and Test Tones
Today, surround sound formats require speakers to be purchased in packs of five, seven or nine speakers, plus a subwoofer (the 5.1, 7.1 and 9.1 formats, respectively). Unfortunately, the overall budget one would spend on speakers hasn't much changed over the years, and if it has increased, it hasn't increased proportionally to the number of channels now supported by the cinematic industry! This situation inevitably leads speaker manufacturers to design their systems around inexpensively small enclosures, complemented by a unique subwoofer.
A subwoofer - or sub - not only is needed to play the low frequencies, but also to output the frequencies that the smaller satellite speakers are unable to reproduce. Such a configuration works quite well in practice, because the human hearing is unable to localize the low frequencies precisely: as long as the subwoofer does not enter the mid-range frequency territory, it can effectively fool the ear to believe that all the sound, including the deep basses, comes from the small satellite speakers.
To gain a better understanding of sound localization in the lower frequencies, please check our Subwoofer Imaging Test.
Using a subwoofer to supply the missing midrange will shift the perceived sound location from the satellites toward the subwoofer. Sounds with a predominant midrange component - such has vocals - won't play from a well defined spot anymore, but from an undefined position, influenced by the position of your sub in your listening room.
Systems that do not use their sub only for what a subwoofer is meant to, will always compromise sound imaging.
In order to check if this is your situation, disconnect your satellite speakers, then play our next bandlimited noise samples (files courtesy of wavTones).
The first file packs the typical subwoofer frequencies (20-80 Hz) into one single bandlimited noise file, and will be used to calibrate sound levels first: play the file, then adjust the volume so that your subwoofer gently rumbles at a moderate level.
The second file exclusively consists of midrange frequencies that should be played by your main speakers only (120-300 Hz). If your sub remains silent while playing this second file, it successfully passed the test! If not, you just confirmed that your subwoofer outputs midrange frequencies as well.
Some manufacturers compromise the performance of their speaker system in order to achieve the smallest satellite speakers possible. Lower midrange frequencies will now become too low to be output from the tiny satellites, yet too high for the subwoofer. These frequencies simply won't play, or play at a much reduced level.
Beware! Those systems often offer "signature" sound - bright highs balanced by round basses - that will impress you at first. They won't offer you the performance you'd expect from quality speakers though. This will be particularly noticeable when playing vocals: the voice will sound thin, and lack warmth, presence and realism.