 The horizontal axis is time, and each division is worth 5 milliseconds as displayed here. So you can see that the period, or the time that it takes for a sound wave to travel its wavelength, is this. The frequency is one on the period, or in this case 250 Hz. The frequency of a sound wave is referred to as the pitch of the sound, that is for example C, or Bb, or G sharp. Simply speaking, music is simply sound waves using pitch, or frequency, and dynamics or loudness and softness, or amplitude, organized in a rhythm, or a mathematical sequence, all given in a set of instructions. No wonder many scientists are also musicians. The famous ones are Brian May, guitarist for Queen, who is an astrophysicist, and Brian Cox, the former keyboard player for Zareem. He's a particle physicist. All instruments can be used to probe how sound is generated. You could do this with percussion, wind or brass instruments, but as I'm a cello player, we'll look at stringed instruments. In a nutshell, when I vibrate a string with my bow, the oscillations of the string create pressure waves, or sound. The frequency of the string vibrations are the same as the frequency of the pressure waves, or the pitch of the sound generated. You can change that frequency by changing the tension of the string, by winding the string tighter with the pegs, or by changing the length of the string by pressing the string with a finger into the fingerboard. The length of the vibrated string is the distance between the bridge and the depressing finger. The string's frequency also depends upon the mass per unit length of the string. There are four strings of different diameter or mass, so by vibrating or bowing different strings, while changing the length of the strings with your finger, you can generate different pitches. You can get the same frequency from a string by plucking it or bowing it with different techniques. This frequency is called the string's natural frequency. That a taut string, fixed at two ends of given length and mass, generates a particular natural frequency is due to the creation of standing waves. We'll describe standing waves later. But for now, understand that the natural frequency of a vibrated string gives rise to a pressure wave of the same frequency. As a reference, what musicians call middle C, that is C of the middle octave of an 88 key piano, is a pressure wave of frequency to 61.6 Hertz. And those of us who play stringed instruments have to be careful of placing the finger to get the correct string length so that we create the right frequency, or so that we're in tune. We check to see if we are in tune by measuring the frequency of the sound, using effectively a digital oscilloscope or tuner. Until the 20th century, musicians didn't have oscilloscopes, to check to see if they were playing the right pitch or were in tune. In the Renaissance and early Baroque period, someone would stand up and play a note, judging it by ear, call it a middle C, even though it might not have the frequency of 261.6 Hertz. And everyone would mimic that pitch and adopt it as C. But in 1711, the tuning fork was invented. A tuning fork is a metal fork that vibrates with unique or natural frequency. Can you see how the tuning fork, initiated with a tap, vibrates with a fixed frequency? These vibrations create a pressure wave of the same frequency or a standard pitch by which musicians could tune their instruments. The tuning fork standardized pitch. Again, we'll talk about tuning forks later when we introduce standing waves and resonance. Next time, we'll look at microphones in a little more detail. It ends up the ears of microphone that converts sound waves to electrical signals that are passed to the brain. Then we'll look at loudspeakers and a simulation of how air responds to pressure waves.