PSAA Curriculum Unit Physical Science Systems

Problem Area Energy and Power Systems

Sound Lesson

How is sound produced? What happens to the sound when we change the position of the source and change the position of the observer recording the sound? What happens to the sound when we change the position of the source and change the position of the observer recording the sound?

Learning Objectives Explain how a sound wave is produced and how it travels through air. Describe the relationship between volume and the amplitude of a sound wave. Explain the Doppler Effect and the reasons for poor sound quality in buildings.

Learning Objectives (cont.) Identify sources of noise pollution and the means that can be used to control it. Explain how sound waves can be used to locate objects and uses of ultrasonic waves.

Terms Acoustics Decibels (dB) Doppler Effect Frequency Noise Noise Pollution Pitch Reverberation SONAR Sonogram Sound Ultrasonic waves Ultrasound Volume

How are sound waves produced and how do they travel through the air? If one strikes a tuning fork, it creates a sound. Prongs on the fork move toward each other and then apart. Air squeezed and then allowed to spread out, causing sound waves to move away from the fork. As the sound waves move, they cause the air molecules in a wider area to vibrate. Sound waves are longitudinal waves because the air molecules move back and forth along the path of the waves.

Sound Waves (cont.) The speed of sound is much slower than the speed of light. One can judge how far away lightning strikes by counting the seconds until one hears the thunder. The distance can be judged because sound waves travel at a certain speed.

Sound Waves (cont.) At zero degrees Celsius, sound waves travel in air at 331 meters per second. At warmer temperatures, the waves travel faster. On a hot summer day, for example, sound waves travel at speeds as great as 350 meters per second.

Wavelength, Frequency, and Their Relationship

What relationship exists between the volume and amplitude of a sound wave? Does the speed of sound vary with temperature? The loudness of sound waves is referred to as volume. The greater the vibration of the air molecules that reach your ears the greater the volume of the sound. The more energy that is used to create a sound wave the greater the amplitude and higher the volume of the sound.

Volume and Amplitude Volume of sound is measured in units called decibels (dB). A sound just barely loud enough to hear has a value of 0 dB. A 90-dB sound has enough volume to damage the eardrums. This sound level is often reached in factories and at rock concerts. A 120 dB sound can cause ear pain. The jet engine of an airplane produces sound with a volume of approximately 130 dB.

Volume and Amplitude The highness or lowness of a sound is called pitch. The frequency at which the sound waves occur will determine the pitch. Frequency is the number of vibrations of a wave that pass a given point each second. Many animals can hear higher or lower frequencies than humans can hear.

Volume and Amplitude Dogs can hear a whistle that produces sound waves of 40,000 Hz (Hertz). Bats navigate in the dark by using sound waves of approximately 100,000 Hz. The bats make the sounds and then navigate by the echoes created as the waves are reflected from objects. Most people can hear sounds with frequencies between 18 Hz and 20,000 Hz.

Characteristics of Sound

How does the motion of a source of sound affect the pitch that is heard and what reasons exist for poor sound quality in a building? As a vehicle moves forward, the sound waves in front of the vehicle are compressed. These waves have a shorter wavelength and a higher frequency.

Doppler Effect The sound waves in back of the vehicle are spread farther apart. They have a longer wavelength and a lower frequency. A change in the frequency of waves caused by a moving sound wave source or moving observer is called the Doppler Effect.

Doppler Effect (cont.) The higher the frequency of a sound wave, the higher the pitch of the sound. Therefore, as the vehicle approaches, the sound has a high pitch. Once the vehicle has passed by, the pitch of the sound decreases.

Doppler Effect (cont.) The Doppler Effect also applies to situations in which the observer, rather than the wave source, is moving. As the observer moves in a direction against the sound wave, the waves appear to increase in frequency. The actual frequency of the wave does not change.

Doppler Effect (cont.) In poorly designed buildings, sounds can get mixed up, distorted, and even blocked or destroyed. Acoustics is the branch of physics that deals with the transmission of sound. Acoustical engineers try to design the shapes of buildings to carry clear sounds throughout.

Doppler Effect (cont.) Reverberation is the combination of many small echoes, occurring very close together. It is caused when reflected sound waves meet at a single point, one right after another. This action causes sounds to take a long time to fade. Reverberation is the main factor preventing good acoustics. Too much reverberation results in sounds overlapping and becoming distorted.

Doppler Effect (cont.) Seats, lighting fixtures, and decorative details all affect the acoustics of an auditorium. These items cause the reflection of sound waves, as do the walls and ceiling. Sound-absorbing materials such as drapes, slotted blocks, or acoustical tiles help control the reflection of the waves to improve the acoustics of the space.

What are some sources of noise pollution and the means that can be used to control it? Noise is sound produced by a random mixture of sound-wave frequencies. Noise is not only irritating to hear but also can be dangerous. Even the sounds of traffic on a busy street can be loud enough to damage your ears.

Noise Pollution Noise pollution applies to noise that can damage your health. There are laws that help protect people from noise pollution. In factories where noise cannot be lowered to a safe level, workers are required to wear sound-deadening earmuffs.

Noise Pollution The Federal Aviation Administration sets standards for airport noise. Cities pass laws controlling loud traffic noises, such as blowing horns. Automobile and truck engines are required to have good mufflers. In designing machinery, engineers make special efforts to design machines that will produce a minimum amount of noise.

How are sound waves used to locate objects and determine the depth of water and what are some uses of ultrasonic waves? Bats find insects by sending out pulses of very high frequency sound waves. The waves bounce off the insects, and the bats find their prey by listening for the echoes. Bats could not use audible sound waves because the wavelengths are too long. Insects only reflect short wavelengths with high frequency.

Ultrasonic Waves Ultrasonic waves are sound waves with frequency too high for humans. Ships have a navigation system that uses echoes of ultrasonic waves to find the depth of water. SONAR (Sound Navigation Ranging) sends short pulses of ultrasonic waves through the water.

Ultrasonic Waves (cont.) When the sound waves hit the sea floor, some of the waves are reflected back to the ship as an echo. The echo is detected by a SONAR receiver. Sound waves travel much faster in water than they do in air, at approximately 1500 meters per second.

Ultrasonic Waves (cont.) Ultrasound is ultrasonic waves used for medical applications. Ultrasound provides a way to “see” inside the body. The ultrasonic waves bounce off high density tissue, such as a tumor. The reflected waves are converted into electrical signals and fed into a computer.

Ultrasonic Waves (cont.) The computer uses these signals to create an actual picture, called a sonogram. The sonogram can be used to examine a developing baby inside its mother, to find out whether it is forming normally and is in the proper position.

Review/Summary How are sound waves produced and how do they travel through the air? How are sound waves produced and how do they travel through the air? What relationship exists between the volume and amplitude of a sound wave? What relationship exists between the volume and amplitude of a sound wave? –Does the speed of sound vary with temperature? How does the motion of a source of sound affect the pitch that is heard and what reasons exist for poor sound quality in a building? How does the motion of a source of sound affect the pitch that is heard and what reasons exist for poor sound quality in a building?

Review/Summary What are some sources of noise pollution and the means that can be used to control it? What are some sources of noise pollution and the means that can be used to control it? How are sound waves used to locate objects and determine the depth of water and what are some uses of ultrasonic waves? How are sound waves used to locate objects and determine the depth of water and what are some uses of ultrasonic waves?