The Listening Room Part II
Your Most Important Component
Loudspeaker Placement: Fix The Bass And Define Your Sonic Space
Article By David Smith
President And Chief Engineer, Snell Acoustics

  Properly placing your speakers, whether for a 2 channel or a 5.1 system, will maximize their performance and define your acoustic environment. In fact, optimizing your speaker placement may give you a lot of improvement and costs you nothing!

Loudspeaker placement affects the sound in two ways. First, the loudspeaker interacts with the room; to a large extent the bass performance is a function of where you put the loudspeakers. Secondly, the relative position of the loudspeakers determines the stereo image (in a 2 channel system), or defines the total sound field in a multi-channel system.

We will cover these two topics separately.

Bass Performance
In Part One we talked about the acoustics of the room and touched on the topic of standing waves. Standing waves happen when a bass note "fits" the dimensions of your room and sets up a pattern of loud and soft locations for that note. Actually, the dimensions of the room will cause this for many bass frequencies. Their levels will become a function of where you are in the room, and also where the loudspeaker is placed. The best way to hear this is to put on a CD of a piece of music with a strong and continuous bass line (use A B repeat to repeat a short section if you want to). Turn the volume up a little and walk around your listening room while observing the varying level and character of the bass. In most rooms you will hear dramatic differences as you change position. The bass will always be stronger in the corners and generally weaker in the center of the room. If you want to hear more bass you can always sit in the corner or against a wall. At the same time moving the speaker into a corner or against the wall will increase the bass even if you are sitting in the middle of the room. But our primary objective is to have smooth bass, rather than just strong bass. This means a note-to-note evenness, without boom or hangover for some notes and a black hole for others. When bass is very even with level then it will have a tight, fast, tuneful quality. Uneven bass leads to droning or turgid or "sloppy" bass. These may be hard qualities to describe, but you will recognize them when you hear them. So here is your first tuning hint: move your loudspeakers (or your listening position) to modify the bass for maximum bottom end smoothness.

The same rules apply in a 5.1 system with the understanding that the bass may be mostly from the subwoofer and not the front loudspeakers. In that case play with subwoofer position to get strong and even bass. (Front LCR positioning can be done solely with regard to soundstage parameters.) For full range loudspeakers, corners are usually bad because this elevated bass output is out of proportion with the rest of the range. For subwoofers however, corner positions will often give maximum output and good smoothness. In the case of a subwoofer, the electronics usually have an adjustment for bass level that will bring the bass back in balance.

Soundstage
And just what is soundstage, I hear you ask? When you have more than one speaker you can start to recreate the three dimensional aspects of sound. A left and a right speaker will let you recreate left sources and right sources and in between sources (called virtual images since they appear to emanate from spots other than the speaker's positions). You can also create some sense of depth, or source distance, and occasionally simulate sounds outside the loudspeakers.

Moving up to a 5.1 system you can completely encircle yourself with real and virtual sources. You can solidify the frontal sounds, even if you sit away from the best listening position. This is a great boon to home theater because it means that the family can join you, unlike with stereo where the person in the middle seat really gets the best "stereo image". Properly placed surround loudspeakers can add a lot of the sense that you have been transported to some sonic environment: a crowded street, a ballpark, and a concert hall. This is good stuff that lets you become much more immersed in the sonic experience of a movie or multi-channel concert.

Front Loudspeaker Placement
Many things are trade-offs. With stereo loudspeaker placement there is a trade-off related to the distance between the speakers. Moving the loudspeakers apart increases the stereo effect. We get more separation between the signals for our two ears and from that, more of a sense of space and separation between instruments. Whether considering a two-channel system or a multi-channel system, we will first consider the main left and right speaker. The essential parameter for proper location is the distance between the loudspeakers relative to the distance to the listening location. The wider the distance between the left and right speaker, the wider the effective "stage" and the more spacious the sound. Unfortunately, too much of a good thing here will lead to the dreaded "hole in the middle". This is when sounds predominantly come from the left and right loudspeaker, with little filling the gap between.

Consider speaker placement this way: viewed from overhead we could draw a triangle with loudspeakers at two corners and the listener at the third. In that case the angle of the corner by the listener should be about 50 degrees. Another way of stating that is that whatever the distance from the listener to either loudspeaker, the loudspeakers should be about 85% of that distance apart from each other. Please see figure below.

One great effect with well set up stereo is the ability to hear sound sources not just at the left and right speaker, but also at positions between and beyond the two speakers. These are the above mentioned virtual sources, a sort of sonic virtual reality. This is a fragile effect though. For a pair of loudspeakers to fool you that there is a source half way between them, both your ears must receive identical sounds from both loudspeakers. In nature, the only way that that generally happens is when there is a real source directly in front of you. By identical we mean identical. The levels must be the same, the arrival time must be the same and the frequency content must be the same. A pair of well-matched loudspeakers will assure the match of frequency balance and level, but the arrival time match can only come from careful setup. The distance from each speaker to the prime listening spot must be identical. (To within and inch). This effect is easy to hear. Find a mono source, either by pushing a mono button if you have one, or perhaps tuning in to some radio news. Use a tape measure from each speaker to the listening position. If the two measurements are not identical, push the nearer speaker back until they are. Now, sitting in that central position, close your eyes and rock from left to right. If you can sway back and forth about a foot, you will notice that the central image swings from side to side. You will also hear a "swishy" effect on the high frequencies. For some lateral position the image will be dead center between the two speakers. If this "sweet spot" isn't over the center of your comfy chair, then you can push one of the loudspeakers back an inch or two and try again. (Pushing the right speaker back will swing the sweet spot to the right, and vice versa.)

With a multi-channel (5.1) system we will add a center channel and instantly do better with our spaciousness versus center image tradeoff. A center channel gives a dedicated loudspeaker for central sources. The fragile virtual center will be replaced by a real center that doesn't require exact distance matching, so the whole family can enjoy a better spread of sound across the front. Musicians in the middle of the stage or actors in the middle of the screen have a hard speaker location, rather than depending on the ears being fooled by two left and right loudspeakers.

Surround Loudspeakers
Before we get to the specifics of surround placement we should make a distinction between the task of the front loudspeakers and those of the surround speakers.

Front loudspeakers should make a precisely focused sound field where every sound or instrument is tightly placed at a point in space.

Surround loudspeakers should make a diffuse sound field where sources come from many directions with no exact locations.

In the case of movies, we have some pretty important things going on on the screen and we want a tight connection between what we see and its sonic counterpart. Noises off the screen tend to be environmental, increasing the illusion that we are in the same locale as the actors. Exact location is less important than a sense of sound coming from everywhere.

There is a practical aspect to this too. Up front we have three loudspeakers to cover the fairly narrow angle of the width of the screen. Two surround loudspeakers cover all the rest of the 360 degrees around us. If our surrounds are not diffuse their location will be obvious with big gulfs of nothing between them.

Surrounds will give a better effect if placed to the sides of the listening area rather than to the rear. Our acoustic discrimination between sources in front of us and sources to the rear is not great, but our sense of lateral position is. Sources to the rear may get confused with the front loudspeakers, whereas side mounted speakers will greatly increase our sense of space. If you are using a 7.1 or EX system and have more than just two surrounds matters improve and can place them to the sides and the rear.

The effect of diffusion that we want to achieve can be built into the design of the surrounds. Dipole or Bipole pattern loudspeakers will give us diffusion by directing less sound straight to the listener and more to reflecting areas around the loudspeaker. This will give the illusion of a broader, more diffuse, sonic source. Some of the diffusion can come from mounting location. The farther away a loudspeaker is, the less obvious its location. So mount your surrounds well up the side walls, perhaps seven feet off the floor as a recommendation. Even on the ceiling, but near the side wall, can be useful. The greater distance to the surrounds this necessitates will also give more even level over a broader listening area.

In Conclusion
Buying good equipment is always just the start of achieving good results in your Audio system. Dealing with the effects of the room and placing your speakers properly will go a long way toward achieving the best results.

David Smith is President and Chief Engineer for Snell Acoustics, a manufacturer of high-end loudspeakers for music and home theater use.

Manufacturer
Snell Acoustics
143 Essex Street
Haverhill, MA 01832

Voice: (978) 373-6114
Website: www.SnellAcoustics.com