Here is some data on the effects of placing a measurement microphone in various positions on or near a boundary. This is the frequency response of a 1/2-inch diameter measurement microphone, compared to the frequency response of a flush-mounted 1/2-inch diameter pressure-field microphone aiming up in an "infinite" boundary such as a floor. The loudspeaker sound source was 30 degrees above the plane of the boundary.

Using a pressure-field mic, the flattest frequency response up to 12 kHz occurred when the mic was .250 inch above the surface, aiming down with the body of the mic perpendicular to the surface. Using a free-field mic, the flattest response occurred when the mic was angled down 45 degrees with the edge of the mic capsule touching the surface.

When you place a 1/2-inch diameter pressure-field mic closer than about .250 inch to the surface, with the body of the mic perpendicular to the surface, the acoustic inertance (mass) and acoustic compliance of the air volume under the mic diaphragm act as a resonator to boost high frequencies above 10 kHz. This effect is smaller if the diaphragm diameter is smaller.