and Test Tones
Although the sound pressure does not decrease much over the distance covered by your ears, level differences primarily exist because our head acts like an absorber and projects its shadow over the ear that is farther from the source. To shadow sound though, the obstacle must have dimensions comparable to half the wavelengths to be absorbed. Applying this physical principle to the dimensions of the human head makes absorption only possible at frequencies higher than 800 Hz. For lower frequencies, inter-aural time differences are used instead.
Inter-aural delay shifts the sound received by one ear as opposed to the other and introduces a phase difference between the ears. Phase denotes the particular point in the cycle of a sound or a waveform. Our ears are very sensitive to phase differences, more than the inter-aural delay itself: inter-aural delay is only perceptible when the sound reaches your ears, while phase differences still exist afterwards.
Unfortunately, phase differentiation fails when the frequencies are too low. For example, at the bottom of the human hearing range, the wavelength is almost 20 meters long. What does 20 cm - the average distance between our ears - represent compared to 20 meters? Not much, as a pure tone waveform won't significantly change over 1/100th of its cycle. As both of our ears will perceive a very similar signal, the sound will simply be interpreted as coming frontally, even though it comes from your side.
So, as the frequency drops below a critical frequency - usually around 80 Hz - it becomes very difficult to determine a sound's location. This current page tests the exact frequency when this phenomenon happens, in your particular audio setup.
The "Left" version plays the sweep from your left speaker; the "Right" version from your right speaker. The voiceover plays from both speakers equally, and should be perceived as coming from the center point between your speakers.
Bypass your subwoofer during this test: we want to use your main speakers across their fullest frequency range in this test.
As the sweep decreases, its perceived location will move from its associated speaker to an undefined position, possibly in between your speakers with the voiceover sitting on top of it. Note the frequency when this happens: this is the critical frequency we were talking about earlier. Repeat the test with the other speaker to confirm your result.
If your main speakers are unable to play frequencies that are low enough to cross the critical frequency, the sweeping sound will vanish before it moves away from its associated speaker. By adding a subwoofer into the configuration, you will add the missing frequencies, but some of them - in the upper bass register - will be perceived as coming out from your subwoofer, not your main speakers, distorting the stereo imaging of your audio system.
Contrarily, if your subwoofer crossover frequency is lower than the frequency you've just measured, you can be assured that your subwoofer won't distort your system's stereo imaging.