What is the fundamental frequency of a particular medium ?

This is the minimum possible frequency generated by the given medium

Explanation:

Let say the medium through which sound is produced is a hollow pipe closed at one end

Now at minimum possible situation the wave that will form in it will be of quarter length so we can say

so we have

now we will have

so fundamental frequency of closed pipe will be

similarly if pipe is closed then in that case the wavelength will be half inside the pipe

now we have

in this way we can say it is the minimum frequency of sound produced by the source

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truck y has a greater velocity than truck x.