Generating inaudible sound waves using MATLAB I want to generate inaudible sound waves (above 18kHz) with 48kHz sampling rate using MATLAB.

Generating a sound wave with 18kHz frequency makes it inaudible. 



I've inserted the data in the 18kHz band in the frequency domain, converted it into time domain signal.
Then I've duplicated the signal, concatenated it, and played it. 
As expected, the played sound was inaudible.

The problem is,
I want to play the inaudible sound with an interval. 
(playing sound wave for 1.75ms and having a silence period for 4.17ms 
 repetitively)



Because of the inserted silence period, the concatenated signal is no longer inaudible because new frequency components are added.



How I can transmit inaudible sound with time interval?
Eventhough the exact cause of those wide-spread line spectra is not very clear to me from the supplied information, it's most probably due to the on-off switching implied by the silence period you have added in between your message signals. 

The on-off waveform is an implicit operation you multiply to your signal. Which has a value of $1$ during the sound interval and a value of $0$ during the silence interval. The fundamental period of that waveform is roughly 200 Hertz.

More technically, the on-off switching with a given duty-cycle will shift the message spectrum by the amount of k-th harmonic frequency for each significant harmonic present in the continuous Fourier series of the on-off waveform.

In fact this is a method of realizing an analog modulation algorithm. Multiplication with a rectengular wave creates a multitude of harmonic centered up-mixed spectrums, of which only one is selected by a suitable bandpass filter.

Therefore as a solution offer, you could try the following:

1- Generate the 18kHz (already bandpass) message signal in time domain.

2- Insert the silence periods accordingly.

3- Apply a BPF (bandpass filter) centered around 18Khz.

When implemented carefully this should yield a solution to your problem I guess. Distortions in the message signal can happen however.