Back in the days of stereophonic sound, one would spend a significant amount of money to buy a single pair of good sounding speakers, opting for the biggest size one's budget could afford. When it comes to speakers, size does indeed matter: big speakers do not only play louder but also benefit from an extended bass frequency response.
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.
Unfortunately, many sound systems nowadays are built on satellite speakers that have become too small to output the lower midrange frequencies. Some systems will rely on their sub to play these missing frequencies; others won't play these frequencies at all (check our second test below).
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?
|Bandlimited Noise 1
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.
|Bandlimited Noise 2
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.
A second problem relates to the existence of a possible hole in the spectrum, right in the middle of your subwoofer and your satellite speaker's frequency responses.
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.
For this test, we are using a filtered noise sweep. The level of the sweep should remain even, from the lowest frequency (50 Hz) to the highest (400 Hz). Any change in level should be interpreted as a mismatch between your subwoofer and satellites. Worse, if the sound suddenly disappears, your system suffers from a prominent hole in its frequency response!
If this happens, you can try flipping the phase of your subwoofer - most subwoofers offer a phase-reversal switch - and listen to see if it helps. This will only work if the frequency dip is caused by overlapping frequency responses coupled with a destructive interference between your subwoofer and main speakers. This won't help in the case of missing frequency components, such as described in this section.
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