Elephants can create and detect infrasonic sounds—sounds of frequencies lower than about 20 Hz—which can travel long distances. They create these sounds using the cavity that extends from their larynx to the tip of their long trunk. Consider this cavity as a cylindrical air column, of length 4.78 m

f = 17.26 Hz

Explanation:

To find the fundamental frequency of the vibration produced by elephants, approaching the trunk as a system closed at one end and open at the other, the fundamental wavelength is

            λ = 4 L              fundamental

            λ = 4L / 3          third harmonic

In this case the melt wavelength is

               λ = 4 4.78

               λ = 19.12 m

Now let's use the ratio of the speed of sound to the frequency and wavelength

                 v = λ f

                 f = v /λ

                 f = 330 / 19.12

                 f = 17.26 Hz

This is the lowest frequency elephants can produce

the answer is a)

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