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– by Bob Hodas
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Sound Advice 1
Modern Home Theater, March 2003 – by Bob Hodas
When the esteemed editor of a web rag emailed me to write an article on acoustic advice for home audio systems, I said "No problem, I've analyzed plenty of small rooms." Then he asked me to write it from the perspective of “do it yourself” and I said "Now you’re making it difficult”. Well, in a world where there are very few rules of thumb, in the articles to come I'm going to try to give you guidance in solving some of your system problems on your own. This is going to require some intensive listening at times but hey, that’s why you’re an audio freak. You are going to make your ears your analyzer. Since this article is aimed primarily at the "do it yourselfer," the acoustic solutions are the cheapest I could think of and "some assembly is required". I will also give advice on off the shelf products you can buy for those of you who didn’t put money into the stock market in the past couple of years.
I expect in the consumer audio world most of you don’t know who I am and wonder why you may want to listen to my advice. I am what the studio guys call a “room tuner”. That is, I travel around the world, from Sony in Tokyo to Abbey Road in London helping people tune their speaker systems into their rooms using acoustic and electro-acoustic solutions. I have also tuned home theaters and audiophile listening rooms for guys like George Lucas, Shaq, Andy Grove and the editor and publisher of HomeTheaterreview.com. I have been doing this for a long time and you can get more info at Clients and Testimonials pages. But enough about me, let’s look at your issues.
Before we can think about acoustic treatments, we need to optimize the speaker positions in you room. This is the single most important thing that you can do to improve your system response. Way better than new cables or components etc. It is very important that you become extremely familiar with the speakers you are going to use. Take a good look at the manufacturer's frequency response charts. Remember that these are anechoic measurements and 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. But 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 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 about this when building your speakers. You want to make sure when you position your speakers and listening position that this alignment point intersects your ear level.
This month I am assuming that your room has four walls with equal spacing, i.e. the common rectangle, no L shapes. 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 rule of thumb as to 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 an easy thing to figure out without an analyzer. It will take some 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 listening height in an approximate left or right speaker position as if you had stereo speakers (remember chicken/egg). Run a mono send from your CD player to one speaker at a time and do some serious listening to the bass. Keep your self-centered between the sidewalls and move forward and back 6 inches at a time. Your distance from the speaker will play a part in the frequency response. You should also move the speaker forward/back and side-to-side 6 inches at a time in this process. You should 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 mid down to low. 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 where that businessman says to him, "I have just one word for you son, Plastics." Well, I have just one word for you, Symmetry. If there is just one thing you do right, it should be to set your room up as symmetrically as possible. What does this mean and why? 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 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.
Why is the above true? Below 200Hz 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, 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, can stop reading my articles. There is no hope) This is also true for first order reflections above 400Hz, which we will address in an upcoming article. Figures 3 & 4 give you a demonstration of what happens to the bass when speakers were placed asymmetrically in a room.
Now, if you don’t want to go through the process mentioned above, you could hire a guy like me to do the math and tell you where your speakers should be placed. Or you could buy a $100 program called Room Optimizer by RPG and try to figure it out yourself (Windows only). I want to be very clear that while these calculations work quite well in theory, they are neither perfect nor foolproof. The program is based on perfectly symmetrical rooms with walls that do not flex or pass signal through them. I often compare my direct measurement results with my calculations and have to say that the program is often very close and I usually only make minor changes to positions to fine-tune the system. But there are times when, due to construction etc. the program doesn’t get the job done and an analyzer or your ears are needed.
Next time we will continue on the speaker placement theme and what happens when the rooms themselves are not symmetrical.
Fig. 1 Speaker response placed along the short wall
Fig. 2 Speaker response placed along the long wall. Note more contiguous bass.
Fig. 3 Left speaker response in a room.
Fig. 4 Right speaker placed in a room asymmetrically with regards to the left speaker. Note significantly different response from fig. 3.