If sinusoidal wave form is a pure frequency, any other wave form is made out of the fundamental frequency, and many other frequencies having a direct relation with the fundamental: Harmonics.

The harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency. Harmonic frequencies are equally spaced by the width of the fundamental frequency and can be found by repeatedly adding that frequency. For example, if the fundamental frequency is 25 Hz, the frequencies of the harmonics are: 25 Hz, 50 Hz, 75 Hz, 100 Hz, etc.

Harmonics are important to us because square waves are made out of a fundamental frequency and many harmonics of this fundamental.

For example, a square wave of 10kHz, with a duty cycle of 50/50 (meaning the top of the square is equal to the bottom of the square), will have a strong 10kHz signal, and 30kHz, 50kHz, 70kHz, 90kHz, 110kHz, with less and less signal, and so on. On this type of wave form, only odd harmonics are present, even are not.

Harmonics are interesting because with a low frequency square wave output, a NEOClark can reach much higher frequencies.   In the precedent example, the 10kHz could reach:

Bordetella pertussis: 329.85kHz to  332.25kHz on the 330kHz with its 33th harmonic (only 1/33 of the fundamental signal)

Gaffkya tetragena 344.85kHz to 352.50kHz on the 350kHz with the 35th harmonic (only 1/35 of the fundamental signal)

Measles antigen 369.50kHz to  373.00kHz on the 370kHz with the 37th harmonic (only 1/37 of the fundamental signal)

And so on...