Unit V: Sound

 

Introduction

Sound waves are all around us most of the time. They provide us with a major source of communication (speech), a source of enjoyment (music), and a source of irritation or distraction (noise). But do you know what sound is? This unit is divide into three sections: vibrations and waves, sound, and musical instruments. It should take two weeks to explore this unit. Intended Learning Outcomes
 
General Instructional Objectives (and corresponding Specific Learning Outcomes) Unit Content
Knows terms

Understands the meaning of terms

transverse and longitudinal waves, sine curve, amplitude, wavelength, frequency, hertz, period, wave speed, interference pattern, shock wave, infrasonic, ultrasonic, compression, rarefaction, reverberation, refraction, forced vibration, standing wave, bow wave, speed of sound, resonance, beats, pitch, loudness, modulation, partial tone, harmonic
Understands scientific concepts wave motion and speed, interference pattern, sonic boom, Doppler Effect, natural frequency, carrier wave, amplitude and frequency modulation (AM and FM), fundamental frequency
Knows all laboratory procedures

Prepares a plan for an experiment

Participates in classroom activities

Respects the scientific process

Displays a scientific attitude

Demonstrates skills in laboratory work

All content

 

Instructional Foci

Experiments: standing waves, speed of sound in air; demonstrations: tuning fork in large room—note interference patterns (rarefactions and compressions add and subtract in different areas to created different loudness), slinky (transverse and longitudinal waves), rope (nodes), look at sound waves on an oscilloscope, two combs to represent beats; homework—make a musical instrument (in class listen to it using the oscilloscope, quizzes, class discussion, questions for thought from the Flying Circus of Physics Teaching Strategies To begin the unit on sound, the teacher should discuss vibrations and waves. The period of a pendulum as discovered by Galileo, should be discussed. Following this should be a description of waves (including simple harmonic motion, sine curves, amplitude, and frequency. Once these basic terms have been taught, wave motion and wave speed may be studied. To distinguish between transverse and longitudinal waves a slinky attached to the wall should be used. By pushing and pulling the slinky you create a longitudinal wave. By shaking the end of the slinky up and down a transverse wave is created. Then interference patterns, where two waves overlap, canceling in places and reinforcing in places, should be discussed. To demonstrate standing waves, which are caused by interference, tie one end of a rope to the wall and shake the free end until standing waves of one, two and three nodes are produced. When the motion of the receiver or transmitter of sound moves there is a change in frequency. This is called the Doppler effect. One relevant example of the Doppler effect is the changing pitch of a car (ambulance, fire truck, etc…) horn as a car drives by a stationary person. If something were to move at a speed equal to the speed of the wave (sound) a wave barrier is produced. This happens when an aircraft travels at the speed of sound. A great deal of thrust is necessary to overcome the wave barrier. However, once overcome the aircraft can fly faster than the speed of sound—supersonic. When something moves faster than its wave speed it produces a pattern of waves know as bow waves, which make the shape of a "v". In three dimensions this is known as a shock wave, which produces waves in the shape of a cone. Supersonic aircraft produce this and when it reaches listeners on the ground they here it as a loud crack or sonic boom.

Now to dissect what we hear, sound. The teacher should begin by exploring where sound comes from. The answer is usually sound comes from vibrations of material objects. He/she should talk about the medium through which sound travels, the type of waves most commonly produced, the speed of sound, and the frequency of waves. Sound can also be reflected, which we call an echo or, in terms of multiple reflections, reverberation. An ultrasound, which most students should have at least heard of, is due to the reflection and refraction of high-frequency sound. The small relatively small amount of energy needed to produce sound should be discussed. Combining forced vibrations that match an objects natural frequency causes a dramatic increase in amplitude known as resonance. An example of resonance sometimes occurs when swinging on a swing. If somebody gives a small push to the swinger in rhythm with the frequency of the swinging motion, the amplitude is greatly increased. On the contrary, when the crest of a wave meets the trough of a wave their amplitude decreased to almost zero. This is called interference. Interference can be demonstrated by striking two tuning forks out of rhythm in a large room. If students walk around or sway their heads back and forth they will hear soft areas and loud areas produced from compressions and rarefactions adding and subtracting. If the two tones are of slightly different frequencies they will vary periodically in loudness. This is known as beats. Overlapping two combs of different teeth spacing will illustrate this phenomenon.

Finally, the last section of this unit will explore musical sounds. Pitch, loudness, and quality should be defined and discussed. You are then ready to study musical instruments and musical scales. Once these topics have been introduced students should be asked to create their own musical instrument, be it a blade of grass or a rubber band guitar. Students should bring their homemade musical instruments as well as any real musical instruments to class to be viewed in terms of frequency on an oscilloscope. A brief study of how compact discs work would be an interesting way to conclude the unit on sound.
 

Special Notes
  1. All labs should be preceded by an in class demonstration by the teacher of the proper procedures. After viewing the demonstration, students’ homework should be to prepare a plan for the experiment. Due to the time constraints of high school classes it is imperative that students have thought through the experiment before class. You, the teacher, need to know that they are aware of proper equipment set-up and safety precautions as well as appropriate hypothesis development, variable controls, and observation and measurement procedures.
  1. Labs can be found in Laboratory Manual in Conceptual Physics by Bill Tillery.

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