Our ears recognize repeated disturbances in the air as sounds. Disturbance is nothing but a change in pressure, imagine pressing and releasing a spring. A single press or a soft press and release will just be change in pressure - the kind you feel when in an aircraft. When you release the pressure suddenly and let it vibrate is when you create repeated disturbances. This repeated vibration can be detected by our ears as sound. How fast or slow they vibrate can also be detected and it is called the frequency or tone.
|Sound waves as change of air pressure.
Sounds in real world are not as simple as this. They are usually not pure frequencies. Instead, they are actually a mixture of many different frequencies. The sound waves are complex, but they can always be broken down to a combination of different simple waves (using maths). Try it yourself, in the small application below try changing the sliders and see the different kinds of waves you can create! (You may need to allow the application to run when the browser asks for it.)
Want to hear how pure tones sound and how it is when sounds are mixed? Try the demonstrations below. (You may need to install this very useful Wolfram plugin though, but it is really interesting.)
The sound produced from instruments have another property called harmonics. When an instrument tries to make sound of one frequency, it is usually mixed with sounds with many other smaller frequencies. These are at frequencies that are factors of the frequency it is trying to create. It gets even more complex from here. The way different instruments produce their sound leaves its mark on the sound. The way the sound starts, ends, the way the strength of sound varies, and so many other things. These are what make each instrument unique!
How can we recreate such complex sounds? It is indeed not easy. Here is what people do, put simply:
- Record and analyze the sounds produced by instruments.
- Try and break it up into as many simpler parts as possible. Mathematics is one thing that is used heavily here.
- Create each of the parts using computers and electronics.
- Mix them and fine tune them with some more wizardry to get the final sound. Something similar to what you did if you tried the demonstrations above, only much more involved!
Below is how a simple string sound is synthesized. Click on each of the sounds below to play. It's amazing how the simple tones gradually change to sound like an instrument!
|Step 1: Oscillators produce two waves
|Step 2: The waves and modified using something called "pulse width modulation"
|Step 3: They are now mixed
|Step 4: Wave appearance (envelope) is changed to match that of a plucked string
|Step 5: Filtered
|Step 6: Filtered again