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Of course there is the placebo effect where one has a predisposition to hearing stereo and therefore believes they hear something in stereo. First, let's establish our definition of stereo. There are three or four 'stereo' descriptions that come to mind.
One) The human auditory system is engineered to locate a sound source by detecting first arrival time or arrival time differences in combination with amplitude differences on both sides of your head. So living in a horizontal world we have an ear on each side of our head. We are adept at locating sounds on a horizontal plane but not very good with vertical localization. Hunting and survival throughout our evolution has been dependent on our auditory system localization ability. In a similar manner two eyes give us depth perception.
So stereo in one sense of the word is designed to trick the human auditory system and eliminate our ability to localize. How does this work? Take two speakers and space them a sufficient distance apart. The source can be either stereo or mono for the effect to work. The position of the listener forms a triangle with the two speakers. Ideally the listener would be 1.5 times the distance away as compared to the distance the speakers are apart. In this scenario the sound can be heard from the space in between the speakers rather than from just two distinct sources. This alters your perception. This stereo image is rather fragile. You have to be facing the speakers. You have to be very close to the middle position so that the distance from the two sound sources are near equal and so that the amplitude is near equal. You have to be a precise distance away. Turn around, sit off to the side, sit too close or too far away and the contrived stereo image collapses. This also takes a fairly linear speaker. If the speaker is too peaky, especially in the midrange and treble region, then the exact speaker location is broadcast diminishing the stereo effect.
Typically in a vehicle cabin this particular stereo effect is not possible without some help. The driver and passenger are off center. One speaker is closer and one is farther. One speaker is more directional and one is more off-axis. The two sources have different responses as a result of their different locations and different surrounding boundaries and paths related to the listener. However, a good DSP processor can equalize the response differences and correct for time arrival differences. Suddenly the image is lifted to eye level (which corresponds to your ear level) and has a profound sense of depth, as some of the material may seem to originate from over the hood of the car. This effect can be obtained with either stereo or mono source material. But the full effect only works in one very precise listening position for either the driver or passenger but not both simultaneously. For instance, angled kick panel speaker locations in a car are vastly superior to high door or dash locations for creating stereo. The longer the distance away and the more similar the orientation, the more the time, phase, amplitude, and response is equalized for both driver and passenger. You don't have to worry about the speakers being too distant in a car.
Certainly a seamless horizontal speaker array across the tower will create a sound source that spans the center region just like two carefully position speakers will. It's impossible that the array wouldn't create a similar stereo effect since there is not a center gap that is not occupied by a speaker. But the panoramic perception has nothing to do with whether the source is stereo or mono. It's panoramic for a simple reason...because the center space is filled. A horizontal array has other issues to be covered later in this draft.
Two) Another description of 'stereo' is when the recording was made in stereo in the studio or live environment. That means that all musicians are in that space with stereo microphones in place. The recording captures the ambience of the room and localizes the various musicians in left to right space and depth. If the playback system is very precise then it can be somewhat of a mirror image of the original environment thereby revealing many of the details of the live environment. The reality is that perhaps one percent or less of contemporary recordings are made in this fashion. And among those select recordings virtually none are from the Pop, Rock, Hip Hop venues. These few recordings are likely to be an intimate jazz setting or a classical orchestra.
Million dollar studio album recordings of the previous era are all but gone. Many recordings are engineered on a Mac laptop. They don't use real back up vocalists. They just layer the voice of the primary artist. It's just an example of the simplicity of most contemporary recordings. The easy internet access has forever changed the music industry. Expensive investments in high fidelity recordings are mostly in the past. And the playback medium has changed. Today it is all about compressed delivery formats such as ipod, MP3, streaming audio, XM/Sirius, etc. There isn't much room for dynamic range and stereo separation.
Three) There is 'invented stereo' on the mixing board where the engineer may pan back and forth between left and right and create various other channel bias effects. Again, in this day and age of maximum compressed recordings (because they sound better or louder on MP3, ear buds, and air play when highly compressed) there is little room for this type of effect. Listen closely to your own recent recordings. There isn't much true stereo information. You will certainly hear the absence of a speaker once eliminated but you will hear very little deviation between the content played form those two speakers.
Four) Ambience. It's the same effect derived from wearing headphones. There is no front to rear depth. The image is in your head. You can get this by listening from in the middle of left to right speakers and front to rear speakers. It's not part of a stereo recording but the non-directional surround sound stereo effect is pleasant none the less. You are 'in' the recording so to speak. You can get this in your vehicle cabin or down in your boat cockpit but not from a tower.
Here are a few generalizations worth considering.
For a stereo image based on music content you must have separation between the left and right speakers. You must have open space in the middle. Without it you are listening to summed mono. It doesn't matter if the summing is done at the amplifier input or acoustically at the speakers. The result is the same. MONO.
For a stereo image based on music content you must have white space in the recording. That means brick wall compression leaves no room for stereo information. Be realistic about the music you are listening to.
For a stereo image, in any form, according to any description, you must be facing the speakers. You must be centered between the speakers. You cannot be too far away in order to perceive an image. It must be quiet. Once you are listening from double the distance or farther than the distance the speakers are apart then the information collapses to mono. So to have stereo from the tower it has to be quiet, you have to be in the middle, and you have to be no farther than 20 feet away if the speakers are 10 feet apart. Keep in mind that at the end of a rope for a rider tower speakers are like two inches apart. But even at rest you have an extremely narrow window to enjoy a stereo effect regardless of the source of that effect. Exactly with who and when does this apply? You'll quickly come to the conclusion that stereo on the tower is hardly worth defending.
The best imaging speakers are those that are extremely linear or are slightly recessed in the upper midrange/lower treble region which is the area where you can best localize the source. That slight lull is helpful. But this has nothing to do with the stereo material content. And sometimes the very best imaging speakers conceal a bit of upper mid/lower treble information.
You cannot have stereo from a tight horizontal speaker array not in your car, home or boat. It's simply summed mono as you perceive it unless the necessary elements to achieve stereo are intact. As mentioned above, a wide and seamless horizontal tower speaker array can create a large image directly behind the tower but it has nothing to do with the music content or whether that content is mono or stereo. On a bit of a tangent a horizontal speaker array has greater problems with the negative effects of comb filtering (off-axis phase cancellations resulting in response peaks and valleys at various listening positions). However, fanning the speakers out creates a wider dispersion pattern and also helps minimize the negatives from comb filtering.
Technically, the ideal scenario for a six-speaker horizontal tower array would be to either run mono or alternate every other speaker left and right.
Do you lose anything by having mono on the tower? Absolutely nothing real is lost once you get past the placebo effect and false preconceived notions. However, it is important that the left and right channels are correctly summed at the input of the tower amplifier. A fullrange mono amplifier will do this. An amplifier with the right input configuration switches will do this. By simply bridging many amplifiers you cannot accomplish a clean and fully symmetrical signal when applied to fullrange material.
Can you gain anything by having mono on the tower? Sure. Let's argue that your program material actually has legit left/right bias. In that case, when you are far outside the beam of the boat at rest or outside the wake then you will hear a single channel dominance and information from the other channel is mostly lost. In summed mono you get ALL audio information regardless of your listening position.
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