Little practical difference exists between down-firing and front-firing subwoofers, according to the Audiophile Review; under most circumstances, it's hard to tell the two types apart just by listening. Other factors, including the quality of the drivers and cabinet and the subwoofer's location in the room, affect audio performance more strongly.
Inside the subwoofer enclosure is the speaker driver, consisting of the cone, frame, coil assembly and magnet. The cone vibrates, producing sound waves in the air. In a down-firing design, the driver points down through a hole in the subwoofer cabinet, toward the floor. In the front-firing design, the sound comes from a cabinet opening on one side, with the driver facing out.
Subwoofers and Low Frequencies
Your ears are designed to locate the direction from which a sound is coming, although this ability works only for middle and high frequencies, not the deep bass from a subwoofer. The rumble from a subwoofer appears to surround you from all sides. This effect also applies to the type of subwoofer: Because you can't tell direction from low-frequency sound, the difference between a down-firing subwoofer and a front-firing one becomes nearly impossible to distinguish.
Although front-firing subwoofers offer no great advantage over the down-firing type, it still pays to spend time listening to different models in an audio showroom. Not all speakers and not all ears are created equal; to find a subwoofer that suits you, try several out and make a decision based on what you hear. You may find a front-firing unit, for example, that sounds good to you. When you audition speakers, bring one or two of your favorite CDs; since you know the music, your ears will do a better job of picking a winner.
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Effects of Positioning
Optimize a subwoofer's sound by trying different locations in your listening room. Move the speaker to a particular spot, then sit where you usually do and listen carefully to the bass; if it is strong and clear, leave the subwoofer in that location. If you place the unit near a wall, the bass frequencies tend to sound louder; the effect is even stronger in corners. For a subwoofer whose bass is already strong, locate it away from walls and corners to prevent bass from "muddying" or overwhelming other sound. Some subwoofers have a control called boundary gain compensation that automatically adjusts the speaker's output, helping alleviate the problem of "boomy" bass near walls.