Lesson 7: Sound

Objectives

By the end of this lesson, students will be able to:

• explain how sound travels

• explain what factors affect the speed and behavior of sound waves

• summarize how sound is measured and identify the units by which it is measured

• apply how sound relates to every day experiences and technology

• define loudness, intensity, frequency, and resonance

• explain the Doppler Effect

Click here for the course glossary

Introduction

Have you ever been to a rock concert? Did the cheering of the fans and the sounds of the music leave your ears ringing? Scientists study the effects of noise on the human ear and take measurements to determine what is safe. Rock concerts measure right on the edge of damaging to the ears. The noise level of a rock concert registers at just one step below that of a jet plane taking off.

The last lesson was about waves, their properties, and how they interact with each other and the environment. This lesson builds on that knowledge with a more in-depth look at sound. Sound waves, intensity, pitch, and the ear are just a few of the topics to be covered in this lesson.

This lesson builds on the information from the previous lesson on waves. If you are unfamiliar with any of the concepts presented here, and would like more explanation than the glossary provides, refer to the Waves lesson.

Concert-goers sing along in Leeds, England

A Brief Review: What Are Waves?

A wave is a disturbance that transmits energy through matter or space. Mechanical waves require a medium through which to travel. In other words, they need a solid, liquid, or gas that vibrates to pass along energy from place to place. Electromagnetic waves do not require a medium, and travel best in a vacuum, such as space.

The last lesson focused mainly on mechanical waves and their properties. There are three kinds of mechanical waves:

In transverse waves, the particles vibrate in an up and down motion. A transverse wave moves like a rope would move if you were to shake one end of it.

In longitudinal waves, the particles vibrate in a back and forth motion. A longitudinal wave moves like a spring toy would move when pushed and pulled.

In surface waves, the particles vibrate both up and down and back and forth.

It is important to remember that in all waves with a medium, the medium does not move. It is just a substance by which energy, in the form of a wave, travels.

What Are Sound Waves?

Sound waves are longitudinal waves that travel through air molecules.

Vibrations cause the displacement of air molecules, creating pressure in a pattern of compressions and rarefactions. Compressions are molecules that are pushed together. Rarefactions are molecules that are spread out. The pressure travels from particle to particle through the medium, and moves the energy along.

For sound waves, the amount of energy transferred through the medium depends on the amplitude of the wave. For example, the harder a guitar string is plucked, the larger the wave amplitude, and the greater the sound.

Properties of Sound Waves

Sound behaves in many different ways. It can travel through many different media. Almost every behavior of sound can be explained by examining its properties. These include speed, intensity, pitch, and frequency.

Speed

Sound waves must travel through a medium. The particles of the medium vibrate to transfer the wave energy. Sound can travel through many different media, but its speed varies. Sound travels fastest through solids, slower in liquids, and slowest in gases. The particles are closer together in solids than they are in liquids or in gases, and vibrations travel faster and better when the particles of the medium are closer together. The speed of sound is measured in meters per second, or m/s.

If you and a friend go scuba diving, you would hear many underwater sounds. You could hear the bubbling of your oxygen tanks and the clanking of equipment on your boat. This is because sound travels fairly well underwater. If your friend were to remove his mouthpiece and call out to you, you would hear his call at a rate of about 1449 meters per second. That is about 40 times faster than a car driving down the highway!

A scuba diver with mouthpiece removed

The speed of sound also depends on other factors. The density of the medium affects how quickly sound moves through it. The elasticity of the medium is important too. Elasticity refers to the way a substance snaps back to its original form after a force affects it in some way.

Think About It
 

Look at the following chart. What can you conclude about how temperature affects the speed of sound? What can you conclude about the density of lead compared to the density of cast iron?

Intensity

The intensity of a sound wave is the amount of energy that passes through a given point in the medium in a given amount of time. As the amount of energy increases, the amplitude of the wave increases. These increase the intensity of a sound wave. 

Intensity also depends on the distance from the source of the sound. For example, if someone whispers in your ear, the sound could be more intense than if they called to you from across a parking lot. The energy used to make the vibrations, however, would be less. As the distance from the source increases, intensity decreases.

Intensity is a measurement of energy. The term loudness is used to describe the physical response to the intensity of a sound. Loudness depends on the individual. One person's perception of loudness may be very different than another's and is based on such factors as health, age, and how the brain interprets sound.

Frequency

The frequency of a sound wave is the number of sound waves produced in a given amount of time. The vibrating speed of the sound's source is what determines the frequency.

You can tell by the shape and size of a musical instrument what type of frequency it will produce. This picture shows alpine horn, or alphorn, players. Compare a trumpet to the alphorn. The trumpet has a much shorter tube through which sound must travel. The alphorn has a long tube down which the sound must vibrate. The longer the tube, the longer the wavelength, and the lower the frequency.

German alphorn players

Like intensity, frequency is a scientific measurement. Related to frequency is pitch. Pitch is the frequency of a sound as heard or perceived by an individual. Pitch and frequency work together. High frequencies have high pitches and low frequencies have low pitches. However, pitch is subjective. Its effect depends on the person doing the listening. Pitch, like loudness, depends on age, health, and how the brain interprets sound.

Behavior of Sound Waves

If you completed the Waves lesson, you already know how waves interact. In case you did not complete the Waves lesson, here is a brief review of wave interactions.

There are two types of wave behavior: interaction between waves and interaction between waves and their environment. Interactions between waves include interference and resonance. Interference occurs when waves overlap or share the same space. When two waves meet and create a bigger wave, it is constructive interference. When two waves meet and cancel each other out to create a smaller wave, it is destructive interference.

Interactions between waves and the environment include reflection, refraction, and diffraction. Reflection occurs when a wave bounces off of a barrier. Refraction occurs when a wave bends as it passes from one medium to another at an angle. When a wave bends around a barrier or spreads out after it travels through an opening, it is called diffraction.

Sound waves experience many of these interactions. Have you ever called out across a valley or down a long hallway and heard an echo? Echoes are caused by sound waves reflecting off of barriers. Bats use echolocation to find their way and to spot insects. To do this, they send out a burst of sound that reflects off of surfaces. The bats listen for the echoes to determine how close barriers are.

Golden-crowned fruit bat

Sound waves commonly diffract as well. When it comes to sound waves, the longer the wavelength, the louder or more distinguishable the wave is. Imagine you are standing on a street corner waiting for a parade to wind its way through the city. Since lower frequencies have longer wavelengths, and longer wavelengths diffract better, you will be able to hear the sounds of the tubas and drums and other instruments with low, deep sounds first. Diffraction carries these sounds around buildings the easiest.

Resonance is also a behavior of sound waves. Remember that resonance is the response of one wave to another of the same frequency. That second wave's amplitude is pushed higher, resulting in a more forceful blast of sound. Concert halls are designed with resonance in mind; they push the music up and out over the audience.

If you have ever been inside a concert hall or a large auditorium, you may have noticed the design of the walls and ceilings. Acoustic engineers design these buildings so that what is happening on stage can be heard by everyone in the audience.

Large, angled panels on the ceiling help stop sound from reflecting over the audience. If sound reflects in the wrong place it can cause a muddied, unclear sound or an echo. The ceiling panels often curve upward to help gather the sound waves and point them toward the audience. There are also sound absorbing tiles, so that the music or sound from the stage is not reflected off of the floor.

Both the ceiling panels and tiles are placed to ensure that there are no dead spots in the hall. A dead spot is a place where destructive interference lowers the volume of the music.

Auditorium with sound-reflecting ceiling tiles in Lleida, Spain

How Do We Detect and Use Sound?

You now know that sound travels in waves, in a series of compressions and rarefactions. You have also learned that sound travels at different speeds in different media, and the difference between intensity and loudness as well as frequency and pitch. But how do we hear a sound? How do we know how loud or intense it is? How fast does it go? This next section answers all of these questions.

The Human Ear

When you hit a drum, you can see the top of it vibrate. The top of the drum, often called the head, is actually called the membrane. The membrane of the drum is the part that the drummer hits to start the vibrations that we hear. Like the drum, your ear has a membrane inside it. The membrane, called the eardrum, vibrates at the same frequency as the sound waves that strike it.

The sound waves pass through the eardrum to the middle ear. The middle ear contains three small bones that also vibrate. This helps to amplify the sound and make it clearer.

The vibrations from the bones in the middle ear travel to the inner ear. The inner ear contains a snail-shaped tube called the cochlea. The cochlea is filled with fluid and nerve cells that move back and forth with the motion of the vibrations. Their movement sends signals to the brain through the auditory nerve.

When sound waves travel to you, your outer ear picks up the vibrations and sends them to your middle ear. The middle ear receives and amplifies the vibrations. Then, your inner ear uses nerve cells to send the information to your brain.

Measuring Sound

The intensity of sound is the amount of energy that is transported from place to place. For sound to be classified at a certain intensity, it needs to be measured. The intensity of a sound wave is measured in watts/meter². Sound wave intensity decreases as the sound wave moves further away from the source. It decreases because the wavelengths are distributed over a greater surface area.

When sound levels are compared, the decibel (dB) scale is used. The decibel is a unit that is used to compare the intensity of different sounds. Decibels are measured in powers of 10. A 0-decibel sound can barely be heard by the human ear. A 20-decibel sound has 100 times the energy as a 0-decibel sound. A 30-decibel sound has 1000 times the power of a 0-decibel sound. Sounds greater than 90 dB can be harmful to the human ear. Rock concerts average about 110 to 120 decibels. A jet plane taking off measures from 120 to 160 decibels.

Tornado sirens, like the one pictured here, can emit blasts of sound that register up to a 138-decibel reading at 100 feet away. These are carefully positioned so that in an emergency they can be heard as far as 10 square miles. Standing under one of these sirens when it goes off can damage the eardrum.

The Federal Signal Thunderbolt siren

alerts residents in and around

Milwaukee, WI when tornadoes are spotted.

Using Sound

So far you have read about sound waves that you can hear, but that is only a small portion of the sound waves that travel around us. There are many ranges of sound that are too high or too low for humans to hear. Infrasound is sound with frequencies too low for humans to hear. These sounds can often be felt as vibrations. Infrasound is produced by nature in whale songs, volcanoes, falling ice shelves, or severe weather. It can also come from manmade sources such as diesel engines, quarry work, and sonic booms.

People who sense infrasound have reported anxious, sad, or scared feelings without knowing why. For this reason, infrasound is sometimes used in movie soundtracks and some music compositions to inspire a certain mood. Some scientists believe the presence of infrasound can explain supernatural phenomena. It can cause anxious feelings and seemingly mysterious vibrations that can even take form as grayish apparitions just out of one's line of sight.

One of the 66 tornados that ripped through Oklahoma on

May 3, 1999. The core of a tornado produces infrasound.

You are probably more familiar with the term ultrasound. Ultrasound is the opposite of infrasound. It is sound with frequencies that are too high for humans to hear. An ultrasound machine sends short bursts of sound, called pulses, that reflect off of barriers. Hospitals use ultrasound machines that work with computer software to take images of the heart or of developing fetuses.

Sonar is a type of ultrasound imaging. SONAR is an acronym that stands for sound navigation and ranging. It is a system that uses reflected sound waves to detect the distance to underwater objects. It sends out ultrasonic pulses and listens through a sensitive microphone for them to bounce back from obstacles. Sonar is used to measure water depth and to find enemy water vessels in times of conflict.

Traveling at the Speed of Sound

Have you ever heard of the Concorde? The Concorde was a plane that flew for British Airways and Air France at twice the speed of sound. The supersonic transport system (SST) was much talked about in the 1970s and 1980s. Progress was halted due to environmental concerns, years of low sales, and financial troubles. The plane was permanently grounded in 2003.

The Concorde takes off for its last flight in 2003.

The speed of sound depends on air density, which is affected by temperature and pressure. For example, as a plane climbs in altitude, the air pressure decreases. As the temperature goes down, the air particles carry sound more slowly. The speed of sound is different at different altitudes. The speed of sound in air is referred to as Mach 1.0. The Concorde traveled at twice the speed of sound, or Mach 2.0. Look at the chart below to see some examples of aircraft that can travel at Mach speed.

When an aircraft flies faster than the speed of sound, shock waves from the nose of the airplane change the air pressure around the plane. From the ground this sounds like an explosion. This is known as a sonic boom. Sonic booms can be harmful, depending on the size of the aircraft and the location of the observer. It can hurt an observer's ears and even break windows. Aircraft that travel over the speed of sound are no longer allowed to fly at that speed over populated areas. They are now required to fly over water at a certain distance from the shore.

What Is the Doppler Effect?

Have you ever stood on a street corner or watched out a window while an ambulance screamed past? You can hear the siren, but as the ambulance passes you, the pitch of the siren sounds lower than it sounded when the ambulance was far away. This phenomenon is a result of the Doppler effect, a change in sound frequency caused by motion.

Discovering the Doppler Effect

Christian Johann Doppler (1803-1853) was an Austrian physicist and mathematician. He taught at universities in Prague and Vienna, and eventually became the director of the Physical Institute of Vienna University. He is known for his theories about sound and motion. Doppler thought that the frequency of a sound seemed to change depending on whether the sound was coming towards or moving away from the listener.

Doppler tested his theory in 1845. He hired two horn players to play the same note at the same time. One player stood still, while the other sat in an open train car moving past Doppler and other observers. Doppler noted that as the horn player passed by, the horn sounded lower in pitch. Doppler related this phenomenon to the properties of sound waves. His theory was applied to the properties of light as well.
 

The Doppler Effect and Sound

The Doppler effect, as it relates to sound, is the change in a sound's frequency caused by the motion of the sound's source, the listener's position, or both. As a sound moves toward a listener, the listener hears a higher frequency. As the source of a sound moves away, the listener hears a lower frequency. This is because when the source of a sound moves toward the listener, the sound waves bunch up in front of it. The waves are more spread out behind a moving source and longer sound waves sound lower. This is why the sound is lower as the source moves away from the listener.

Suppose you are on the street and a car goes by, honking its horn repeatedly. As the car comes toward you, the sound waves are bunching together in your direction, shortening the wavelengths. As the car passes, you hear the sound waves from behind it. They are spread out, and so the pitch of the horn sounds lower as a result of the longer wavelengths.

Making Connections

Sound waves have their own special properties and behaviors, but they are still longitudinal waves. They exhibit the same properties of longitudinal waves found in the previous lesson on waves. As you continue through this course, you will also read about light waves. Keep in mind that all waves have basic characteristics in common. These include behaviors such as reflection, refraction, and interference.

In the last few decades, sound waves have become an important environmental concern. Humpback whales sing songs at low frequencies to communicate and look for mates. In the last 50 years, however, ocean traffic has increased dramatically. Ship engines are bigger, and their propellers stir up more bubbles. This produces a greater amount of noise than ever before. All of this activity emits low frequency noise into the ocean that some say negatively affects the whales. Since both whales and ships emit low frequency sounds, it is possible that these sounds will interfere with each other when they are close enough. However, other factors lead some researchers to question the degree to which whales are affected by ocean noise pollution. This topic continues to be debated across international lines.

Humpback whale