The interaction of a loudspeaker and a reflective wall or floor can lead to cancellations at certain frequencies.
At a distance of 1/4 wavelength (λ/4) of the the loudspeaker from a reflective surface, the sound travels 1/4 wavelength to the surface, another 1/4 wavelength back, for a round trip when it meets the loudspeaker again of 1/2 wavelength. The reflected sound and the sound emitted by the loudspeaker are 1/2 a wavelength out of phase when they meet, (180 degress out of phase) resulting in theoretically complete cancellation.