The Audio File: The Low Down

The Low Down

Well, I finally added a pair of subwoofers to my music system. This entailed considerable personal risk on my part, as my extensive experience over the past fifteen years with this most wily of species, Lowbassus Frustratus, has resulted mainly in premature hair loss. With the recent proliferation of mid-fi home theater systems, a separate subwoofer has become more common, but the task of enhancing the low bass performance of a well-tuned music system is considerably more difficult than that of conveying the crashes and booms of the latest Hollywood blockbuster DVD.

Pick up any audio/video magazine on the newsstand, and you're likely to find an article about subwoofers. Recently I spied an entire magazine devoted to the subject, including reviews of 25 different models; a technologically savvy Danté must have threatened the editors with banishment to some special circle of hell in order to coerce them into undertaking such a project. Some of these articles have offered useful information regarding where to place a subwoofer and how to adjust its controls for the "most bass" or "smoothest bass" (which will never occur at the same placement or settings in a real-world room). However, they invariably focus on the interaction between the subwoofer and your listening room's "resonant modes--which are a function of the room's dimensions--rather than on the interaction between the subwoofer and the main speakers. This approach may very well result in "good bass" from the subwoofer, but will rarely result in that elusive holy grail of a "seamless transition" from the subwoofer to your main speakers.

In order to integrate a subwoofer into an otherwise capable music playback system, you must think like a speaker designer. This means quickly discarding some of the most common myths prevalent in subwoofer lore, e.g. (1) you only need one subwoofer, (2) bass is omni directional so you can put a subwoofer anywhere in the room, and (3) for the most bass, put the subwoofer in the corner.

You'll notice that I opened this article by mentioning that I added a PAIR of subwoofers to my system. Unless you're one of those historical performance fetishists who play only mono recordings, two subwoofers will always outperform one, especially with classical music recorded in large acoustic venues such as concert halls or opera houses. Why? When we listen to live music, our perceptions of the volume and dimensions of the performance space are derived from the complex pattern of reflections and reverberations within that space. Because we have two ears, we rely on differences perceived by the left and right ears to make sense of what we hear, especially in the lower frequencies. A well-produced stereo recording will preserve those psycho acoustic cues that allow us to perceive the volume (as in spatial dimensions, not loudness) of the original recording venue.

Now, if we "mono" all the low frequency energy of such a recording by re-routing the left and right bass to a single subwoofer, we destroy most of those precious psycho acoustic cues: at some frequencies the left and right channel bass signals will add together and reinforce each other, while at other frequencies they will cancel each other; in all cases the mono subwoofer will be playing a signal that IS NOT IN THE ORIGINAL RECORDING. In contrast, by using a dedicated subwoofer for each stereo channel, we can preserve and enhance those cues.

It takes only seconds to demonstrate this effect. Play a selection of music with two subwoofers connected in true stereo, then disconnect the cable from the right subwoofer and plug it into the second input of the left subwoofer. Upon re-playing the same selection through the single mono-bass subwoofer, two things occur: (1) the bass range sounds thick, heavy, opaque, and "dead," and (2) the listener can much more easily identify the location of the subwoofer in the room. The latter problem is not at all unexpected once we understand that a mono-bass subwoofer cannot possibly blend properly with the separate main left and right speakers since it's no longer reproducing the actual low frequency content of either channel! For this reason, if you must use a single subwoofer due to budgetary or physical placement constraints, CONNECT ONLY ONE CHANNEL, not both (I generally use the right channel, since the double basses are usually "over there" in most recordings, though this choice is arbitrary). The audible benefits of feeding a single subwoofer with only one channel of the stereo signal have been utterly consistent in my experience; I no longer even bother setting up a mono-bass subwoofer, other than to briefly demonstrate the unacceptable compromises inherent to any such configuration.

With a variety of currently available subwoofers designed to play music rather than explosions, it is indeed possible to integrate subwoofers with a high performance speaker system. Next month, we'll investigate the challenges of optimal speaker and subwoofer placement in real-world rooms.