Homework Helpers: Physics

8 Waves and Light


Lesson 8–3: Sound Waves

As I mentioned earlier, sound waves are longitudinal mechanical waves. They require a medium to travel through, and the molecules of the medium will be displaced parallel to the direction in which the wave travels. It is also interesting to mention that sound waves radiate in three dimensions, allowing us to hear sources that are above us, below us, in front of us, behind us, and so forth.

Now that we have discussed many of the properties and characteristics of waves in general, it will help you to apply these to a specific type of wave that you are very familiar with, such as sound. In the last lesson, we gave an example of how sound waves exhibit diffraction, allowing sounds to spread out beyond barriers. Let’s look at how some other properties of waves can be applied to sound. Making these real-life connections will help you understand and remember the many terms associated with waves.

Which property of waves would be associated with the loudness of a sound? How does a whisper differ from a shout? A loud noise represents a greater disturbance, in the same way that a large rock creates a larger disturbance than a small rock when thrown in a pool of water. Large rocks create bigger waves in the water, and loud noises create bigger waves in the air. In this case, when we say “bigger” waves, we are talking about the amplitude. The amplitude of a sound wave determines how loud we perceive it to be. The decibel (dB) scale is often associated with the amplitude or intensity of sound waves.

It is easy to imagine a higher frequency wave traveling through a rope or through water. High frequency would mean that many wave cycles would pass through a given point every second. How would our ears perceive a high frequency sound wave as opposed to a low frequency sound wave? We perceive higher frequency sound waves as high-pitched sounds, and lower frequency sound waves as low-pitched sounds.

Human ears can only detect sound waves in a specific range—from around 20 to 20,000 hertz. As you may know, dogs can hear high-pitched sounds that humans can’t. A dog whistle that produces sound waves with a frequency of 35,000 hertz is quite inaudible to the human ear, yet still well within the hearing range of dogs. Bats are capable of producing and hearing sounds that are too high-pitched for even a dog to hear them, giving them the echolocation, or “radar,” for which they are famous.

How would you explain an echo in terms of waves? An echo is a result of the characteristic of waves called reflection. When a wave hits a certain barrier, such as a wall, it can be reflected or turned back. A bat can produce a high-pitched sound and if it hears that it is reflected back very quickly, it knows there is something in front of it.

Much like a bat, your ears can give you information about nearby objects that you may not even pay attention to. You probably use information about the amplitude of sound waves to determine if an object is coming toward you. For example, an approaching airplane or truck seems to grow louder as it grows closer. What you might not realize is that you can often tell if the source of a sound is getting closer to you by listening for a change in the pitch of the sound.

The Doppler Effect

The apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves.

The Doppler effect is responsible for the change in the pitch of a sound as it moves relative to you. When the source of a sound is approaching you, the sound waves that it produces are reaching you faster, resulting in a higher perceived frequency and a higher perceived pitch. When a sound source is moving away from you, the sound waves reach you at a slower rate, resulting in a lower pitched sound. The next time you hear a siren in the street, try to listen for changes in pitch, as well as changes in amplitude.

Lesson 8–3 Review

1. _______________ is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves.

2. The ________________ of a sound wave is responsible for its perceived loudness or intensity.

3. The higher the _______________ of a sound wave, the higher the perceived pitch.