Polar pattern of typical speakers and microphones found in smartphones
After doing some research I've found that:

MEMS microphones have typical diaphragms of 0.5 mm (I've checked Knowles which seems to be one of the world leaders in the area):

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Assuming that we are working with a MEMS microphone with a diameter of 0.5 mm and a frequency of 18 kHz, from the frequency and the speed of sound we can determine λ = speed/frequency = 340/18000 = 0.018888888888889 m. Substituting in the formula ka = 2πa/λ we find that ka = 0.166319611072401 which means that a MEMS microphone with such a small diaphragm is omnidirectional at 18 kHz.

The typical speaker used in smartphones/tablets is the micro-speaker. These devices according to this link have a typical size of 11-mm × 15-mm.

Assuming that we are working with a microspeaker with a diameter of 11 mm and a frequency of 18 kHz, from the frequency and the speed of sound we can determine λ = speed/frequency = 340/18000 = 0.018888888888889 m. Substituting in the formula ka = 2πa/λ we find that ka = 3.659031443592817 which means that a microspeaker operating at 18 kHz is very directive. In general a speaker with a ka >2 is considered to be directive according to this link

For those who want to know how does the ka affects directivity see this image: