Before delving too far into the physics of calculating sound waves (studying things like the Doppler Effect, for example), lets address some basics of sound. Sound is a wave—a longitudinal wave of pressure that travels through compressible medias (i.e., solid, liquid, gaseous, or made of plasma). There is no sound in a vacuum; by definition, a vacuum is a space free of any particles or matter. Thus there in a vacuum, there is no media through which sound waves can travel. Following are some characteristics of sound:
- Sound travels in longitudinal waves. When drawn these are also called sinusoidal waves, a visual example of which is shown in (we will cover this in more detail in a different section).
- Sound waves have frequency; that is, the pitch of sounds goes up or down.
- The amplitude of a sound determines its volume (loudness).
- Tone is a measure of the quality of a sound wave.
- Sound travels faster in a hot meduim, or in a solid. It also travels faster at sea level (where air pressure is higher).
- Sound intensity is the energy transmitted over a certain area. Intensity is a measure of the sound's frequency.
- Ultrasound uses sound waves with high frequencies to see things normally hard to detect, like tumors. Animals, like bats and dolphins, use ultrasound (echolocation) to navigate and locate things. Ships also use a similar technique (known as SONAR) to locate things underwater. (This point will be discussed further in a more advanced Atom. )
Sound Perception
Every sound wave has properties that define its frequency, such as wavelength, amplitude and intensity. Calculating these properties is outside the scope of this atom and will be addressed later. For now, it is important to know the basics of sound. As with light waves, sound frequencies have a range. Each living creature has a different level of sound perception. For example, consider the following examples of sound ranges (in Hz, Hertz):
- Humans 20 - 20,000 Hz
- Dogs 50 - 45,000 Hz
- Bats 20 - 120,000 Hz
By this comparison, humans have a relatively low sound perception.
Sound Speed
As mentioned previously, the speed at which sound travels depends on the media through which the sound is traveling. It is much faster in a solid than in a liquid or gas. The general formula for calculating the speed of sound is given as:
where K is the coefficient of stiffness of the material (also called the Bulk modulus) and p is the density of the material. We will examine this further in another section. Generally, the expression 'faster than the speed of sound' refers to 344 m/s. is an image demonstrating a plane moving faster than the speed of sound. This general measurement is taken at sea level—at a temperature of 21 degrees Celsius under normal atmospheric conditions.
Breaking the Sound Barrier
This familiar image is of a plane that is moving faster than the speed of sound.