Sounds are just vibrations that we can hear. These vibrations happen when some object moves up and down or back and forth rhythmically. Usually, we hear sounds in air, but we can also hear sound in solid objects. Put your ear to the wall sometime. The air vibrates sound waves similar to the kind of waves you see on the ocean. The difference is that sound waves go back and forth whereas water waves go up and down. We hear different sounds depending on how far apart these waves are. The different sounds are called frequencies. The ear can hear sounds as low as 20 waves per second up to about 20,000 waves per second. For a musician, the note A on the piano is 440 waves per second. Most sound is a mixture of many frequencies. Some mixtures sound good, like a violin, and some not so good, like a jackhammer.