SUBWOOFERS. The perfect placement.

The subwoofer has become the sword of Damocles of every good multichannel system. Placed correctly, it can complete the audio spectrum and reveal a whole new sound. Badly placed, on the other hand, it can become a nightmare for us… and for our neighbors. In this article we offer you help so that you can enjoy your subwoofer at its full potential.

In a multichannel system, everyone is very clear about where to put the front speakers, where the center channel goes and where the rear speakers go, but what should we do with the subwoofer? Or, more importantly, where do I put it so that my home doesn't turn into a nightclub full of out-of-control bass? Obviously, the answer depends on several factors, some of which we cannot know, such as the dimensions of the room or its insulation. However, we can offer a series of general guidelines to help us position it.

There is a great deal of bad press surrounding the subwoofer: about its usefulness, about the impossibility of placing it correctly or even about whether it is needed at all in home theater systems. It should be made clear that it is simply indispensable in a multichannel system. This is something that has always been very clear to serious enthusiasts, but there is a legion of consumers who have struggled a great deal trying to fit it into the right spot and, all too often, without much success. And since there was an unbearable amount of low frequencies roaming around the room, nobody stopped to think that the problem might lie in poor build quality. Or that its power far exceeded the needs of the room. It was simply banished to the storage room. We are going to make an effort to restore confidence in it and in everything it can bring us.

WHAT IT IS

The first step is to know exactly what a subwoofer is. It is a loudspeaker that covers the lowest frequencies, usually between 20 and 80 Hz, which conventional loudspeakers do not cover (in this case, we are referring specifically to the woofer, which covers from 40/80 Hz up to 1000 Hz). We stress that these figures are approximate since, given the large number of quality loudspeakers on the market, they vary enormously. The aim is to cover the part of the audio spectrum below the woofer's reach. These figures, although only a guide, are important, because in an installation certain frequencies may be reproduced twice. Let's imagine that our subwoofer works between 20 and 100 Hz, and our woofers between 90 Hz and 9 kHz. In that case, the frequencies between 90 and 100 Hz are reproduced twice, which would give rise to the undesirable booming effect, the characteristic BOOM, BOOM, BOOM that we sometimes hear from ours and that affects the whole room. Bear in mind that bass waves are omnidirectional, which can cause "collisions" between them. We can solve this with a crossover, a frequency-dividing filter in which we can set the woofer's cutoff at 100 Hz, thereby completing the audio spectrum without the frequencies interfering. Most current models incorporate a crossover so that they can be adapted to any stereo or multichannel system. This allows us to achieve a complete sound image and make the sound more natural and true to life. Also, since the crossover does not cut off the frequency abruptly but progressively, reducing the level in dB, we should set that crossover point at a frequency slightly higher than the one set on the subwoofer.

CONNECTIVITY

As far as connectivity is concerned, most have two types of input. The so-called Line Level Input or LFE (Low Frequency Effects) inputs carry a preamplified-only signal from the amplifier or receiver, which delivers the .1 channel of 5.1 or 7.1 recordings and the frequencies of the other channels below the crossover point selected on the AVR. This is the one used most often, with the highest-quality coaxial cable possible to avoid interference from other radio frequency signals. If there are two LFE inputs (for an 11.2 configuration, for example), manufacturers usually recommend the left one, although in my experience the result is usually exactly the same with either.

The other inputs, generally called Speaker Level Input, are for amplified signals and come directly from the binding posts of an amplifier that has no LFE output; they are located next to the corresponding outputs to which the loudspeakers are connected. The subwoofer picks up the frequencies we select on the crossover (which we have already discussed) and also passes the amplifier's signal through its outputs to the loudspeakers. On some models it is possible to choose which frequencies reach the loudspeakers by means of a mid- and high-frequency crossover.

PLACEMENT

It is precisely the omnidirectional nature we referred to earlier that can help us with placement. This means we don't need to have the subwoofer in front of us, since its position is influenced by other factors that are also important. It is essential that it has as much free distance as possible, so the ideal starting point is the longest diagonal of the room. Be careful, because placing it in a corner will generate sound waves that bounce off the walls and floor, providing an average of 9 dB of extra gain (3 dB for each of the walls and another 3 for the floor). As we move it away from the corner, this extra gain will diminish, but it can still be present up to one or two meters away. Beware if you still have a CRT, since electromagnetic waves will be generated that interfere with the display, which will constrain placement.

Once it is in the corner and angled along the diagonal, we can start our tests, always beginning with multichannel. There is a huge range of movies to choose from, but it is a mistake to think that a movie full of explosions and crashes is the ideal way to test our subwoofer. Explosions have such low frequencies that they are "easy" or "effective" to reproduce under any circumstances. The aim is for the result to be as natural as possible, so we should look for a title with a constant presence of bass (for example, a soundtrack with jazz-like rhythms), without big rises and falls in the low frequencies. Dreamgirls could be a very interesting option, as it has musical numbers with a constant bass presence, and the BD edition offers extraordinary sound. Million Dollar Baby could also serve our purposes, since the fights also let us check how spectacular our guest can be. Don't wait until the movie ends to make the necessary changes. It is imperative to move the loudspeaker during the film, and even to note down the changes. Obviously, subjectivity also plays a part, since not everyone likes the same bass intensity. The first move should start from the corner, angled along the diagonal, moving it away from the wall. This will progressively reduce the 9 dB we mentioned earlier, resulting in less bass.

When working with two subwoofers, the approach should be similar, placing the two loudspeakers facing each other along the longest diagonal of the room and gradually reducing the distance. Once we have found the setting we like, we can vary the orientation, but bear in mind that because bass waves are omnidirectional, these changes will make very little difference. It is also important to test all changes from our usual viewing position, since the tones generated will not sound the same throughout the room, which can lead to delays or phase losses. Let's not forget that these waves will hit the floor directly, so if we have neighbors, and also for strictly acoustic reasons, it is particularly worthwhile to use carpets or rugs under the subwoofer, especially with models whose driver faces the floor.

STEREO AND THE SUBWOOFER

When it comes to stereo use, the question arises immediately. If we have been listening to music for so many years with two loudspeakers, in strict stereo, is it necessary to use a subwoofer now? The answer is that, in the strictest sense of the word, no, it is not necessary. As always, and the same applies to multichannel, you have to listen in order to decide. At Sarte's Alta Fidelidad Absoluta event held in Valencia last May, we had the opportunity to listen to a REL Accoustics setup with Vienna's Grand Beethoven, in which two subwoofers were placed facing each other on a diagonal. It left me with the feeling of listening to music in a different way, perhaps more complete (in the most frequency-related sense of the word), but somewhat colored compared with the original recording. We felt it was an option that home theater fans will like, but that will surely displease the most purist audiophiles and the stereo diehards. Let's also take into account our loudspeakers' ability to deliver bass. Although they work on different frequencies, it is clear that loudspeakers with a somewhat limited bass presence will benefit much more from the use of a sub than loudspeakers that handle powerful bass.

As for setup using automatic systems such as Audissey, which are increasingly common in new receivers, it has its pros and cons. Bear in mind that the approach is exactly the opposite of what we have done so far. Since with Audissey you have to set the distance and then carry out the calibration, the result is that the system is really effective when, for aesthetic or space reasons, we have to leave the subwoofer in a particular location. Only in that case can Audissey help us get the best performance. But in any case, the ideal environment is one in which we can move it as we please so that we can set it up ourselves.

The subwoofer is undoubtedly very complex to set up. For that very reason, it can also be very rewarding when its arrangement and placement are correct. Bear in mind that using a subwoofer is not just about increasing the presence of low frequencies, but about giving the whole system coherence and harmony.