CENTER CHANNEL SPEAKERS
Prepared by Big Daddy
The center channel speaker is perhaps the most important speaker in a home theater setup. Close to 75% of the soundtrack is directed to the center channel. Ideally, the sound should come from behind the screen like in a movie theater. Unfortunately, the design criteria for a center channel speaker requires that it fits under or above the TV or projector screen.
The primary job of a center channel speaker is to reproduce the midrange frequencies of the human voice in a multi-channel encoded movie. Chances are the center speaker isn't able to produce enough midrange or the midrange has some dispersion problems. One of the most common complaints about center channel speakers is that dialogue gets drowned out when the action or musical score gets loud.
A lot of these problems are associated with the design of the center channel speaker. With their height constrained, center channel speakers must be able to produce sound down to at least 80Hz where the subwoofer takes over. To design a speaker that is only a few inches tall to go down to 80Hz or lower without distortion and good sound level requires careful design and expense. The most common approach is to use two midrange drivers to divide the bass and midrange frequencies and a single tweeter for the upper frequencies. Almost all center-channel speakers have an MTM (Midrange, Tweeter, Midrange) orientation or sometimes called D'Appolito array.
PROBLEMS WITH HORIZONTALLY ORIENTED CENTER CHANNEL SPEAKERS
MTMs were designed to be vertically oriented because they have very good horizontal dispersion and less than ideal vertical dispersion.
When a center channel speaker that uses MTM orientation is laid on its side, you will get good vertical dispersion and a very poor and narrow horizontal dispersion. Although both should be wide, horizontal dispersion perhaps is more significant than vertical dispersion. Off-axis horizontal response is the sound that the listener hears while moving around the speaker.
Remember, if a speaker is operated horizontally, the vertical dispersion becomes the horizontal dispersion and vice versa.
If you sit exactly in the middle in front of the speaker, you will probably hear good sound. If you sit slightly off-axis (as little as 30° off the primary listening position), you will realize that some of the frequencies are canceled (mostly in the midrange frequencies) and some are reinforced.
In addition, you may experience that imaging has shifted away from the middle and that the speaker sounds hollow and this completely defeats the purpose of a center speaker.
WAVE INTERFERENCE (COMB FILTERING AND LOBING)
Sound systems are broad band and contain many different frequencies. When two sound waves are combined together, they will form a resultant wave that we hear. At a given point of observation, the combination of the two waves will result in some in-phase (synchronous) frequencies that are added constructively and cancellation of some out-of-phase (asynchronous) frequencies. Between these two extremes there are many intermediate cases. This can produce uneven sound coverage over an audience area.
This phenomenon is the result of wave interference. The adding and subtracting of various frequencies at various angles cause lobing and comb filtering. When similar independent waves are combined, the result can be either constructive or destructive interference, depending on whether the waves are in phase or out of phase. This interference occurs when the waves have the same or nearly the same frequency. Constructive interference will enhance sound. Destructive interference will weaken sound. If two identical waves are 180 degrees out of phase, they will cancel out. Whether the interference is constructive or destructive, the individual waves continue to exist separately. The interference itself is merely the effect of the waves together at one point in space.
The animation in the following shows two waves traveling in the same direction. The phase difference between the two waves varies with time so that we see constructive interference when maximum points are aligned (peak) and destructive interference when minimum and maximum points are aligned (null). This illustrates the problem with horizontal center channel speakers and also why it is important to adjust the subwoofer’s phase control with respect to the main speakers so that we obtain maximum output.
Interference of two circular waves can also be seen in the following diagram. The frequencies decrease from top to bottom and the center of the two waves (the distance between drivers) increases from the left to the right). As time progresses, the wave fronts will move outwards from, but the dark regions (destructive interference) stay fixed.
In general, there are four problems with MTM speakers:
A narrowing of dispersion at higher frequencies. This is the problem of all speakers.
A series of severe fluctuations in level as you move in front of the speaker. For most common center speakers, lobing begins around 10 degrees and by 25 degrees is quite strong. Lobing error, or the dispersion of treble sounds from a different axis than midrange or bass frequencies, create a sound imbalance that varies from side to side. Equalization cannot help correct this problem.
The following animations demonstrate lobing in two and three dimensional space.
This is caused across your listening positions by having the same frequencies reproduced by multiple drivers. When you are dealing with a horizontally placed MTM speaker, an increase in sound output is obtained on the main axis of the array, while at some points off the main axis the driver array creates a cancellation at varying wavelengths (frequencies). This phenomenon is known as comb filtering which leads to another phenomenon of loudspeaker arrays called lobing.
In the diagram above, the frequency response is not represented by a straight horizontal line, but by a series of peaks and nulls. This results in tonal coloration of the sound.
The human auditory system uses arrival time differences of sound between the two ears to determine where it is coming from. When there is no interference, the sound will arrive to both ears at the same time and level, so the brain images the sound source to be directly in the middle in front of you. With phase interference and comb filtering, poor imaging will result.
Low frequency and high frequency sound waves have different radiation patterns. As a result low frequency drivers (midrange woofers) and high frequency drivers (tweeters) have intrinsically different radiation patterns. At the crossover frequency point, the directivity of the two drivers changes. Interference between the two drivers at this point causes a dip in the level around the crossover frequency, with a peak above it. This will result in muddiness of the sound and movement of sound stage.
In the following diagram positive and negative wave forms are represented by red and green respectively. The dark areas are places where there is no displacement. The similar waves from two sources interfere with one another. When the two waves are in phase, they add to make stronger waves, shown by the alternate red and green blobs. When the waves are out of phase, they will cancel each other, shown by the dark lines .
POSITIVE APPLICATIONS OF PHASE INTERFERENCE
Phase interference can be used to do useful things like reduce the off-axis sensitivity of a microphone (directional microphones), noise cancelling headphones, or design speakers with multiple driver arrays.
In a similar way, phase interference can be used to increase the sound level from the main speakers in an auditorium to the audience and reduce the spill of sound onto the stage.
The wave interference can be used as a tool to improve the sound and imaging of a speaker by including multiple drivers in a line array. That is the reason why Yamaha’s highly rated sound-bar speaker has 40 drivers.
LOW FREQUENCY AND HIGH FREQUENCY RADIATION PATTERNS
The radiation pattern of all speakers varies according to the frequency of the sound they produce. Lower bass frequencies have much wider off-axis response and higher frequencies have a much narrower radiation pattern. Speakers with wider dispersion will most likely sound better to the listener in the primary position and to listeners who are sitting off-axis on the sides. Also, a wide dispersion keeps sound more even when you are sitting or standing.
This can be observed by standing behind a speaker. You will notice that there is a significant drop in higher frequencies, but no change in the bass response. The higher frequencies only radiate in front of the speaker and bass frequencies tend to be non-directional.
The following diagram demostrates that high frequency sound waves have a narrower off-axis response and low frequency sound waves have a much wider off-axis response.
An ideal speaker system will have even dispersion at all frequencies. However, conventional speakers are unable to distribute sound equally because of horizontal dispersion patterns in addition to side wall and floor/ceiling reflections.
If the MTM center speaker is not optimized for horizontal orientation, it likely will exhibit lobing and off-axis combing filtering effects. Vertical floor speakers with multiple drivers will also have wave interference, but vertical variation in frequency response is much less of a problem than horizontal variation. Vertically oriented speaker interact less with the floor and the ceiling. Horizontally oriented speakers will interact more with the floor and ceiling, and suffer from a poorer off-axis response.
Different Center Channel Designs:
Advantages and disadvantages of each design is disccussed in the following article: Pros & Cons of Various Center Channel Designs
Two-way speakers make excellent center-channel choices, often at much lower cost. The optimal center-channel speaker choice will usually be a good-quality, vertically deployed 6.5" two-way speaker.
Place the horizontal center channel speaker vertically.
Placing the tweeter above the horizontal line of the midrange drivers such that an approximate 45 degree angle is formed will lessen horizontal lobing.
Gently curving a speaker can help in covering a wider area. Radically curving the driver arrays may introduce problems.
A lower crossover frequency point, higher order crossover network, or other designs that have better off-axis performance can minimize the effect of wave interference.
Let us summarizes some of the the problems that may occur with standard MTM horizontal center channel speakers.
- Increase in sound at certain frequencies and cancellation at other frequencies.
- Poor off-axis response and narrowing of sweet spot.
- Poorer vertical response as you stand up or sit on a higher chair.
- Horizontal lobing with hot and cold spots.
- Coloration of the tone.
- Poor comprehension and intelligibility of speech.
- Lack of music clarity.
- Poor imaging and its shift from the middle.
You do not have to limit yourself to a traditional horizontal MTM center speaker. You have several options:
- Turn your center speaker vertically for improved frequency response performance across your home theater.
- Use a two-way speaker for the center channel.
- Buy a center channel speaker that has drivers that are not on the same horizontal axis.
- Buy a slightly curved center channel speaker.
- The best and final solution is to use three identical vertical speakers in the front.
To get the best performance out of the center channel, use a speaker that is identical to the two main speakers. If that is not possible, look for bookshelf speakers that have identical drivers as the mains, center speakers that have a vertical arrangement of their tweeters and midrange drivers, a lower crossover frequency points, higher order crossover networks, or other designs that have better off-axis performance and can minimize the effect of wave interference.
ADDITIONAL INFORMATION AND REFERENCES
Interference (wave propagation) - Wikipedia, the free encyclopedia
Seymour AV | Center Stage screens
Pros & Cons of Various Center Channel Designs
Vertical vs Horizontal Center Speaker Designs — Reviews and News from Audioholics
Live Sound International | Tech Topic: Practical Realities of Phase Interference
Can Line Arrays Form Cylindrical Waves? A Line Array Theory Q & A
Sound & Communications - Audio
Acoustics and Vibration Animations
Interference (wave propagation) - Wikipedia, the free encyclopedia
HeadWize - Technical Paper: The Elements of Musical Perception by HeadWize
Mono Compatibility and Phase Cancellation - Tutorials
Speaker Articles (Alphabetical Sort) - eCoustics.com
Loudspeakers, Loudspeaker Technology