Sub-audio signals are used as controls (since we can't hear them) in synthesis to produce effects like vibrato. The lowest 32' organ pipes also produce fundamental frequencies below our ability to hear them (the lowest, C four octaves below 'middle C' is 16.4 Hz) we may sense the vibrations with our body or extrapolate the fundamental pitch from the higher audible frequencies (discussed below), but these super-low ranks are usually doubled with higher ranks which reinforce their partials.
Example: A440 Hz (the frequency many orchestras tune to) in a dry, sea level, 68°F room would create a waveform that is
2.5 ft. long (2.56 = 1128 (feet/sec) / 440). Be certain to measure the speed of sound and wavelength in the same units. Notice how if the speed
of sound changed due to temperature, altitude, humidity or conducting medium, so too would the wavelength.
One particularly interesting frequency phenomenon is the Doppler effect or Doppler shift. You've no doubt seen movies where a police siren or train whistle seems to drop in pitch as it passes the listener. In actuality, the wavelength of sound waves from a moving source are compressed ahead of the source and expanded behind the source, creating a sensation of a higher and then lower frequency than is actually being produced by the source. This is the same phenomenon used by astronomers with light wavelengths to calculate the speed and distance of a receding star. The light wavelengths as stars move away are shifted toward the red end of the spectrum, hence the term red shift .