Tim Peake uses an alarm clock to help explain how sound travels. When a sound wave meets a boundary it may be:. Whether a sound wave is reflected, refracted, or absorbed depends on the densities of the materials either side of the boundary. If the densities are very different then reflection is more likely.
When sound waves move from one medium to another, there will be changes to the velocity or speed , frequency and wavelength of the sound wave. Scale is important throughout science, from biology to physics, though not all disciplines give it formal treatment.
Scale is relative. A pebble that may seem huge compared to an ant would be tiny next to an elephant! But imagine for a second that you were an ant. That rock would seem like a hill! If you were a mouse, the rock would be like a small boulder.
If you are a human, the rock is merely a pebble. And if you were an elephant, the rock is like a tiny piece of gravel. Scale applies to more than just physical size, though. Almost every quantity can cover a wide scale, be that distance, pressure, time, or even money. If something is two orders of magnitude larger, that would be times the size. The order of magnitude is so important that it is part of scientific notation. For example, 5,,, meters m might be written as 5. A famous video commissioned by IBM runs through the scales of the universe in orders of magnitude, from the largest to the smallest.
While there are certainly exceptions, two quantities are considered comparable when they are within one order of magnitude of each other.
While this may seem like just a semantic difference, in many physics equations having one quantity much smaller or much larger causes the math to clean up to much simpler forms, which corresponds to much simpler physical behavior. While scale is important throughout science, there are few places where it is more apparent than with wavelength and sound. Wavelength of audible sounds, as it turns out, covers a very large range of scales.
On the large end, you have low frequency waves with wavelengths of up to 17 meters 20 Hz , while the highest frequencies can be as small as 1. Compare this to the wavelengths of visible light nanometers , and not only do you find that sound covers a much wider range of scales four orders of magnitude , but it also covers a range that is squarely at the scale of human experience.
For an example of how wavelength determines the behavior or sound, consider living in an apartment with a noisy next door neighbor. Sound is a wave. More specifically, sound is defined to be a disturbance of matter that is transmitted from its source outward. A disturbance is anything that is moved from its state of equilibrium. Some sound waves can be characterized as periodic waves, which means that the atoms that make up the matter experience simple harmonic motion.
A vibrating string produces a sound wave as illustrated in Figure This creates slightly higher and lower pressures. The higher pressure The pressure disturbance moves through the air as longitudinal waves with the same frequency as the string. Some of the energy is lost in the form of thermal energy transferred to the air.
You may recall from the chapter on waves that areas of compression and rarefaction in longitudinal waves such as sound are analogous to crests and troughs in transverse waves. The amplitude of a sound wave decreases with distance from its source, because the energy of the wave is spread over a larger and larger area. But some of the energy is also absorbed by objects, such as the eardrum in Figure Figure From this figure, you can see that the compression of a longitudinal wave is analogous to the peak of a transverse wave, and the rarefaction of a longitudinal wave is analogous to the trough of a transverse wave.
Just as a transverse wave alternates between peaks and troughs, a longitudinal wave alternates between compression and rarefaction. Ask them why the sound of thunder is heard much after the lightning is seen during storms. This phenomenon is also observed during a display of fireworks.
Through this discussion, develop the concept that the speed of sound is finite and measurable and is much slower than that of light. The speed of sound varies greatly depending upon the medium it is traveling through. The more rigid or less compressible the medium, the faster the speed of sound. The greater the density of a medium, the slower the speed of sound. The speed of sound in air is low, because air is compressible. Because liquids and solids are relatively rigid and very difficult to compress, the speed of sound in such media is generally greater than in gases.
Table Students might be confused between rigidity and density and how they affect the speed of sound.
The speed of sound is slower in denser media. Solids are denser than gases. However, they are also very rigid, and hence sound travels faster in solids. Stress on the fact that the speed of sound always depends on a combination of these two properties of any medium. Sound, like all waves, travels at certain speeds through different media and has the properties of frequency and wavelength. Sound travels much slower than light—you can observe this while watching a fireworks display see Figure The relationship between the speed of sound, its frequency, and wavelength is the same as for all waves:.
Recall that wavelength is defined as the distance between adjacent identical parts of a wave. The wavelength of a sound, therefore, is the distance between adjacent identical parts of a sound wave. Just as the distance between adjacent crests in a transverse wave is one wavelength, the distance between adjacent compressions in a sound wave is also one wavelength, as shown in Figure The frequency of a sound wave is the same as that of the source.
For example, a tuning fork vibrating at a given frequency would produce sound waves that oscillate at that same frequency. The frequency of a sound is the number of waves that pass a point per unit time. Fret placements on instruments such as guitars, banjos, and mandolins, are mathematically determined to give the correct interval or change in pitch.
When the string is pushed against the fret wire, the string is effectively shortened, changing its pitch. Ask students to experiment with strings of different lengths and observe how the pitch changes in each case. One of the more important properties of sound is that its speed is nearly independent of frequency. If this were not the case, and high-frequency sounds traveled faster, for example, then the farther you were from a band in a football stadium, the more the sound from the low-pitch instruments would lag behind the high-pitch ones.
But the music from all instruments arrives in cadence independent of distance, and so all frequencies must travel at nearly the same speed. Hold a meter stick flat on a desktop, with about 80 cm sticking out over the edge of the desk.
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