f.5 physics sound

Treat the ground as a "mirror" that reflects sound wave coming from the speaker. Interference is thus produced between the direct sound wave and the reflected sound wave. This situation is the same as another speaker (the image speaker) is placed 20 cm below the ground (i.e. the mirror).

Because the "image" of the speaker (the sound source) is 20 cm below the ground, i.e. separation between the two sound sources d = 40 cm

Use equation: d.sin(theta) = m入 for constructive interference to occur.
where angle (theta) is the angular position of a maximum, m is an integer and 入 is the wavelength.

Hence, at point Y, m = 2 (since m = 0 at X, and there are 3 maxima from X to Y).
tan(theta) = 40/100
angle (theta) = 21.8 degrees

Therefore, 40.sin(21.8) = 2入
入 = 7.43 cm

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