The speed of ultrasound is approximately 330 m/s in air and 1500m/s in water. Many materials exhibit surface-acoustic-wave velocities in the 300-1500 m/s range. For this reason, air appears to be the ideal coupling fluid for exciting and detecting flexural waves, which are generally slower than surface acoustic waves, and shear waves. Air coupling may also be ideal for launching and detecting the quasi-longitudinal phase velocities, such as balso wood (800 m/s). In this paper, we describe an experimental, air-coupled, ultrasonic measurement system that can be used to study the propagation of MHz signals in wooden plates ranging in thickness from 3-25 mm. Also, we report experimental measurements of the amplitude of transmitted, narrow-band 0.5 MHz signals as a function of the abgle of incidence for different grain orientations. We compare the experimental results for balsa and poplar with results obtained on plates made of poly-methyl-metacrylate (PMMA). Finally, we interpret our results using a slowness-surface approach.