Study Guide: Propagation of Sound G6

Main Ideas

1. Vibrations cause waves of vibrations in a medium, and sound is transported by these vibrations.
2. Sound requires a source, a medium and a receiver.
3. We hear sounds because vibrations in the air cause our ear drums to vibrate.
4. Resonance occurs when a object vibrates in harmony with a sound wave.
5. Sound travels at about 343 m/s in air.
6. Sound travels at different speeds in different mediums and conditions.
7. Reflection phenomena (echos) are commonly heard with sound waves. We can use echos to measure distance, for example in auto-focusing cameras.

Vocabulary

• Vibration
• Sound Wave
• Pitch (Frequency)
• Volume (Amplitude)
• Frequency (Tone)
• Timbre (Qualities of sound)
• Resonance
• Echo (Reflection Phenomena)
• Speed of Sound

What is Sound?

A sound is a form of energy, similar to electricity, heat or light. Sound waves are the result of the vibration of objects. These vibrations propagate as acoustic waves through a medium such as a gas, liquid or solid.

Vibrations and Waves

Sound is all about vibrations. To make a sound, there needs to be a source—something that vibrates—whether it’s a musical instrument, the larynx (voice box) of a person, or the movement of Tectonic Plates. But there’s more to it than that.

Sound vibrations need a medium to travel through, a substance capable of transmitting vibrations from one location to another.

Finally, there must be a receiver—something capable of receiving and interpreting the vibrations as sound, such as our ears or a microphone.

Sounds Waves

Vibrations cause waves. Sound waves are made of vibrating particles that bump into other particles, causing those particles to vibrate and bump into more particles, and so on. This is why sound travels away from its source. A wave can be thought of as energy traveling through a medium.

Medium

The medium is a series of interacting particles through which the energy moves. This medium might be a gas, fluid, or solid. By the way, sound can not travel through a vacuum because there are no particles to bump into each other.

Compression Wave

Source: Christophe Dang Ngoc Chan (cdang), CC BY-SA 3.0, via Wikimedia Commons.

Human Hearing

Humans can hear sounds ranging from 20 Hz to 20 kHz. Sounds with frequencies above the range of human hearing are called ultrasonic. Sounds with frequencies below the range of human hearing are called infrasonic. The typical sound produced by human speech have frequencies in the order of 100 to 1,000 Hz.

Our two ears and brains are so sensitive to sound vibrations that they can tell when a impacts one ear before it impacts the other. This is why we can tell the direction that a sound comes from.

Sound vibrations reaching the human ear.

Characteristics of Sound

Some sounds are pleasant; some are irritating. When we strum a guitar, or thump on a drum each creates a unique sound that we can recognize. Each instrument has its own voice. Each material is only able to produce specific sounds. This is why banging on metal sounds like banging on metal, and banging on wood sounds like banging on wood.

Volume (or Amplitude): Volume is a measure of how loud a sound is.

Pitch (or Tone): Pitch is how how “high” or “low” a sound is. The faster an object vibrates (frequency), the higher the pitch of the sound.

Timbre (Qualities of sound) the quality of different sounds (e.g. the thud of a fallen rock, the whir of a drill, the tone of a musical instrument or the quality of a voice)

Wavelength: The distance between two consecutive peaks or trough of a sound wave are it’s wavelength.

Frequency: The number of cycles per second is the frequency of a sound. Frequency is measured in cycles per second, or Hertz.

Resonance: Many objects will resonate in harmony with sounds. A piano is a complex instrument designed to resonate at many different pitches at once. Human ears drums resonate between 20 Hz to 20 kHz

Reflection of Sound (Echos)

Echos are a reflections of sounds that bounce (reflect) off of a distant object and arrive back at our ears after a while. Sound travels slow enough in air that we can often notice the time it takes for the sound to return.

Speed of Sound

We usually think of the speed of sound in air, which is about 343 m/s (r 1,235 km/h), however the speed of sound is not the same everywhere. It depends on the type of material the sound vibrations are passing through.

Sound travels more slowly in gases, faster in liquids, and fastest of all in solids. Sound travels fastest in solids because the molecules are packed more closely together. Sound travels slowest in gases because the molecules are spread farther apart.

The speed of sound is also be affected by the temperature and elasticity of the medium.