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Setting up audio

Aug 1, 2005 12:00 PM, By Bob Hodas

Figure 1. Nonsymmetrical speaker placement — L/R speaker traces overlaid with phase, 8Hz to 200Hz. Click here to see an enlarged diagram.

Speaker positioning

In the past, not much attention has been paid to audio in smaller broadcast rooms. With the advent of HDTV, 5.1 surround sound and the computerization of audio, more facilities are starting to take a harder look at how they might make sound improvements. Proper speaker and equipment positioning make a significant impact on a room's audio performance.

You can make huge improvements simply by putting the listener and the speaker into the proper position in the room. Take a good look at your speaker manufacturer's frequency response charts. Remember that these are anechoic measurements. As soon as you put your speaker in a room, the bass response will start to change significantly.

Bass response will build up even more when you place the speaker against the wall or in a corner. But the response charts are useful for knowing what the speaker's limitations are. For example, Yamaha NS-10s can roll off dramatically after 100Hz, so you don't have to be too concerned about deep bass problems when positioning them. Using a wall or corner may even be to your benefit.

You also want to pay close attention to the recommended position for proper phase alignment. You want to make sure when you position your speakers and listening position that this alignment point intersects your ear position.

I'm going to make a couple of assumptions here. One is that your room is already up and running, and two is that the room is a rectangle. Normally, I would put you through a process of finding which wall the sound system wants to be on (short or long), but if you're already up and running, we can only hope that you got the orientation correct.

Figure 2. Response with speaker on meter bridge. Click here to see an enlarged diagram.

To help you in the above speaker placement endeavor, I recommend a €100 program called Room Optimizer by RPG (www.rpginc.com). I want to be clear that while these calculations work quite well in theory, they are neither perfect nor foolproof. There are times when, due to construction and other factors, the program doesn't get the job done, and an analyzer and your ears are needed.

I can't stress the idea of symmetry enough. If your speakers are not placed symmetrically in the room, they will have different frequency responses. This means that your music will sound different in the left and right speakers, your center image will be off center, and your depth of field will collapse. So, make sure that the left and right speakers are equidistant from the sidewalls. The same applies to the speakers in regards to the front wall.

Why is the above true? Below 200Hz, your speakers are fairly omnidirectional. The signals that bounce off the walls and ceiling are going to mix in with the direct speaker signal. This delayed bounce will cause comb filtering. The time delay and, therefore, frequency of interaction is dependent on the speaker distance from the walls. If the left and right speakers are different distances from the walls, the cancellations will occur at different frequencies.

This is also true for first-order reflections above 400Hz, but the high frequencies are much more directional. Figure 1 shows what happens to the bass when speakers were placed asymmetrically in a room. One speaker looks great, but the other is in bad shape. If you try to treat the above bass problem acoustically, you will find that a treatment that works for one speaker will not work for the other and perhaps make it even worse. Therefore, symmetrical placement is critical.

Figure 3. Response with speaker on stand 8in behind console. Click here to see an enlarged diagram.

You should also pay attention to the placement of your equipment in the room. Most people don't consider this factor, but a bunch of gear on the right side of the room and nothing on the left is going to change the way the speakers behave. Try to design a layout that is balanced side-to-side as much as possible.

Equipment placement

First-order reflections are the initial signal reflections that bounce off the walls, floor and ceiling, and mix in with the direct speaker signal. In most rooms, the reflections will be short enough that your brain cannot separate them from the direct signal. These destructive reflections cause holes in the frequency response, so you miss parts of the music. They also affect the phase response, so it creates problems in the soundstage and imaging, both side-to-side and front-to-back.

The above also applies to the reflection off the console if your speakers are sitting on the meter bridge or too close to the back of the desk. (See Figure 2) Your high end will be much smoother and more coherent if you put the speakers on stands and move them back from the console to a distance where the tweeter doesn't interact. (See Figure 3.)

Many of you probably have no real console, perhaps just a desk with a controller or keyboard. The flat surface of a desk can be worse for reflections than the raked surface of a console, so you should be especially aware of speaker placement.

You also must consider the positions of your computer monitors and how they interact with the speakers. If the monitors are right in between the speakers, you could get low-frequency loading into the surface of the monitor. You also will experience a loss of front to back depth imaging. I suggest placing the monitors below the speaker level. If possible, build them into the desk and place them at an angle so they are easy to view but don't interact with the speakers.

Figure 4. Use mirrors to check for first-order reflection points.

Sound and light act a lot alike above 400Hz, so you can use a mirror and simple geometry to find these unwanted reflections. Invest about €30 in a frameless 0.6m × 0.6m plastic mirror. (You can get one at a plastics store.) Have someone sit in the listening position while you hold the mirror absolutely flat against the sidewalls and ceiling. Slide the mirror all around to see if the listener can see the speaker components (not the side or top of the speakers) in the mirror.

Each time the speaker components are seen in the mirror, that is a first-order reflection point. You will want to put a treatment in that place.

Remember that you must keep the mirror perfectly flat against the boundary surface while doing this test. You will probably be able to outline one large area on the sidewalls that shows the reflection of both speakers. But be judicious with the absorption. (There is nothing worse to my ears than an overly damped room.) Just treat the areas that need it, and leave the rest alone.

I like to absorb the ceiling and side reflections rather than diffuse them because this increases the coherence of the system. I like to diffuse the rear wall reflections because it adds space to the room. The diffusion gets rid of the discrete reflections but leaves most of the energy intact. The energy is just spread out over time. Don't let anyone try to equalize first-order reflections; they are completely dependent on your position and will change at different seating positions throughout the room. Equalization will not fix a high-frequency reflection problem.

Bob Hodas is an acoustic consultant and owns Bob Hodas Acoustic Analysis.