First of all, a frequency is measured in Hertz. A frequency in theory is how many waves pass a certain point in a certain amount of time. So, basically, the lower the interval, the higher the frequency.
Now, the cool thing is, adjusting the frequency. A higher frequency is a higher pitched tone, while a lower frequency is a lower pitched tone. If you listen to music and hear the bass, the frequency is lower. You can also feel it in your body a lot more than high frequency tones, because generally those are harder to perceive in the first place. The neat thing is, that this allows us to alter the effects of sound techniques by altering the frequency. That being said, you probably see how versatile sound actually can be, and how it can create different effects. Even if you can't hear something, the sound waves are still there and can still have effects on your body
humans can only hear certain frequencies, or a range in between 2 frequencies. Generally, this is about 20 - 20 000 Hertz. Now, as you grow older, your body gets older as well, and you start to have trouble hearing high frequency tones. This is why younger people generally hear a lot better than old people. I was wondering if you speak in a lower voice, if people with hearing problems would understand you better, but I'm unsure (though it makes sense).
This does not mean that higher/lower frequencies do not effect the body! Remember that!
About the ultrasonic: Ultrasonic are simply sounds above 20 kiloHertz (20 000 Hertz) and thus above what humans can actually perceive. Other animals, like dolphins and dogs can perceive these (think, the dog whistle). These range from 20 KiloHertz into several MegaHertz. This is also what they use for sonograms (you understand what this is?) and such. Infrasound is the opposite (below 20 hertz)
Oh, one more thing I'd like to add, and this is particularly useful against Mugi's Radio Frequency/Wave CE. Low frequency sound waves generally move easier through rocks/earth than higher frequency ones. Think about your cellphone, when you move through a tunnel, the reception sucks.
Now, realize that sound can move through any medium. Don't let anyone tell you otherwise! Though we needs some sort of limitations, and it's reasonable to say wind is a good counter to sound if it can be seen (doujutsu) or heard, but at that point it might be too late (depending on how a technique is used and how the effects are). Earth, however, depending on the situation (and the level of understanding of the usage by the opponent) can be a devastating counter as well.
Now, look below:
-Speed of sound:
solid > liquid > gas.
Sound moves fastes of all through solids. Generally, this is because the particles and atoms in earth are more tightly packed than they are in liquids or gas. So, the closer the atoms/molecules, the faster sound moves.
- Elastic properties
There is a catch in between different solids. Sound moves faster through rigid solids (i.e. steel, iron) than other things like jelly. This is because solids that are rigid generally mean that the atoms and molecules have a much stronger force of attraction towards each other than in other solids. This is because of the fact that, once moved, they move back to their original position/form more quickly (and thus resulting in a much higher vibration speed). jelly in this case has much weaker attraction between the molecules, that's why it's so "elastic" (not to be confused with elastic properties), and much more flexible.
- Density
Next to this, there is the density. Now, more dense substances = more mass. More mass=bigger molecules. Because sound is based on kinetic energy that travels from one point to another, moving through denser objects means it has to use more energy, seeing as it has to move (vibrate) larger molecules. This however is used for substances with the same elastic properties.
To explain this take two examples with nearly the same elastic properties. Gold and aluminum (Gold = 10.8 psi; aluminum = 10 psi). While they nearly have the same elastic properties, gold is much denser, so it will travel faster through aluminum than gold.
Another funny thing is, this effect does not quite work the same in gaseous media.
Let's say it's hot hot hot in one side of the room, and cold cold cold in the other side. The logic explanation is that sound would move faster in the cold air (since it's denser), but that isn't the case!
Heat, like sound is a source of kinetic energy. More heat = More kinetic energy = more energy for the sound. This leads up to the fact that they can vibrate faster in hot air.
. I even said wind which is oxygen and that is a gas I'm slow.