Editor's note: This article and the subsequent articles provide an introduction to Sound. It is excerpted from the book Audio in the 21st Century published by Intel Press. This excerpt is from Chapter 2 of the book. All material is copyright Intel.
If a tree falls in the forest, does it make a sound? Because it is a macroscopic event free of quantum mechanical effects, the answer is yes. But what pre-cisely is sound?
A glib answer is that sound is what we hear. But then what is hearing? Hearing is the perception of sound. These circular definitions are a bit worri-some, but have no fear; we'll spend some time in this chapter and the next describing the physical characteristics of sound—acoustics—and how humans perceive that sound—psychoacoustics.
This chapter has a fair number of equations and graphs in it. These are intended to provide a com-prehensive reference for audio engineers. If you are reading this book just to become familiar with dif-ferent audio technologies, it may behoove you to skip to the summary at the end of the chapter. You should still be able to parse the remaining chapters without too much difficulty. However, I strongly en-courage everyone to try to make it through this math chapter. Like the foundation of a house, a strong understanding of the fundamentals is required to support future concepts.
Intro to Sound
Sound is simply vibrations in matter. As such, sound requires a medium through which to propagate. By comparison, light requires no transport medium and can travel through vacuum. No matter what you may hear in most science fiction films, sound does not travel through a vacuum, no matter how big the ex-plosion is.
Audible sounds are those vibrations that we hu-mans can hear. Most of the sounds humans hear are transmitted through the air, although audible sounds can also be transmitted through liquid and solid matter. Airborne sound travels in the form of longi-tudinal waves.
Consider a long spring or Slinky' stretched out along a table. One end of the spring is fixed in place. The other end is attached to a crank-driven piston. If the crank is quickly turned, the piston compresses the coils of the spring nearest it. This region of compression propagates to the other end of the spring. As the crank continues to turn, the pis-ton moves away from the spring, creating a region of decreased coil density in the coils nearest it. Again, this region of expansion or rarefaction propagates along the spring. This process is shown in Figure 2.1.
Figure 2.1 Longitudinal Waves in a Slinky'