Re: for dogs only
Well, actually, it is important to have those sounds. I'm not an audiophile by any means, in fact I actively try to debunk the bollocks the audiophools punt about (previous comments include dipping audiophools in liquid nitrogen to lower the noise they emit) but in that particular instance they're right, supersonic and subsonic frequencies can be important.
It's basic theory (and practice), if you mix two fundamental frequencies together you will have an output that contains the two fundamentals *and* the additive and the subtractive frequencies of both.
So even though the fundamental frequency of such a sound is outside your hearing range, it will heterodyne with other frequencies and produce within your hearing range, they contribute to a fuller, more rounded sound.
For an example of how harmonics can make something sound different, grab a copy of audacity and have it generate a sine wave and a square wave at 440Hz. Mute one track and listen to the other.
Then swap tracks.
The square wave track will sound 'smoother', more rich, a fuller sound. performing a fourier transform on the square wave will reveal that it's rich in harmonics.
A little light reading, might want to take a pencil, notepad and calculator: