IN THE KNOW: JAN/FEB 2002

One of the most often repeated questions that we hear at Autosound 2000 is about speaker placement. Since finding a place to put large speakers in a car is difficult at best, this is no surprise. Unfortunately, one of the first things we learn about building a quality audio system is that speaker placement is at the top of the list. I’ve always reminded installers of what I call the golden rule of car audio: “A good speaker in a great location can always outperform a great speaker in a poor location.”

Speaker location is important to virtually every parameter of speaker performance. Most installers learn very early that good imaging is impossible if the speaker location is not optimum. Maintaining equal distances and mounting symmetry from the left and right channels to the listener is a certain way to not only get a good center image, but to get a balanced stage as well. Yet, while most installers wouldn’t think of installing the left speakers different than the right speakers, many think little of large separations between speakers of the same channel.

Separating Mids and Tweets
Large distance separations of components such as tweeters and mids can wreak havoc to the fidelity of any system. Unfortunately, no speaker can do a very good job of reproducing the entire audio spectrum. And, since our goal is to reproduce real sounds with fidelity, we must attempt to reproduce those sounds the way nature originally produced them. In nature, sound is produced when an object vibrates, and when objects vibrate, all the frequencies from the lowest to the highest emanate from the same source. Since we can’t do this with a speaker, we need to combine several units to cover the entire spectrum. With the use of a crossover we’re able to “divide” the audio spectrum into narrow bands and distribute the frequencies to the speakers that handle each bandwidth the best.

Getting several speakers to work in perfect unison is impossible. However, if everything is done correctly, we can come pretty close. In car audio, a lot of attention is given to the crossover frequency and the slopes of the networks, but little, if any, is given to the actual placement of drivers. In virtually any application, the best results are to be obtained when all the speakers of a single channel are located as close as possible together. Even small separations can create errors that are not easy to resolve by any means but closer placement.

Test Results
To illustrate an example of just how placement-sensitive a pair of mids and tweets can be we performed a little test. To begin the test, we chose a high quality pair of separates and mounted them on a baffle as close as they could be mounted. Then we took a high quality equalizer and adjusted the response to just about perfectly flat. The virtually flat line on the test graph shows the reference measurement at “0” from 100 Hz to 20 KHz (see Figure 1). After taking the reference measurement, I separated the mid and tweet by a distance of one foot. It’s important to realize that I didn’t change the measurement distance of either speaker from the measuring mic as the speakers remained on the same measurement plane. I also repeated this same measurement at a separation distance of two feet.

The two resulting curves indicate the effect on the measured frequency response of the same speakers with only the separation of the mids and tweets changed. In each case, the crossover, of course, remained the same — only the distance between the speakers changed. Notice how the greatest response errors are near the 2500 Hz crossover frequency and just above. The frequency range where both speakers are reproducing the same frequencies are most affected by the increased separation. The simple fact is that once a signal is divided, it’s always difficult to combine it exactly like it was in the beginning. Even with the best of speakers and crossovers, things don’t always come out as good as we would like. I’ve always liked to compare sound to meat by saying, “It is easy to turn a cow into hamburger, but it’s pretty hard to turn hamburger back into a cow!”

Crossover Slopes are Secondary to Placement
The funny thing is that I always hear installers worrying about crossover slopes and frequencies being correct, yet they think little of how much separation they have between the components. The fact is that the physical placement of the speakers is considerably more important than the exact frequency or slope of the crossover. When the mids and the highs come from different locations, there’s not much hope that the sound is going to be coherent like the original waveform. With nearly any speaker combination, once the physical separation is equal to one wavelength at the crossover frequency, there’s not much hope of a good transition at all possible listening angles.

If the distance between drivers can be kept to a quarter wavelength or less, things are always going to be much better overall. In the real world, what do these distances relate to? At the typical mid/tweet crossover of 2.5 to 3 kHz, this would equate to a separation of 2 inches or less. This would make a pretty strong case for a coaxial mounting arrangement. So, what does this all boil down to? The situation becomes the classic tale of “why speakers are like real estate.” In speakers and real estate there are three important things that make a good sounding system or valuable property: location, location, and location.