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Posted by Edp on April 05, 1999 at 09:20:27:

Last year I placed this info over on the Audio Review site. I did not write it but copied and posted anther's findings. It sounds correct, but you could FAQ it until you get the "official one" or not.

****************** Begin Include *****************
This information is based on scientific research and observations of Audio Physic and some of their most successful dealers.

The solution presented here is aimed at limiting the interference of the room, with speaker placement and listening position, through the application of psycho acoustics and physics. This method can give superb results through experimentation without the use of special room treatment. However, it may be possible to get even better results by carefully using room treatments in addition.

The way we locate sonic events in space is by the brain measuring the time delay of the sound between the two ears. if there is no delay the sound emanates from directly in front of us, if it reaches the right ear first it is to the right, etc. This spatial information is determined by the brain in the first 800us of the transient because this is the maximum time delay between the two ears. It is after this initial recognition of the occurrence of a sonic event, that the hearing of tonality starts. This has recently been proven in scientific studies and is believed to be a critical part of our survival historically. In other words, we first locate the source of a sound, a potential danger for example, and then try to identify what made the sound.

So, the first step to getting a good stereo soundstage is to eliminate early reflections of the leading transient as much as possible. Or, in practice, to have the sound from the speakers arrive at your ears before any reflections. This also has a secondary benefit. According to a psychoacoustic phenomenon called the Haas effect, the brain prioritizes the first sound wave to avoid confusion, if the reflections are low enough in amplitude. The result is that, if the speakers measure flat under anechoic conditions, the brain will register flat behavior even if instruments measure severe deviations in frequency response due to reflections.

To illustrate the ideal set-up using these principles lets start with a well proportioned room that is dedicated to listening. The ideal location for the speakers is at the two center points of an ellipsoid touching the walls of the room. The best listening position is 1 to 3 feet from the rear wall.

Here are some of the advantages with this arrangement:

In this position the sound from the speakers reaches the ears before any reflections coming from the side walls resulting in better soundstaging and an unaltered perception of the speakers tonal balance.

Setting the speakers along the long wall allows maximum possible speaker separation for the widest desirable soundstage.

The close proximity of the head to the rear wall has two effects. At the room boundaries (walls) the room nodes are suppressed - because the sound pressure is high and the velocity is low. Sitting in the maximum pressure area gives the best perception of deep bass. Secondly, the reflections are shorter than the circumference of the head, so the brain can not measure the time delay between the ears. When the brain cannot localize reflections it ignores them. Here is a simple example of how the brain ignores unwanted or unessential information. Imagine the situation of being in a somewhat noisy public place and conversing with the person next to you. Even though a recording made from your listening position would sound like random noise, you can follow the conversation. If you hear your name spoken several feet away, you can change your focus, and "listen in" on the other conversation. Our brains do this automatically all the time to, for example, filter out the annoying natural resonance of a room to facilitate speech, or to identify potential dangers.

To sum up, it is usually best to locate the listening position so the first information to arrive at the ears is from the speaker and the secondary reflections arrive much later and at a much lower volume. Place the listening chair near the rear wall, because the distance ( 1 to 3 feet ) is too short for the brain to measure the time delay and locate the source of the reflection. Also, it places you at the room boundary where the perception of bass is greatest. In regards to the bass reinforcement advantage, we will expand on that in the next section.

Lets move on to some room physics information. This is a method Audio Physic calls room mapping. The principle of this technique is that of wave phenomenon. Since it is uncommon to have a dedicated listening room, and one of ideal proportions at that, here is how to locate the speakers according to room dimensions. Accurately measure the room and draw a simple floor plan. Divide the room into even divisions. At even points in the room, bass frequencies are reinforced.

|------------A-------------B-------------C-------------|
|-------------|--------------|--------------|--------------|
|-------------|--------------|--------------|--------------|
|-------------|--------------|--------------|--------------|- 1
|-------------|--------------|--------------|--------------|
|-------------|--------------|--------------|--------------|- 2
|-------------|--------------|--------------|--------------|
|-------------|--------------|--------------|--------------|- 3
|-------------|--------------|--------------|--------------|
|-------------|--------------|--------------|--------------|

Referring to the above example, the cross points 2A and 2C with the listening chair at B and the wall, are the ideal speaker/listening situation. If you want to set the speakers on the short wall in a more traditional arrangement, you can get almost identical results for bass reinforcement by placing the listening chair against the wall at line 2 and placing the speakers at B1 and B3. This will NOT give you the advantage of wide speaker placement with the maximum distance from reflective surfaces. The next best results would be obtained at A1 and A3 or C1 and C3. You can also place the listening chair at 2B and the speakers at A1 and A3 or C1 and C3. Another option is the chair at 2A and the speakers at C1 and C3. The practical decision of where to place the listening chair and speakers can be determined by the room size and furniture layout. The point is that, by placing the speakers and/or listening chair at an even division of the room, you will get natural bass reinforcement from the room.

A method of tuning the bass and midbass is by using the same principle to cancel, rather than reinforce, low frequencies. To do this you move the speakers into odd divisions of the room.

An important point to remember is that the room can be divided into far more than just quarters or thirds. At the even divisions the bass is reinforced and at the odd divisions the bass is canceled.

DAVE,ROBERT( BY OVERLAYING TO FORM A GRID OF THE EVEN AND ODD DIVISIONS OF THE ROOM YOU CAN SEE THE SMALL MOVEMENTS THAT CAN HAVE A LARGE EFFECT ON THE SOUND)

For tuning, consider that the tendency is for lateral movements to effect the midbass and forward and backward movements effect lower bass.

The basic set up process is as follows and should be done in the order presented.

After determining the general placement for deep bass with the above room mapping technique, the next step is to determine the distance between speakers. Using a recording with strong center information, a voice works well for this, listen to the center fill with the speakers about 6 feet apart and pointed slightly behind the listeners head. Move them further apart, about 6in. each, and listen again. When the center image thins out and becomes diffuse you've found the point where the separation is to great. You then know how wide you can place the speakers and still get good center fill. Our preference is to get the widest possible soundstage without losing center fill energy.

The next step is to adjust balance. If the system balance is adjusted for equal output, and the center image is not centered, it could be that one speaker is closer than the other. This is compensated for by moving one speaker slightly forward or backward. For example, if a lead vocal that should sound centered sounds slightly to the right, the right speaker may be pushed back or the left moved forward. Usually one inch movements are audible.

The final step is to focus the sonic "image". The basic technique involves rotating the speaker face to change the dispersion pattern. This is easier to do with 2 people. Start with the speakers aimed slightly behind the listeners head and the same distance from ear to tweeter on each side. Put on some music with a voice or violin. One person should listen for focus. The other person should rotate the speaker around the inside front spike (for reference ). The listener signals to indicate the best speaker location. When this is done neither speaker has to be readjusted to "look" like the other. The reason the speakers are not usually symmetrical is that rooms are not symmetrical and these differences affect dispersion.

IF YOU SIT ON THE WALL YOU SHOULD STILL USE SOME SORT OF LIGHT DAMPING DIRECTLY BEHIND YOUR HEAD. YOUR BRAIN MAY NOT BE ABLE TO PROCESS THE REFLECTIONS BUT BELIEVE ME, THEY CAN STILL A SLIGHT EFFECT ON THE SOUND. I CAN EXPLAIN THIS IF YOU ASK. I HOPE THIS INFO HELPS YOU UNDERSTAND THE PRINCIPLES OF STEREO SOUND A LITTLE BETTER. LET ME KNOW WHAT KINDS OF RESULTS YOU GUYS GET IF YOU TRY IT.

MARC
MLAIBSON@EROLS.COM
MLAIBSON@UMD.EDU