AV Education on RHT:

Bob Hodas on Proper Speaker Placement

May 2003

Introduction
When the esteemed editor of AudioRevolution.com, Bryan Southard, contacted me to write an ongoing column on acoustic advice for home audio systems for RevolutionHomeTheater.com, I replied, "No problem, I've analyzed more consumer level systems than I can remember." Southard then added in the challenge of writing my column from the perspective of a “do-it-yourselfer” who has made some significant investments in his gear but is tired of hearing that he needs to constantly invest more in gear to get better sound. We all know your room and your system setup is essential to the overall success of your sonics. In many cases, it is more than the gear you have purchased.

In a world where there are very few rules of thumb, in my upcoming features, I'm going to give you my professional guidance in solving some of your system problems on your own. This is going to require some intensive listening and a little legwork at times, but isn’t finding that last percentage of performance in your system part of the fun?

Ultimately, you are going to use your ears as your analyzer, as opposed to using a tool like I use in the Meyer Sound SIM System II. This article is aimed primarily at the "do-it-yourselfer," so the acoustic solutions are the least expensive options that I could think of and "some assembly is often required." I will also advise you on effective products that you can purchase from dealers.

Background
Professionally, I am what the pro audio guys call a “room tuner.” I travel around the world, from Sony Studios in Tokyo to Abbey Road in London, helping audio professionals tune their sound systems in their rooms, using acoustic and electro-acoustic solutions. I also tune high-performance home theaters and audiophile listening rooms. Some of the more notable home clients include George Lucas, Shaq and Intel Chairman Andy Grove, as well as much smaller projects for both the editor and publisher of AudioRevolution.com. I do both a system consultation via remote, which starts at $900 per room, and on-site tuning and EQ, which ranges from $1,500 to $3,500 per session. For readers of RevolutionHomeTheater.com and AudioRevolution.com, my advice is, of course, free.

Setting Up Your Speakers
Before we can think about acoustic treatments, we need to optimize the speaker positions in your room. This is the single most important task you can accomplish to improve your system’s performance. It is very important that you become extremely familiar with the speakers you are going to use. You will also want to take a good look at the manufacturer's frequency response charts. Remember that these are anechoic measurements. As soon as you put your speaker in a room with boundaries (walls), 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. However, the response charts are useful for knowing what the speaker’s limitations are. For example, bookshelf speakers can roll off dramatically after 125Hz, 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’ll also want to pay close attention to the recommended position for proper phase alignment. For some speakers, it's directly aligned with the tweeter. For others, it's a point between the woofer and tweeter. It depends on the design, so check the manufacturer's literature. Hopefully they cared enough about this when building your speakers. You want to make sure, when you position your speakers and listening position, that this alignment point intersects at your ear level.

For this month’s model, I am assuming that your room has four walls with equal spacing, i.e., the common rectangle rather than an “L” shape.


Step one is to determine which wall your speakers should be on. If your room is square, this doesn't matter. If your room is rectangular, there is no magic formula to determine whether you should put your speakers on the long wall or the short wall. It all depends on the dimensions. Please remember that this is not a simple solution to figure out without an analyzer, but it is far from impossible. It will take both time and effort, but the payoff is tremendous. The speaker distance from the front and sidewalls will also dictate the listening position. I also want to point out that this is one of those “chicken or the egg” situations. All of the factors are interactive, so plan on moving things around a lot.

To figure out which wall to use, you should place one speaker on each wall. Place the speakers at your listening height in an approximate left or right speaker position, as if you had stereo speakers (remember, chicken/egg). Run a mono signal from your CD player to one speaker at a time and listen seriously to the bass. Keep yourself centered between the sidewalls, and move forward and back six inches at a time. Your distance from the speaker will play a part in the frequency response. You should also move the speaker forward and back, and side to side, six inches at a time in this process. You should then be able to get a feel for which speaker has a flatter bass response. More bass is not necessarily better. Listen for smooth and connected bass from the midband down to lowest frequencies. Now you’ve done the hard part. While finding the proper wall, you also found the proper speaker and listening position in the process. Figures 1 & 2 are examples of just how different the walls can be and what you should listen for. This room measures 15' x 9'.

Remember Dustin Hoffman's college graduation party scene in “The Graduate?” In the scene where the businessman says to him, "I have just one word for you son -- plastics." Well, I have just one word for you: “symmetry.” If possible, it is imperative that you set your room up as symmetrically as possible. What does this mean and why is it important? 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, making your center-image off center and making your depth-of-field suck. So your job, should you choose to accept it, is as follows: get a tape measure and 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. (For reference, the front wall is the wall behind the speakers when viewed from your listening position.)

Why is this necessary? Below 200 Hz, your speakers are fairly omni-directional. 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 thus the 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. You wouldn’t put one speaker on the floor and the other on a stand, would you? (At this point, those of you who would do this can stop reading my columns - there is no hope.) This is also true for first order reflections above 400Hz, which we will address in an upcoming column. Figures 3 & 4 give you a demonstration of what happens to the bass when speakers are placed asymmetrically in a room.

The process mentioned above may seem basic but it is the foundation for a well-tuned speaker system. In periodic articles, we will address additional solutions to making your room sound its very best. If either you don’t have the patience or, after attempting this section’s tips and those to follow, you feel that your sound system is capable of so much more, you can hire me or someone like me to do your room. I am not suggesting this to promote my business as much as to suggest as subtly as possible that there is fantastic improvement to be made by proper speaker placement, yet there is only so much that can be done with the ear. In big-dollar sound systems, you will find that the price for professional setup vs. performance to be the best ratio in the high-performance audio/video world.

Next time, we will continue on the speaker placement theme and what happens when the rooms themselves are not symmetrical.

Figure Descriptions

Figure 1


Figure 2


Figure 3


Figure 4


Bob Hodas tunes many of the worlds finest recording studios and mastering labs as well as many of the best private home theater and music playback systems. His clients include: George Lucas, Abbey Road London, Sony Music Tokyo, Paul Stubblebine Mastering and many more http://www.bobhodas.com/clients.html.

Based in Berkeley California, Hodas travels the world to tune audio systems, designs acoustically excellent rooms and implements acoustical treatments. Bob is available for consultation as well as in-home tunings starting at $500. To contact Bob Hodas, email bobhodas@bobhdas.com or call (510) 649-925




Have you tried out Bob Hodas' speaker placement tips yet?
Tell RHT what you think about this feature.
Bob Hodas Acoustic Analysis
P.O. Box 9485
Berkeley, CA 94709
phone - 510.649.9254
web - www.bobhodas.com
email - bobhodas@bobhodas.com




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