Multichannel audio

     gain and pain

The greatest advantage of stereo sound is that the entire music selection is recorded, mixed, and designed for stereo. Thus, when listening to stereo, or two-channel audio, one can be sure - the sound is exactly what the artist intended to deliver. However, two channels also have their clear drawbacks. The biggest drawback is the central focus. If the artist performs in the center of the stage - equally from both channels, then when listening from the correct position, the artist is precisely between the channels and in the center of the stage. But a slight movement to the side is enough to shatter this illusion. Instead of remaining between the speakers in the center, where the artist is intended to be, the artist suddenly jumps to the nearest speaker. And so, as the listener moves around the listening room, the artist constantly jumps back and forth between the speakers. Another problem with stereo is related to the spatial sound image. The sound reflected back from different directions in the room, which is definitely present in high-quality audio production, is still presented from those same two speakers. And only the listener's interpretation in determining the locations of the sound arriving at different times allows us to perceive the sound not as stereophonic but as spatial.

Multichannel theory is beautiful. In the center of a circular room is the listener, surrounded by identical speakers at specific distances in certain directions... I have never seen such a listening room in my life. In real life, you want to listen to good cinema sound (where all those Dolby standards originated) in the room where the television is. And most often, the television is in the living room. The television is on one wall and the viewing sofa is on another. None of the viewing and listening spots are in the right place for multichannel system theory. The distances are wrong, doors and windows and other furniture interfere with speaker placement. There are interesting resonances in the room, and those who want to watch and listen prefer to do so in multiple places. When looking around the room and evaluating directions, it becomes clear that the first three speakers can definitely be placed. With a little more exploration of the space, you figure out where two more can be placed. Then that's it, because most people in the living room are not willing to place a speaker in the middle of a door or window. Even less willing are other family members. Surprisingly, in blind tests, the difference between 5 - 7 - 9 - X channels is very small, and essentially a well-executed 5-channel setup can be equivalent to a much larger system. So don't lose hope - if you found a place for 5 speakers, then good movie sound is entirely possible.

The front channel speakers are fortunately an easy task. It's not different from setting up a regular stereo at all. The speakers are far enough apart, so you can comfortably use multi-driver speakers as desired, accommodating multiple listeners, and varying heights are still within the sound focus. As usual, the ideal setup would have the tweeter seated at ear level (120 cm from the floor), and the horizontal placement angle relative to the central listening position should be 22 - 40 degrees sideways. For example, at a distance of 5 meters, they should be approximately 3.5 meters apart. The speakers can also have a bass component, but generally, better bass can be achieved in the room with separate subwoofers, allowing the stereo channel to go down to 80 Hz, which permits the use of relatively small speakers. The directional profile of the channel speakers should cover all listening positions with equal sound, and in terms of distance, 30 degrees should be sufficient.

The center speaker is usually placed below the screen in the center. The center speaker must be of at least the same quality as the stereo channels because often up to 80% of the movie sound comes from the center channel. A common setup is in D'Appolito configuration: Midrange - Tweeter - Midrange horizontally positioned speaker. This is a very suitable choice for a single central listening position. It's a nightmare if you have a sofa with multiple listening spots. Such a speaker has a very wide directional profile perpendicular to the speaker itself and very narrow along the axis of the speaker itself. This means that such a speaker creates a very uneven sound image for all adjacent listeners and adds additional floor and ceiling reflections in the room. A better choice is if your screen accommodates a completely ordinary speaker with speaker drivers placed one above the other. Then the width of the sound profile is similar at all listening positions and narrower in height, which reduces ceiling and floor reflections. A vertical D'Appolito is a suitable choice for all front speakers if all listeners and viewers share more or less the same height position. However, such a speaker does not tend to fit well vertically under the screen, and a passive filtered D'Appolito is certainly not timed over the entire frequency range. Thus, the best compromise is to solve all front channels with three identical "bookshelf speakers". In other words, smaller speakers with two channels, which can be mounted on the wall, placed on a stand, or placed on equipment furniture.

Surround speakers are a much more complicated issue for the listening room. The surround speaker should create a unified sound image from behind/the sides and must not reveal its location. But how to hide the location when for those sitting at the ends of the sofa, the closest speaker, placed at the right height, looks directly and very closely into their ears? The solution is simple and complex. To hide the location, the high-frequency part of the sound must be reduced for the listener sitting closer. Simple: the speaker needs to be raised, even up to a meter higher than the listener's ears if necessary. Complex: to hide the location, the speaker must also be directed towards the farther listening position so that the diffuse sound in the room weakens the high frequencies as much as the volume weakens for the listener sitting below due to the angle. For example, for a 4-inch speaker, this means a 55-degree angle at 1500 hertz, 1 meter away from the listener, if the farther listener is 3 meters away (note that in this example, the numbers do not match, and a suitable compromise should be sought). Fortunately, listeners from behind do not distinguish height information very well, so you can freely play with the angle and height of the surround speakers. The sound from the surround speakers is quite minimal. Although Dolby claims that all speakers must be equal, considering the actual sound image, this is not necessary. The sound system plays only two things from the surround speakers: spatial sound effects and the same sound echo present in the recording. Sound effects are usually too short to perceive the quality of the speaker in any way, and the echo is much weaker than the direct sound. Therefore, a smaller and more space-efficient speaker fits very well as a surround speaker, and the sound image does not suffer from it. If you have the money to invest in a good sound system, there is certainly a temptation to buy multi-driver surround speakers. In this situation, it would mean chaos in sound frequencies for listeners closer and farther away, as the distance between different frequency speakers would vary at different listening positions. Therefore, rear channels, which have a single full-range speaker, maintain the timing of the sound and a complete sound image in every listening position - thus, a single-driver speaker solution is not a cost-saving measure but one of the best choices for good sound in this example scenario. Of course, there are both worse and better full-range speakers. The guide is simple: choose the better ones. Of course, you can choose any speakers if you have only one listening position and the speakers can be placed at the right height and distance.

If there are multiple rows of listening positions, then the rear and side channels are solved with a slightly different speaker. Two drivers, one on each side of the speaker, one slightly forward, the other slightly backward. It is correct to use them in the same phase (measurements indicate that out of phase, i.e., a dipolar speaker does not create a good sound image in the room). I haven't observed multiple rows of viewing in living rooms at home very often. For those interested, you can Google "bipolar surround speaker."

Overhead speakers are an interesting new topic with the arrival of Dolby Atmos. The Dolby Atmos standard dictates that they should be equivalent to all other speakers. In practical terms, they are often very small, ceiling-mounted speakers with a wide directional profile, often limited by a filter to frequencies above 250 Hz. Alternatively, another option is to use speakers mounted on top of the main speakers, aimed at the ceiling, where the ceiling reflection then creates height sound. These speakers require a narrow directional profile and are also limited in the low frequencies. Therefore, for good sound, it is more sensible to place two smaller single-driver speakers near the ceiling on the front wall, aimed either using the ceiling reflection or directly diagonally towards the farthest listening position. This way, you don't have to run cables to the ceiling. As for cable management, you can decorate the cable area with a flower arrangement or hide it behind a narrow and stylish vase shelf on the front wall.

While most AV receivers also adjust the sound profile correctly, it's worth mentioning a recommendation regarding the speaker sound profile. Since most multi-channel speakers are placed either on the wall, in a corner, or near the ceiling, the final result for a straight profile should have the bass portion freely measured in the initial profile, according to 3 - 6 - 9 dB weaker than the high frequencies. The exact starting point and how much - it depends on the speaker's directional profile and the distance from the walls. A simpler guideline is not to hesitate during test listening to speakers placed away from the walls with a brighter sound or weaker bass if they otherwise sound pleasing. Proper channel system speakers consider wall placement, and thus, the measured profile may be strangely rising, rather than the usual straight or declining. The rule discussed earlier also applies here, suggesting to avoid symmetrical placement relative to corner walls. Therefore, an Atmos speaker on the front wall could be farther from the side wall than from the front wall, and the distance to the ceiling could also differ from both measurements.

Timing alignment seems like a daunting task for a multi-channel system. Fortunately, modern AV receivers handle this entirely automatically. After completing the setup, the speakers' timing is aligned at the microphone's location. Unfortunately, AV receivers do not yet handle internal speaker timing. Therefore, for timed, cohesive, and truly spatial sound, all speakers used should have internally timed sound. This requirement applies in the provided example to the three front speakers with two drivers. Fortunately, single-driver full-range speakers are inherently timed. Thus, a timed home theater sound system may not necessarily be more expensive than a system without timing alignment. Things get more complicated if you have a very large room or need very loud sound.

Timing is also one of the most inconvenient drawbacks of a multi-channel system. The timing alignment of stereo sound channels is ensured at least along the axis passing through the centers of the speakers throughout the room, and often, the spatial image can still be heard when deviating from this axis. In the case of multi-channel audio, the timed sound point for all channels is one and only, where the measurement microphone was during setup. Often, this point is very small, and the sound detail immediately decreases when you sit even slightly to the side of the couch. Of course, this doesn't make the sound bad; it just loses the sparkle that characterizes well-timed sound. Film effects are distributed widely enough to be heard in other places, so watching a movie doesn't pose a problem. The more challenging issue is music recorded and mixed for stereo. Although modern AV systems can distinguish and redirect delayed reflection sound elsewhere, the end result often doesn't match stereo sound quality. Additionally, the recommendations for positioning good stereo speakers are different, so even good front speakers may not sound as good as properly positioned stereo speakers in the room. Therefore, audiophiles still prefer stereo speakers for listening to music, and if a multi-channel home theater system needs to be added, compromises are usually made on the multi-channel solution, while stereo speakers are left in their correct positions. Thus, it's worth considering when installing the first left and right speakers of a multi-channel system whether it's possible and reasonable to place them slightly away from the front wall closer to the best stereo speaker spot. For example, they can be attached to side walls or placed on a separate stand according to the room's possibilities. The movie sound is minimally affected by this, but when listening in stereo, the sound quality can be much better.

At the time of writing, there are also issues with the audio quality of AV receivers. Since sound needs to be processed and amplified for all channels, to keep costs under control, some components of AV systems are rather of mediocre quality and lag far behind even budget stereo amplifiers. Naturally, there are also high-quality AV systems on the market, but the prices of proven high-quality AV receivers are high. Of course, as discussed earlier in the blog, the weakest link in an audio system is always the speaker itself, and while the sound quality of AV receivers may be weaker in comparative measurements, it doesn't matter when watching a good movie. If the sound is spatial, covers all frequencies, and is reasonably loud, then everything is fine, and if you're thinking about the sound system during a movie, it's a bad movie.

As with any audio system, you can recognize a well-placed and tuned multi-channel speaker system by the fact that no speaker is audible. It sounds contradictory, but I have yet to see anyone who wants to listen to a speaker. All we want is for the events of the movie or the music performers to be present right here in the room, all in their proper places. If you hear the speaker, the system still needs development.