically a deep bass channel. The 5 main channels will all have a full range signal. Most Dolby Digital processors will have a function that allows the bass from the 5 main channels to be redirected to the subwoofer if desired. This allows for having bass in a system even when using small surround speakers.
Dolby Digital 5.1 encoding is used primarily in DVDs. There are other mediums that use a 2 channel Dolby Digital format, such as CDs, TV satellite, digital cable, video games, and older laser discs. Although it is Dolby Digital, it is not the 5.1 encoding. The 2 digital signals are decoded into a 4 channel surround sound signal. This is used by companies that use a digital signal to save bandwidth or increase quality, but have not yet upgraded to the full Dolby Digital 5.1 system. Dolby Digital does not necessarily mean 5.1 encoding.
Dolby Digital and DTS signals are compressed. The compression on Dolby Digital signals is higher than that of DTS, so a DTS signal retains more of the original signal and therefore is of higher quality. While Dolby Digital is standard on almost every DVD, many still do not use DTS.
Both Dolby Digital and DTS continue to add channels to their format. You can now get 6.1 (adds a rear center), 7.1 (uses 2 side channels), and 8.1
Should all 5 speakers be driven with the same amplifier power?
When Dolby Surround and Pro Logic came out, the typical home theater receiver had less power for the center and rear channels than it did for the fronts. Since the rear speakers were not full range speakers, this wasn’t much of a problem. With Dolby Digital, all 5 speakers have their own full range channel. Many recordings are now designed for systems that have the same power for all 5 speakers. It is suggested that you get an amplifier with equal power to all channels.
The subwoofer in a Dolby Digital system usually has its own amplifier built into the sub. Since a single 15″ or 18″ sub can require 400 watts of power, it makes sense for a sub to have its own amp. Although powered subs come with many features like built in crossovers and polarity switches, a regular sub with a dedicated amplifier can work just as well.
What are dipolar and bipolar speakers?
The terms bipolar and dipolar both refer to speakers with drivers that are fired in two different directions. With dipolar speakers, the two sets of drivers are on the sides of the speaker aimed in opposite directions and are in reverse phase causing a cancellation of sound waves in front of the speaker. This is usually done in rear speakers that are mounted on the wall, where the front of the speaker is aimed at the listening area. This causes all of the sound to bounce off the walls before it is heard. This makes it almost impossible to determine where the speaker is, causing a true surround sound effect.
With bipolar speakers, the drivers are fired in opposite directions, but are in phase causing an increase in bass output. In this case, the drivers are in the front and back of the speaker. Sometimes bipolar speakers have side firing woofers, technically making it a tripolar speaker because it has drivers on three sides. These types of speakers are still referred to as bipolar. There are also speakers that can be used as bipolar or dipolar. In these speakers a switch is used to change from one mode to the other.
Another type of surround sound bipolar speaker is where the side drivers fire at an angle of 90 degrees or less from off the main drivers. This allows the speakers to be mounted in the corner of the room. The side drivers do not need to be identical to the front speakers in this case.
Are horn drivers better for home theater?
I have actually seen speakers that had horns and dome tweeters, with a switch to alternate between the two for home theater and music. Horns originally became popular because of their high sensitivity. A horn driver can play much louder that a dome speaker using the same amplifier power. Since amplifiers have gotten cheaper horns have become less necessary. The big problem with horns are that the are too efficient. There are no horn woofers, and you don’t want your treble 5 times louder than your bass. This problem could be avoided by using different amplifiers for the treble and bass drivers. The other benefit to horns is that their output can be easily directed. Remember that the front speakers in a home theater system are more directed than typical stereo speakers.
When deciding on whether to purchase a system with horns, let your ears be the judge. There are a lot of high-end systems that still use horns. Also, remember that movie soundtracks are designed for movie theaters, which usually use horn drivers.
How far away should the listening area be from the rear speakers?
Ideally, the listening area should be in the center of the room at equal distances from all speakers. If this is not possible, then there are ways to improve the situation. If the listening area is near the back wall, then put the surround speakers high on the side walls facing each other. If dipole speakers are used, then the can be mounted further forward in the listening area and lower on the wall, as long as the front of the speaker is aimed toward the listening area.
Should I play music using only the front speakers?
This is up to the individual’s tastes. Most likely, there will be some music that sounds better in a surround mode, and other music that sounds outright horrible. When experimenting with playing music on a Dolby Digital system, try it with and without the center channel.
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