Long waves are canceled more.
Work Step by Step
When two speakers playing out-of-phase tones are brought very close together, the low-frequency, long-wavelength waves experience more cancellation. For example, when the speakers are 5 centimeters apart, all waves longer than a meter long are about 170° out of phase, but shorter wavelengths are not as far out of phase, and may even be in phase. In summary, the low frequencies (longer wavelengths) suffer the most volume reduction in this experiment, making the music sound "tinny", as though the bass were turned all the way down. You should try it out, and demonstrate the selective cancellation of the low-frequency waves.