Wavelength = speed/frequency = 2/15 Hz = roughly 0.133... Hz.

2 400 000 000 Hz

Let me put it this way:

20 years ago, I bought a 2-Hz microprocessor for $2.

An octave is a factor of 2 in the frequency. So, just divide 1200 Hz. by 2, then divide the result by 2 again.

Higher notes have higher frequencies. A typical tuning fork vibrates at 440 Hertz. That's the tone of the A above middle-C on a piano. The A one octave higher is 880 Hz (2 x 440 Hz). The A one octave above that is 1760 Hz (2 x 880 Hz). The A below middle-C is 220 Hz (440 Hz ÷ 2), the next lower A is 110 Hz, and so on. The lowest note on a piano is 27½ Hz, and the highest is 4186 Hz.