Optimum Placement of Dipole Speakers

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Optimum Placement of Dipole Speakers

Postby softspeaker » 08 Apr 2014, 22:29

Almost all audiophiles acknowledge the importance of speaker placement and yet very few really care to experiment with it. An audiophile’s speaker placement is typically based on intuition, influenced by factors such as initial listening, manufacturer’s recommendation, WAF, consideration to the rest of the objects in the room including furniture, windows, doors, etc. Once the initial position is decided, whether right or wrong, the speakers often remain there until replaced, precluding experimentation and hence missing the possibilities of additional optimization.

The fact is that speaker placement is as essential as the speaker itself because the room and speakers operate as one entity to produce sound. Imagine how much time and energy people spend, sometime going out of way and even to a point of obsession, in trying out cables, vibration control, and power treatment. However, the speakers’ positions and their relationship to the room, which is what actually produces sound, are hardly tempered with. It just goes to show yet another peculiarity of audiophiles and their priorities drawn in a wrong order. I often find people hesitant to move their speakers but keen on trying out a $2000 speaker cable or a “sucking life out of music” power conditioner. When offered a friendly advice, the typical answer is “Oh, I think this position is just fine,” or “spikes make it difficult to move.” Not with standing such excuses, the hesitation primarily stems from a goodwill belief which is at best unsubstantiated. What they do not understand is that the room and speaker need to be viewed just as one entity that should function in a perfect harmony. The correct position wherein that harmony occurs is to be determined through moving the speakers, experimenting with the distance to the back wall, distance from the speakers to the side walls, toe-in angle, and the position of the listening seat including its distance from the speakers and the rear wall. Here, I provide a guideline that can serve as a starting point that will need to be further refined by the listener because every room is different. Specifically, the suggested guideline is aimed for planar speakers like Apogees and Maggies that will achieve optimal speaker placement by determining the focal point for room coupling for achieving best bass response, and finding optimal soundstage synchronization for best imaging and soundstage depth and width.

The effect of speaker placement is more profound for planar speakers compared to cone speakers. The reason being their dipole radiation pattern simply adds one more parameters to the complexity. The dipole radiation pattern can be a blessing because this is what adds the third dimension to the sound, but only if properly harnessed. Several sound characteristics get affected from speaker placement. The foremost is the bass. If you get the bass right, usually the midrange and highs do get right as a side effect. Bass can be done right by determining the focal point of the room. The other things that get affected are the soundstage (the depth and width) and imaging, which can be done right by soundstage synchronization. I expound both of these points below.

Speaker placement is done primarily in one of the two ways. The first is the conventional position in which the speakers are 90-150 cm from the front wall, with sufficient distance from the side walls. The listening seat is typically at a good distance from the speaker as determined by the convenience. Some people place their seat too close, using a position called near field listening. I have never been a fan of near field listening as it can get hard on ears and is good only for some music.

There is apparently nothing wrong with the conventional position as it does give a “good” sound. The only problem is it is far from the optimal! First of all how could this be an optimal position for every room? Every room has its own acoustic character, with peaks and dips, cancelling and exaggerating certain frequencies across the audible band and producing phase cancellations. Second, dipole speakers are firing to the front and back. While the conventional position provides an ample room to the front wave, the back wave has only a fraction of it. The myth is that by keeping the speakers 1-1.5 m, the back wave reinforces the front wave and if you move the speakers further, this reinforcement disappears. The reality is that in this position, the back wave does not optimally reinforce the front wave as it is just an assumption (some people even “scientifically” support it by calculating the time for the back wave to return, which does not make any sense). The second myth is that if you move the speakers further into the room beyond 1-1.5 m, the bass disappears. The bass does start loosening up but the sound wave is a cycle, and if you keep moving the speakers further, the bass will get stronger again.

Thus, this position is a gross compromise and is taken for granted. Minor changes such as a few inches here and there, toe in, and acoustic treatment, only incrementally alter the sound for better or worse, giving a false perception that you have achieved the best for your room. In fact, you got the back and front wave totally out of alignment and thus are missing the essentials of the maximum soundstage expansion that your speakers are capable of.

The second option is the non-conventional placement, thinking out-of-the-box, using an approach in which the speakers are pulled way forward into the room. The objective is to determine the focal point and soundstage synchronization for a given speaker-room combination.

First, for determining a room’s focal point, the room should be a rectangle. The commonly popular 1.6 by 2.6 ratio works fine, even though longer lengths or other ratios would be fine too. The rule of thumb is that no two room lengths should be multiple of each other. The notion of room focal point is pioneered by Ivan Lee, a friend from the Far East friend (the one who introduced me to this concept). According to Ivan, every room has a focal point, a spot where you get the minimum phase cancellation and hence minimum distortion.

To start the speakers should be placed along the length of the room. The wall behind the speakers is referred to as the front wall while the wall behind the listener is termed as the back wall. The speakers should be at about 40% of the room length from the front wall (thus leaving about 60% length in front of them). The speakers should also be close to the sides wall, ideally, leave a distance of about 15-25 cm from the outer edged of the speaker to the side wall; of course, you need to have the same spacing on both sides. Note that this is only a guideline and incremental changes will still be needed for every room.

You will need to experiment with plus minus 15-25 cm of that position. Ivan recommends 1/3 of the room length to start with and slowly increasing it but I think 40% is a better starting point because Ivan’s experience has been with Maggies, and also because I think if you start with 1/3, you will eventually move to close to 40% anyway.

Further optimizations should be done from this initial starting point because every room is different and has its own peaks and valleys in sound. Again, Ivan’s method is very effective for this purpose. Once the speakers are placed at 40% of the room length, you play some loud music with all kind of instruments, especially bass. Then you walk from the back wall to the front wall slowly following a straight line that runs along the exact central axis, passing between the two speakers. You will need to bend down a little such that your ears are at a height that is near the center of the mid/high tweeters. As you walk like a monkey (do it when your wife and kids are not home, or else they might think you gone totally mad) hear the loud music carefully and then stop at the point where it is the loudest. That is the focal point of your room. Place the speakers on that line across the room.

The next biggest gain in sound that the non-conventional position brings you is what I call front-back soundstage synchronization. This synchronization is the alignment of the two parts of sound. The first part of the soundstage is the one being created by the front wave and the second part by the back wave. When the two parts are perfectly aligned in a focused manner with your sitting position is also at the correct point, you achieve the best imaging, and deepest and widest soundstage.

A common misconception is that with the speakers placed in the middle of the room, the back wave disappears or the back wave is mostly out of the equation. The fact is on the contrary: The back wave is rejuvenated and plays a central role in imaging, sound staging, layering and depth, and defining the boundaries of musical instruments. But the following principles need to be observed for proper synchronization.

First, absolutely no toe-in should be employed. Another common misconception is that the central image will be tighter with toe-in. This is simply not true. The only thing toe-in does is to increase the level of tweeter energy because highs are very directional. The negative effect of toe-in is that the back wave is out of synch with the front wave and that is contrary to our goal. With the non-conventional placement, there is a sound image at the front wall if you stand near the back wall and face the speakers. Incredibly, there is also a sound image on the back wall if you stand at the front wall and look back at the listening seat.

Second, place the speakers as apart as possible such that each speaker’s outer edge is close to the side wall. Depending upon the width of the room and the width of the speakers, in a reasonably sized room, that should leave good 1.8-2.7 m inner distance between the speakers. If you start losing the highs with a distance less than 1.8-2.7 m, then there is something missing in your electronics (assuming the speakers are not rolled off). The point is to place the speakers as far apart such they are close to the side walls, or until you start losing the highs. Note that this is in contrast to the conventional position wherein a larger distance from the side wall is preferred. But now, with the non-conventional position, all parameters are different – you need the side walls to reinforce the bass, and the mid/highs reflections will not have a negative impact.

For testing, play some music in which there is a right, left and central image. For example, play Holly Cole Trio, Don’t Smoke in the Bed. Holly Cole will be right in the middle. Right, there is nothing new here. But now is the time for a monkey walk again. Stand up from your listening seat, bend down a little (such that your ears are at the level of the mid-point on your speakers’ mid/high driver, and walk on the straight line (midway between the speakers) towards the front wall, going between the speakers. You will feel you are floating into Holly Cole’s image. Keep going until you touch your face to the wall -- you will feel as if you are kissing the central image. Now, turn around and look back. The central image will now be on your listening seat near the back wall. Now walk back and again you will be floating into the central image. In fact, the central image exists in the whole central axis front to back and back to front. And when you sit in your listening chair, the whole energy of the central image in that axis will be focused into the middle but with a three-dimensional effect. You have achieved soundstage synchronization. If you do not get this effect then you need to move your speakers bit by bit, left and right, and front and back until you achieve this effect maximally. I am not sure if cone drivers can give you that effect. By now you can probably guess that with toe-in, the back image gets out of focus and hence offsets the soundstage synchronization.

The next step is to determine the position of your listening seat. The starting point I recommend is 40% of the room length measuring from the speakers (thus leaving 20% from the back wall). There is no need to maintain any special ratio between the speakers and the distance from the speakers to the listening position, soundstage synchronization will be achieved by moving your seat plus minus one foot and stopping where you get the best image and mid/high/bass, all to be determined by you. Do experiment with it and make a final determination yourself.

This will give you the deepest, widest soundstage coupled with best imaging and best bass. The images from music will be painted on the front wall, side walls, in front and back of the speakers and many of them will give a three dimensional effect. Do experiment as it does not hurt to try. If it does not work for you, your room is not a good rectangle. If your wife or girlfriend gets in your way, try to convince her or bargain with her. And if that remains a problem the chances are she is not sensitive to your needs, implying that you may have to think of some more serious options.

Very important: Remove all acoustic treatment from your room before doing this. You can experiment with putting those things back after you are done with your positioning. The chances are you will not need them because the effect of room coupling and soundstage synchronization is so powerful with the results are so stunning that you will not really need any of those acoustic things, especially the tube traps because they will do more harm than good. But do experiment if you like and you might be happy with putting some of those panels back but they will probably require new positions.

My Fullranges are placed at about 3.3 m feet from the front wall in my 4.8 by 7.8 m room. Their outer edges are about 23 cm from the side walls. Of course, I use no toe-in. My listening seat is about 6 cm from the back wall. This is where my focal point and best soundstage synchronization is. I do have a 60 cm by 120 cm DIY skyline diffuser on the front wall (in the middle) though and it gives me a tighter central image. I did try placing 6 feet tall tube traps between the speaker outer edges and side walls. It gives me a bit more weighty bass presumably because it is virtually extending the effect of the Fullrange side wings but it also sucks some highs and mids. Ultimately, I prefer listening without the traps.

Before I discovered the non-conventional position, I perceived the sound in the traditional position to be great but now there is no way I could listen to that again. The layered soundstage, imaging, and bass that I have now is sheer magic. It gives me what the planar speakers are meant to do. The bottom line is that your room and speakers are just one entity and hence need to be perfectly mated. Makers of Apogees and Magneplanars probably knew of it but would not recommend it because it would give them the stigma that their speaker need to be placed in the middle of the room which is not wife- or furniture-friendly.

Do try it because it does not cost anything.

Originally posted on audioworld:



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