Sound

Sound

Introduction

Sound is all about vibrations. The source of the sound vibrates, bumping into the air molecules nearby, which in turn bump into their neighbours and so forth. This results in a wave of vibrations through the air to the eardrum, which in turn vibrates. What the sound wave will sound like when it reaches the ear depends on a number of things such as the medium it travels through and the strength of the initial vibration.

In the following activities, students will use simple supplies to create, visualize, and feel sound waves, investigate vibration and its role in producing sound, and make their own percussion instruments.

Objectives

• Describe how sound is produced.
• Understand how our inner ear contributes to hearing.
• List some properties of sounds.
• Describe what pitch is and how it varies.

Background

Sound is a type of energy made by vibrations. When an object vibrates, it causes movement in the air molecules surrounding it. These molecules bump into the molecules close to them, which causes them to vibrate as well. This, in turn, makes them bump into more air particles. This movement, called sound waves, keeps going until the molecules run out of energy. As a result, there is a series of collisions as the sound wave passes through the air, but the air molecules themselves don't travel with the wave. Each molecule moves away from a resting point and then eventually returns to it.

Pitch and Frequency

If your ear is within range of the vibrations, you hear the sound. However, the vibrations need to be at a certain speed to be able to hear them. For example, we would not be able to hear slow vibrations that are made by waving our hands in the air. The slowest vibration our human ears can hear is 20 times a second. That would be a very low pitched sound. The fastest vibration we can hear is 20,000 times per second, which would be a very high pitched sound. Cats can hear even higher than dogs, and porpoises can hear the fastest vibrations of all (up to 150,000 times per second). The number of vibrations per second is referred to as an object's frequency, measured in Hertz (Hz).

Pitch is related to frequency, but they are not equivalent. Frequency is the scientific measure of pitch. While frequency is objective, pitch is completely subjective. Sound waves themselves do not have pitch; their vibrations can be measured to obtain a frequency, but it takes a human brain to map them to that internal quality of pitch.

The pitch of a sound is largely determined by the mass (weight) of the vibrating object. Generally, the greater the mass, the more slowly it vibrates and the lower the pitch. However, the pitch can be altered by changing the tension or rigidity of the object. For example, a heavy E string on an instrument can be made to sound higher than a thin E string by tightening the tuning pegs so there is more tension on the string.

Nearly all objects, when hit, struck, plucked, strummed, or somehow disturbed, will vibrate. When these objects vibrate, they tend to vibrate at a particular frequency or a set of frequencies. This is known as the natural frequency of the object. For example, if you ping a glass with your finger, the glass will make a sound at a pitch that is the natural frequency. It will make this same sound every time. This sound can be changed by altering the vibrating mass of the glass. For example, adding water causes the glass to get heavier (increase in mass) and harder to move, so it tends to vibrate more slowly and, therefore, at a lower pitch.

What is Sound?

When we hear something, we are sensing the vibrations in the air. These vibrations enter the outer ear and cause our eardrums to vibrate (or oscillate). Attached to the eardrum are three tiny bones, the hammer, the anvil, and the stirrup. They also vibrate when the eardrum moves, but they make larger vibrations within the inner ear. Essentially, they amplify incoming vibrations before they are picked up by the auditory nerve.

The properties of a sound wave change when it travels through different media: gas (e.g. air), liquid (e.g. water), solid (e.g. bone). When a wave passes through a denser medium, it goes faster than it does through a less-dense medium. This means that sound travels faster through water than through air, and faster through bone than water.

When molecules in a medium vibrate they can move back and forth or up and down. Sound energy causes the molecules to move back and forth in the same direction that the sound is traveling. This is known as a longitudinal wave. (Transverse waves occur when the molecules vibrate up and down, perpendicular to the direction that the wave travels).

Speaking (as well as hearing) involves vibrations. To speak, we move air past our vocal cords, which makes them vibrate. We change the sounds we make by stretching those vocal cords. When the vocal cords are stretched we make high sounds and when they are loose we make lower sounds. This is known as the pitch of the sound.

The sounds we hear everyday are collections of simpler sounds. A musical sound is called a tone. If we strike a tuning fork, it gives off a pure tone, which is a sound of a single frequency. But if we were to sing or play a note on a trumpet or a violin, the result is a combination of one main frequency with other tones. This gives each musical instrument a characteristic sound.

Fun Facts

• The speed of sound is around 1,230 km per hour (767 miles per hour).
• The loud noise you create by cracking a whip occurs because the tip is moving so fast it breaks the speed of sound!

Entire Lesson

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Other Resources

Science World at TELUS World of Science | School Workshop | Sound
Science World at TELUS World of Science | Mitchell Odyssey Foundation Gallery: Eureka!
Engaging Science | Online Games | Make a Note